GB2071117A - Elastomers and tyres containing them - Google Patents

Abstract

Novel copolymers of aromatic vinyl compounds and conjugated diolefins, especially styrene-butadiene copolymers having a vinyl content of at least 30%, have a differential content of the aromatic vinyl compound such that in at least one of its end portions said differential content shows a sharp and substantial increase in the direction of the outer extremity of said end portion. Styrene-butadiene copolymers of special interest are those whose percentage styrene content changes in a portion of not more than 5% of the copolymer chain (as determined by monomer conversion) from a first value to a second value, said second value being at least 25 percentage points greater than said first value; and said portion lies within a terminal 10% portion of the copolymer chain (as determined by monomer conversion). The copolymers may be obtained by appropriate control of the conventional anionic solution copolymerization process. The copolymers of the invention are useful in the tread portion of tyres in that they provide improvement in rolling resistance and/or grip on wet road surfaces.

Description

SPECIFICATION Elastomers and tyres containing them This invention relates broadly to elastomeric copolymers of conjugated dienes and aromatic vinyl compounds, having novel structures and to processes of making them, to unvulcanised and vulcanised elastomeric compositions containing them and to their use in tyres, in particular in the tread portion of tyres. The invention is particularly concerned with copolymers derived from a styrene and a butadiene, for example styrene and 1 ,3-butadiene, and it is with reference to such polymers that the description hereinafter particularly relates.

The novel structure of the copolymers of this invention can be defined with reference to the distribution of the styrene or other aromatic vinyl compound in the copolymer molecule. For example, if the proportion of styrene in successive incremental portions of the molecule (differential styrene content) is plotted againsttotal monomer conversion (expressed as a percentage of the monomers reacting to form copolymer) there is obtained a line which represents the respective amounts of styrene present along the length of the molecule. By way of comparison, a graph of this sort in respect of a true random copolymer having, for example, an average styrene content of 23% by weight, is a straight horizontal line indicating that the styrene content of successive incremental portions of the molecule is substantially 23% throughout.

We have found, according to the present invention, that the properties of wet grip and/or rolling resistance of tyres whose tread comprises a styrene-butadiene copolymer are significantly improved if the copolymer is one having in at least one of its end portions of the molecule a significant styrene-rich component or zone such as is shown in the accompanying drawings.

The styrene-rich component can be considered to have two dimensions: length (that is, the proportion it constitutes of the total length or size of the molecule) and height (that is, the maximum differential styrene content in said end portion). In general, it appears that the more important feature is the maximum differential styrene content occurring in the end portion and especially where this maximum is reached over a shorter rather than a longer length of the end portion.

Accordingly, in a first aspect of the present invention provides an elastomeric copolymer of an aromatic vinyl compound and a conjugated diene, suitable for use in the tread portion of a pneumatic tyre, said copolymer having a vinyl content (as herein defined) of at least 30% by weight and having a differential content of the aromatic vinyl compound such that in at least one of its end portions said differential content shows a sharp and substantial increase in the direction of the outer extremity of said end portion.

In a second aspect the invention provides an elastomeric copolymer suitable for use in the tread portion of a pneumatic tyre, whose percentage differential styrene content changes in a portion of not more than 5% of the copolymer chain (as determined by monomer conversion) from a first value to a second value, said second value being at least 25 percentage points greater than said first value; and said portion lies within a terminal 10% portion of the copolymer chain (as determined by monomer conversion).

Copolymers of particular interest are those in which the percentage differential styrene content changes in a zone of not more than 21% of the copolymer chain (as determined by monomer conversion) from a first value to a second value, said second value being at least 14 percentage points greater than said first value; and said zone lies within a terminal 10% portion of the copolymer chain (as determined by monomer conversion). The 5% or2 portion or zone referred to above can, for example, be a terminal portion of the copolymer chain.

As can be seen from the accompanying drawings, the styrene tail in some of the copolymers of the invention is most pronounced in a very small portion of the copolymer chain, for example Wa, 1 Wa or 2% of the chain (as determined by total monomer conversion).

The term "vinyl content" as used herein refers to that portion by weight of the 1,3 butadiene or other diene component of the copolymer which has polymerised at the 1,2 positions. Where the diene is 1,3 butadiene the 1,2-polymerisation results in the formation of pendant vinyl groups; where the diene is other than 1,3 butadiene corresponding pendant groups are produced by 1,2-polymerisation.

The vinyl aromatic compound is normally a styrene or other mono-vinyl aromatic compound, for example: styrene, 1-vinyl naphthalene, 3,5-diethylstyrene, 4-n-propylstyrene, 2,4,6-trimethyl styrene, 4-phenyl styrene, 4-p-tolystyrene 3,5-diphenylstyrene, 3-ethyl-1-vinylnaphthalene, 8-phenyl-1-vinyl naphthalene.

Where, for example, branching or cross-linking is desired a polyfunctional vinyl compound can be used. For example, suitable polyfunctional vinyl compounds are divinyl compounds, for instance divinyl benzene.

The conjugated diene is one capable of polymerisation with styrene atthe 1,2 positions and such that, when polymerised with styrene or other selected aromatic vinyl compound or compounds, it provides a polymer having desired elastomeric properties. The diene can, for example, be a butadiene or pentadiene, for example: 1,3-butadiene 2-methyl-1,3 butadiene (isoprene), 1 ,3-pentadienes (piperylenes), 2,3-dimethyl-1,3-butadiene, 2methyl1 ,3-pentad iene, 2,3-d imethyl-1 ,3- pentadiene, 2-phenyl butadiene.

The styrene-butadiene copolymers of the invention can, for example, be ones whose average styrene content is at least 10% (for example 15, 20, The drawing(s) originally filed were informal and the print here reproduced is taken from a later filed formal copy.

25, 30 or 40%) by weight of the copolymer. However, the invention also includes copolymers whose aver age styrene content is less than 10% by weight, for example copolymers whose portion or portions other than the styrene-rich portion or portions has little or no styrene. Such a copolymer is that of Experiment 20 below. Mixtures of aromatic vinyl compounds and/or mixtures of dienes can be used, The copolymers can have at one or more of their extremities a block of polystyrene or polybutadiene, particularly the latter. A polybutadiene block will normally reduce rolling resistance at some expense to wet grip, and a polystyrene block will, to a lesser extent, enhance wet grip at expense to rolling resistance. Therefore a block can be used to provide (or at least to assist in providing) a desired combination of wet grip and rolling resistance.We have found, however, that in general any polystyrene block should not have a molecular weight as much as 50,000 when the molecular weight of the copolymer (excluding the block or blocks) is 300,000.

The preferred size of any polybutadiene block is in the range from 20,000 to 50,000 or, expressed as a percentage of a 300,000 molecular weight copolymer, 7% to 17% by weight. If a polystyrene block is used for its contribution to wet grip the size of the block is preferably in the range 20,000 - 40,000, or expressed as a percentage of a 300,000 copolymer, 7% to 13%.

The copolymers of the invention can be obtained by a solution polymerisation process using a suitable initiator, for example, a lithium-hydrocarbon compound. Suitable mono-lithium initiators (providing linear polymers) are, for example: methyl lithium, ethyl lithium, n-propyl lithium, isopropyl-lithium, n-butyl lithium, sec-butyl lithium, tert. butyl lithium, n-amyl lithium, isoamyl lithium, n-hexyl lithium, n-octyl lithium, phenyl lithium.

If it is wished to produce a double or branched polymer a dilithium or other polythithium initiator can be used, for example: tetramethylene dilithium, pentamethylene dilithium, hexamethylene dilithium, phenylethylene dilithium, tetra phenylethylene dilithium.

Polylithium initiators containing more than two lithium atoms per initiator molecule, are for example, those derived from lithium divinylbenzene of isoprene; such compounds are those commerically available under the trade names DILI-3 and DILI-1 (Lithium Corporation of America).

Suitable solvents (which can be two- or other multi-component solvents) for the reaction are, for example, alkanes, alkenes, cycloalkanes and cycloalkenes, for instance: benzene, toluene, xylenes, ethylbenzene, iso-butane, n-pentane, iso-pentane, n-heptane, iso-octane, n-decane, cyclopentane, methyl cyclopentane, dimethylcyclopentane, cyclohexane, methyl cyclohexane, dimethylcyclohex ane, 1-butene, 2-butene, 1-pentene, 2-pentene, cyc- lopentene.

Where it is wished to provide enhanced wet grip fortyres, there is preferably used in the polymerisation reaction a substance (structure modifier) whose use results in an increased amount of the butadiene being polymerised at the 1,2-positions. Such polymerisation results in the formation of vinyl groups (or corresponding groups where other conjugated dienes are used) which enhance wet grip of tyres whose tread contains the polymer.Examples of suitable modifiers are as follows: (1) Ethers, thioethers, cyclic ethers, tertiary amines; for instance diethylether, dimethylether, tetra hydrofuran, dioxane, orthodimethoxy benzene, Monoglyme, Diglyme,trieihylamine; (2) Hexamethylphosphortriamide; (3) Difunctional Lewis bases, for instance, tetramethyl, ethylenediamine; (4) Organic potassium or sodium compounds, for instance potassium-tert. butoxide.

The use of structure modifiers such as those referred to in the preceding paragraph results in 1,2polymerisation at the expense of 1,4 or other alphaomega polymerisation. Furthermore, the copolymerisation reaction rate of styrene with butadiene increases resulting in a more random copolymerisation. Thus, by suitable choice of reaction conditions it is possible to produce a copolymer having a content of butadiene, albeit small, extending as far as that portion of the polymer produced at or towards full conversion. By use of a suitable modifier no polystyrene blocks normally form at the end of the molecule unless additional styrene is added.

Various means can be adopted to provide an enhanced and/or an increasing proportion of styrene in that portion of the copolymer formed towards the beginning and/or end of the reaction. One such means is by use of a structure modifier as previously discussed. A second means is by the addition of one or more further portions of styrene (usually with butadiene) during the reaction, either in a continuous, semi-continuous or stepwise manner. A suitable semi-continuous process is one in which, during the reaction, the concentration of reactants changes and/or one or more of the other reaction conditions changes at sequential time intervals.It will be seen that some of the processes of the Exam ples are semi-continuous in that they employ a mixture of styrene and butadiene in the reactor before adding the modifier, and a further one or more portions of styrene is added subsequently. Such subsequent addition can be at the same time as introduction of the modifier (or a first portion thereof) into the reaction zone and/or at a later time.

Where said second means is used the proportion of styrene which is added in subsequent additions can usefully be greater than that in the reactor initially and one or more of the subsequent additions can be of styrene without butadiene.

Suitable temperatures for the reaction are, for example, in the range from 20 to 700 or 800C. Care has to be taken in using the highertemperaturesto speed up the conversion rate because highertemperatures favour 1 ,4-polymerisation and thereby can lead to a reduction in vinyl content below 30%.

If desired, the copolymers of the invention can be in the form of composite molecules obtained by coupling together two or more smaller copolymer molecules. Accordingly, in a third aspect the invention provides an elastomeric copolymer obtained by coupling together two or more copolymers of the first or second aspects of the invention. The coupling can be effected conveniently by use of a coupling agent; a difunctional coupling agent, for example, dibromoethane, provides a linear coupled copolymer, and a tri- or other poly- functional coupling agent, for example the tetra-functional compounds silicon tetrachloride (SiCI4), diethyladipate (DEAP), dimethyladipate our stannic chloride (SnCI4) provides a non-linear or branched coupled copolymer.

It is to be noted that the styrene content depicted in all the accompanying drawings is that of uncoupled copolymer.

The coupling is normally preferably performed so that the coupling copolymer molecules couple with one another at the, or an an, end of the molecule not having a tail. Coupling normally is effected at the end of the copolymerisation reaction and therefore is the copolymer is one having an end tail the coupling results in the tail ceasing to be "free" but becoming linked to another copolymer molecule. Where the coupling is only partly complete, for example at 50% of the theoretical amount, the product of the coupling reaction consists partly of coupled copolymer and partly of uncoupled copolymer and the product of such coupling when used in tyre treads provides, at least to some extent, a useful combination of wet grip and rolling resistance values, though increasing degrees of coupling at end tails is detrimental to said combination of values.

On the other hand, coupling of copolymers having begin tails does not reduce the amount of free tails and in consequence there is no such detriment to said combination of values. Therefore in general it is preferred, when a coupled product is required, to use a begin tail copolymer as precursor.

Another way of providing branched polymers is to use an initiator having three or more active functions.

The polymerisation process can be terminated by use of an end-stopping agent; this can be a protonreleasing compound, for instance water, an alcohol or an amine.

This invention is illustrated by the following Examples (also referred to below as "Experiments").

These Examples describe the production of copolymers of styrene and 1,3 butadiene, some of which were subsequently coupled to produce coupled copolymers. The formulation of the copolymers in elastomer compositions and the use of those compositions in (model) tyres to demonstrate wet grip and rolling resistance properties of the elastomers is also described.

It will be seen that all the processes described are solution polymerisation processes using a lithium hydrocarbon compound and a structure mod ifier/randomiser. In some Examples the conditions are such that an increasing proportion of styrene is present in the reactor in the later part of the reaction; and in some Examples there is an excess of styrene in the reactor in the earlier part of the reaction, which falls towards the end of the reaction.

It is believed that the high proportion of styrene (in admixture with butadiene) at the early or late stage or stages of the reaction is related to the superior properties shown by the product copolymers. It will be seen that in the Examples a very full conversion was obtained, and this is believed to be of particular importance especially where a high proportion of styrene is presenttowards the end of the reaction.

Experiments 1 to 4 and 6 to 10 below describe the production of nine copolymers of styrene and 1,3butadiene, two of which were subsequently coupled to produce coupled copolymers.

Experiment No. 1 A 10 1. stainless steel reactor was charged with 2000 grams of cyclohexane and 2000 grams of isopentane. Hereafter were added 69 grams of styrene and 231 grams of butadiene and 2,25 grams of orthodimethoxybenzene. The contents were brought to a temperature of 60"C. Next the impurities in the solvent/monomer blend, capable of reacting with sec. butyllithium were deactivated by titration with a sec. butyllithium solution in cyclohexane (conc. 100 mmol/l). After a temperature rise of 0.5"C was observed, indicating that all impurities capable of interfering with the polymerisation reaction had been deactivated, a sec. butyl lithium solution (33.3 ml of a 100 mmol/l solution in cyclohexane) was added to initiate the polymerisation.

Simultaneously with the addition of the initiator 700 grams of a styrene/butadiene blend (weight ratio styrene/butadiene 23/77 was pumped into the reactor at a rate of 12 grams/min.

The temperature of the reactor contents were kept at 60C by cooling.

After the addition of these monomer quantities the polymerisation reaction was allowed to proceed for a further 60 minutes, by which a monomer conversion of > 99.9% was obtained. Hereafter 0.5 grams of methanol was added to kill the polymerisation reaction.

Finally 0.5 grams of 2.6 di-tert. butylparacresol were added as a stabiliser. The polymer was recovered from the solution by coagulation with steam.

Experiment Nos. 2 and 3 Were carried out as experiment No. 1, except that in experiment No. the amountoforthodimethox- ybenzene (ODMB) added was 1,13 grams and experiment No.3 the amount of ODMB was 0,50 grams.

Experiment No. 4 A 10 I. stainless steel reactor was charged with 2000 grams of cyclohexane and 2000 grams of isopentane. Hereafter were added 15 grams of styrene, 85 grams of butadiene and 2.25 grams of orthodimethoxybenzene. The contents were brought to a temperature of 60"C. Next the impurities in the solvent/monomer blend, capable of reacting with sec. butyllithium were deactivated by titration with a sec. butyllithium solution in cyclohexane (conc. 100 mmol/l). After a temperature rise of0.5'Cwas observed, indicating that all impurities capable of interfering with the polymerisation reaction had been deactivated, a sec. butyllithium solution (33,3 ml of a 100 mmol/l solution in cyclohexane) was added to initiate the polymerisation.

Simultaneously with the addition of the initiator 400 grams of a styrene/butadiene blend (weight ratio styrene/butadiene 15/85) was pumped into the reac tor at a rate of 20 grams/min.

The temperature of the reactor contents were kept at 60"C by cooling.

Immediately after the addition of monomers had been completed another portion of 500 grams of styrene/butadiene blend (weight ratio styrene/butad iene 31/69) was pumped into the polymer solution likewise at a rate of 20 grams/min.

After the addition of these monomer quantities the polymerisation reaction was allowed to proceed for a further 90 minutes, by which a monomer conversion of > 99.9% was obtained. Hereafter 0.5 grams of methanol was added to kill the polymerisation reaction.

Finally 0.5 grams of 2.6 di-tert. butylparacresol were added as a stabiliser. The polymer was recovered from the solution by coagulation with steam.

Experiment No. 6 A 10 I. stainless steel reactor was charged with 2000 grams of cyclohexane and 2000 grams of isopentane. Hereafter were added 115 grams of styrene, 38.5 grams of butadiene and 2.25 grams of orthodimethoxybenzene. The contents were brought to a temperature of 60"C. Next the impurities in the solvent/monomer blend, capable of reacting with sec. butyllithium were deactivated by titration with a sec. butyllithium solution in cyclohexane (conc. 100 mmol/l). After a temperature rise of 0.5"C was observed, indicating that all impurities capable of interfering with the polymerisation reaction had been deactivated, a sec. butyl lithium solution (33.3 ml of a 100 mmol/l solution in cyclohexane) was added to initiate the polymerisation.

Simultaneously with the addition of the initiator 654 grams of a styrene/butadiene blend (weight ratio styrene/butadiene 17.5/82.5) was pumped into the reactor at a rate of 43.6 grams/min. The temperature of the reactor contents were kept at 60"C by cooling.

Immediatelyafterthe addition of monomers had been completed 192.5 grams of butadiene was pumped into the polymer solution at a rate of 2.00 grams/min.

Afterthe addition of the last monomer quantity the polymerisation reaction was allowed to proceed for a further 40 minutes, by which a monomer conversion of > 99.9% was obtained. Hereafter 0.5 grams of methanol was added to kill the polymerisation reaction.

Finally 0.5 grams of 2.6 di-tert. butylparacresol were added as a stabiliser. The polymer was recovered from the solution by coagulation with steam.

Experiment No. 7 A 10 1. stainless steel reactor was charged with 2000 grams of cyclohexane and 2000 grams of isopentane. Hereafter were added 30 grams of styrene, 70 grams of butadiene and 2.25 grams of orthodimethoxybenzene. The contents were brought to a temperature of 60"C. Next the impurities in the solvent/monomer blend, capable of reacting with sec. butyllithium were deactivated by titration with a sec. butyllithium solution in cyclohexane (conc. 100 mmol/l). After a temperature rise of 0.5"C was observed, indicating that all impurities capable of interfering with the polymerisation reaction had been deactivated, a sec. butyllithium solution (33.3 ml of a 100 mmol/l solution in cyclohexane) was added to initiate the polymerisation.

Simultaneously with the addition of the initiator 900 grams of a styrene/butadiene blend (weight ratio styrene/butadiene 22.2/77.8) was pumped into the reactor at a rate of 15 grams/min. The temperature of the reactor contents were kept at 60"C by cooling.

The polymerisation reaction was allowed to proceed for a further 90 minutes, ba which a monomer conversion of > 99.9% was obtained. Hereafter 0.5 grams of methanol was added to kill the polymerisation reaction.

Finally 0.5 grams of 2.6 di-tert, butylparacresol were added as a stabiliser. The polymer was recovered from the solution by coagulation with steam.

Experiment No. 8 A 10 1. stainless steel reactor was charged with 2000 grams ofcyclohexaneand 2000 grams of isopentane. Hereafter were added 115 grams of styrene, 38.5 grams of butadiene and 2.25 grams of orthodimethoxybenzene. The contents were brought to a temperature of 60"C. Next the impurities in the solvent/monomer blend, capable of reacting with sec. butyllithium were deactivated by titration with a sec. butyllithium solution in cyclohexane (conc. 100 mmol/l). After a temperature rise of 0.5"C was observed, indicating that all impurities capable of interfering with the polymerisation reaction had been deactivated, a sec. butyllithium solution (66.6 ml of a 100 mmol/l solution in cyclohexane) was added to initiate the polymerisation.

Simultaneously with the addition of the initiator 654 grams of a styrene/butadiene blend (weight ratio styrene/butadiene 17.5/82.5) was pumped into the reactor at a rate of 81.7 grams/min. The temperature of the reactor contents were kept at 60"C by cooling.

Immediately after the addition of monomers had been completed 192.5 grams of butadiene was pumped into the polymer solution at a rate of 1.9 grams/min.

After the addition of these monomer quantities the polymerisation reaction was allowed to proceed for a further 40 minutes, by which a monomer conversion of > 99.9% was obtained. Hereafter 0.33 grams of diethyladipate was added to couple the polymer chains to a polymer with a branched structure of a fourfold increased molecular weight (coupling efficiency 50%). Next 0.5 grams of methanol was added to kill the polymerisation reaction.

Finally 0.5 grams of 2.6 di-tert. butylparacresol were added as a stabiliser. The polymer was recovered from the solution by coagulation with steam.

Experiment No. 9 A 10 I. stainless steel reactor was charged with 2000 grams of cyclohexane and 2000 grams of isopentane. Hereafter were added 115 grams of styrene, 38.5 grams of butadiene and 2.25 grams of orthodimethoxybenzene. The contents were brought to a temperature of 60"C. Next the impurities in the solvent/monomer blend, capable of reacting with sec. butyllithium were deactivated by titration with a sec. butyllithium solution in cyclohexane (conc. 100 mmol/l). After a temperature rise of 0.5"C was observed, indicating that all impurities capable of interfering with the polymerisation reaction had been deactivated, a sec. butyllithium solution 66.6 ml of a 100 mmol/l solution in cyclohexane) was added to initiate the polymerisation.

Simultaneously with the addition of the initiator 654 grams of a styrene/butadiene blend (weight ratio styrene/butadiene 17.5/82.5) was pumped into the reactor at a rate of 81.7 grams/min. The temperature of the reactor contents were kept at 60"C by cooling.

Immediately after the addition of monomers had been completed 192.5 grams of butadiene was pumped into the polymer solution at a rate of 1.9 grams/min.

After the addition of these monomer quantities the polymerisation reaction was allowed to proceed for a further 40 minutes, by which a monomer conversion of > 99.9% was obtained. Hereafter 0.66 grams of dibromoethane was added to couple the polymer chains to a linear polymer of a two-fold increased molecular weight (coupling efficiency 50%). Next 0.5 grams of methanol was added to kill the polymerisation reaction.

Finally 0.5 grams of 2.6 di-tert. butylparacresol were added as a stabiliser. The polymer was recovered from the solution by coagulation with steam.

Experiment No. 10 A 10 1. stainless steel reactor was charged with 2000 grams of cyclohexane and 2000 grams of isopentane. Hereafter were added 55 grams of styrene, 445 grams of butadiene and 2.25 grams of orthodimethoxybenzene. The contents were brought to a temperature of 60"C. Next the impurities in the solvent/monomer blend, capable of reacting with sec. butyllithium were deactivated by titration with a sec. butyllithium solution in cyclohexane (conc. 100 mmol/l). After a temperature rise of 0.5"C was observed, indicating that all impurities capable of interfering with the polymerisation reaction had been deactivated, a sec. butyllithium solution (33.3 ml of a 100 mmol/l solution in cyclohexane) was added to initiate the polymerisation.

After 60 minutes another portion of 500 grams of a styrene/butadiene blend (weight ratio styrene/butadiene 35/65) was pumped into the polymer solution at a rate of 33.3 grams/min.

After the addition of these monomer quantities the polymerisation reaction was allowed to proceed for a further 60 minutes, by which a monomer conversion of > 99.9% was obtained. Hereafter 0.5 grams of methanol was added to kill the polymerisation reaction.

Finally 0.5 grams of 2.6 di-tert. butylparacresol were added as a stabiliser. The polymer was recovered from the solution by coagulation with steam.

The Examples are accompanied by graph prints showing a plot of percentage differential styrene in the copolymer part of the molecule against total monomer conversion (which corresponds to percentage molecular size of the copolymer molecules).

The differential styrene contents at various conversions were calculated from the copolymerisation kinetics of styrene and butadiene using the following equation (1) S1 - S R1 S + B B1 B R2B+S (1) in which S1 = weight fraction of styrene in copolymer B1 = weight fraction of butadiene in copolymer S = weight fraction of styrene in monomer blend B = weight fraction of butadiene in monomer blend R1 = reactivity ratio of styrene R2 = reactivity ratio of butadiene The styrene and butadiene weight fractions in the copolymer in relation to their weight fractions in the monomer blend were determined by polymerising styrene and butadiene feeds of varying compositions to conversions below 5% and measuring the styrene and butadiene content of the resulting copolymers by infra red analysis.

Knowing this relation, R1 and R2 were calculated according to the method described by Fineman and Ross inJ. Polymer Science,5, (1950) page 259.

For example, under the polymerisation conditions applied in the majority of the Examples, viz. at 60"C in cyclohexane as a solvent and using a modifier (ODM B) concentration of 450 PPM, we measured for R1 and R2 values of 0.73 and 1.40 for styrene and butadiene respectively. Thus, starting with a monomerfeed of known composition one can calculate the copolymer composition at the onset of copolymerisation (approaching zero 0/0 conversion) with the aid of equation (1). The composition of polymer fractions formed at further points in polymerisation time or conversions can then be calculated, using the equation, from the monomer composition remaining after a preceding polymer fraction or increment has been formed.

The graph prints are included in the accompanying drawings and are referred to as Prints 1 to 4 and 6 to 10, corresponding to Experiments 1 to 4 and 6 to 10, respectively.

In the prints total monomer conversion is referred to, for the sake of convenience, as "monomer conversion" the horizontal lines represent the average styrene content (%) of the polymer.

Referring to print 1 of the accompanying drawings, it will be seen that the molecule of the polymer of Example 1 can be regarded as being composed of three portions: a first end portion comprising about 40% of the molecule, in which the styrene content is not greaterthroughoutthan the average styrene content of the copolymer (23%), rising from about 17% to 23%; a second portion comprising about 49% of the molecule in which the styrene content is substantially the same throughout as the average styrene content; and a second end portion comprising about 10% of the molecule, in which the styrene content is greaterthroughoutthan the average styrene content of the polymer. This last-mentioned portion has a styrene-rich component or "tail" in which the styrene content rises at an increasing rate from about the 90% monomer conversion mark towards 100%, and the styrene content at the outer end of the end portion (or later end in terms of the production of the polymer) reaches a maximum of 57%.

The various values for the sizes of portions of the molecule and the differential styrene contents thereof can be obtained by reading off the respective print. For example, referring again to Print 1, it will be seen that the differential styrene content rises in the last 5% portion of monomer conversion (i.e. from 95% to 100% conversion) from a first (threshold) value (T) to 27% to a maximum value (M) of 57% at 100% monomer conversion, a rise (M-T) of 30 percentage points; and that in the last 2Wa portion of total monomer conversion the differential styrene content changes from a threshold value (T') of 31% to the maximum value of 57%, a rise (M-T') of 26 percentage points. The values M, T, (M-T), T' and (M-T') are given in Table B below.

Prints 2 and 3 show that the polymers of Examples 2 and 3 have a structure broadly similar two that of Example 1, having a first end portion, a middle portion and a second end portion, the last-mentioned portion having a styrene-rich component in which the styrene content rises steeply from the average styrene content.

From Print 2 it can be seen that the threshold value (T) of differential styrene content at 95% monomer conversion is 32%, the threshold value (T) art 971% monomer conversion is 39 and the maximum styrene content is 72%. Thus, the differential styrene content rises by 40 percentage points over the last 5% portion of monomer conversion and by 33 percentage points over the last 29% of monomer conversion.

In print 6 it is the first end portion (referred to herein as a "begin tail") of the molecule that has the styrene-rich component, and the second or later end portion has a styrene content less than the average styrene content of the polymer; in between the styrene content is substantially the same as the average styrene content of the polymer.

It will been seen that the differential styrene content of the copolymer of Experiment 6 changes over that portion of monomer conversion between 0% and 5% conversion from a first (threshold) value (T) of 46 to a second (maximum) value (M) of 71 % at 0% monomer conversion, that is, a change (M-T) of 25 percentage points; and that over the terminal 2Wa zone of said 5% portion the differential styrene content changes from a first (threshold) value (T') of 56% to the second (maximum) value (M) of 71 % at 0%, that is, a change (M-T') of 15 percentage points.

The respective values of M, T, T', (M-T) and (M-T') for all of the copolymers of Experiments 1 - 10 are given in Table B from which it will be seen that all the values for (M-T) are 25 percentage points or more and that all the values for (M-T') are 14 percentage points or more. In that Table under the heading "Tail" it is stated whether the copolymertail occurs at or towards the beginning of the monomer conversion (a "begin tail") or at or towards the end of the monomer conversion (an "end tail").

Prints 8 and 9 illustrate the polymers of Experi ments 8 and 9 which are formed as the result of coupling a polymer obtained substantially as in Experiment 6, and it will be seen that those two prints are similar to that of print 6. The polymer of print 8 is believed to be branched and comprises four polymer linear chains coupled together whereas that of print 9 is believed to be linear and to comprise two polymer linear chains coupled together.

As is made clear from the prints, reference above to certain percentage proportions of the "length of the molecule" are derived from the corresponding figures for monomer conversion, and it is not to be inferred that the molecules of any given polymer are all of the same length.

It will be seen from the prints that in the various end portions the differential styrene content rises rapidly towards a high value corresponding to approximately 100% (or 0%) of the length of the polymer molecule as measured in terms of mono merconversion; and in some of the polymers the increase is particularly greatoverthe last few (for example, one, two or three) per cent of the polymer length.

It will be seen from Table A that the vinyl content (that is the amount of the butadiene component of the polymerthat has polymerised at the 1,2positions) is in each case 30% or more and that most of the values lie in the range 40 to 50%, especially 45 to 50%. The styrene content of each polymer is in the range 20 to 30%.

TABLEA Styrene Content % Vinyl Chain Experiment (average) Content % Structure 1 23 48 L 2 23 42 L 3 23 30 L 4 23 49 L 6 23 47 L 7 23 47 L 8 23 47 B (DEAP-coupled) 9 23 47 L (DBE-coupled) 10 23 47 L Notes: DEAP = diethyl adipate L = linear DBE = dibromoethane B = branched TABLE B Threshold Experiment (T) Maximum Threshold No.Tail* at5% at95% (M) M-T (T') M-T' 1 E - 27 57 30 31 26 2 E - 32 72 40 39 33 3 E - 36 72 36 46 26 4 E - 37 97 60 41 56 6 B 46 - 71 25 56 15 7 E - 27 56 29 31 25 8 B 46 - 71 25 57 14 9 B 46 - 71 25 57 14 10 E - 43 79 36 48 31 11,12 E - 37 95 58 42 53 13 E - 38 95 57 42 53 14, 14A, 15,16 E - 37 95 58 43 52 16B B 31 - 82 51 38 44 16C B 44+ - 94 50 71 23 16D B 29 - 83 54 35 48 17 E - 34 97 63 38 59 18 E - 39 97 58 34 63 19 E - 24 97 73 27 70 20 E - 13 94 81 19 75 21 E - 37 95 58 42 53 22 E - 41 97 56 46 51 23 E - 47 97 50 51 46 24 E - 51 98 47 56 42 26 B 19++ - 92 73 27 65 27 B 21 - 48 27 21 27 28 B 21+++ - 47 26 21 26 35/36 B 30 - 83 53 38 45 37 B 37 - 83 46 50 33 + =based on 5% band at 3-8% conversion + + = based on 5% band at 2-7% conversion + + + = based on 5% band at -5% conversion The compositions of Examples 1 to 10 have been tested to assess their wet grip and rolling resistance properties on a road surface.Each of those composi tions was used as the tread compound of model tyres of size 2.25-8 (dimensions in inches). These model tyres were subjected to two tests as follows.

Grip on a wet Delugrip road surface (Delugrip is a Registered Trade Mark) was measured using the variable speed internal drum machine (VSIDM) described in a paper by G. Lees and A. R. Williams in Journal of the Institute of the Rubber Industry, Vol. 8, No.3, June 1974. Measurements of the wet grip were made for locked wheel sliding friction. Rolling resistance was measured using the rotary power loss machine described in Transactions of the Institution of Rubber Industry 34, No. 5, October 1958, applying Equation 3.1 given below.

The results obtained are shown in Table C below.

Equation 3.1, as applied to SP4 steel radial tyres of Dunlop Limited, referred to in the table is as follows: rolling resistance = 8.75E" + 0.83 (Em2 + 66 where E" and E" have the following meanings: (E E" = loss modulus expressed in MPa P = complex modulus expressed in MPa The expression (E)2 is expressed in GN/m2.

TABLE C Wet Grip Rolling Resistance Sliding calculated according Example No. Friction to Equation 3.1 Comparisons: S-1502 100 100 S-1712 116 101 OEP/LTP 112 115 1. 108 93 2. 105 91 3. 104 90 4. 116 93 6. 105 94 7. 109 92 8. 110 99 9. 108 95 10. 108 95 It will be seen from Table C that the tyres tested have a good combination of wet grip and rolling resistance. It is believed that the end portion of the polymer molecule having the styrene-rich component makes a major contribution to the good wet grip and that the remainder of the molecule having a substantial vinyl content and free from a styrene-rich component makes a major contribution to the good rolling resistance.

The production of further copolymers of the invention, their formulation in elastomeric compositions and use of those compositions in the tread portion of tyres is described below.

Abbreviations used have the following meanings: S = styrene B = butadiene S/B = mixture of styrene and butadiene SBR = styrene-butadiene copolymer ODMB = ortho-dimethoxybenzene DEAP = diethyl adipate DVB = divinyl benzene S-BuLi = secondary butyl lithium Diglyme = diethylene glycol dimethylether PS = polystyrene MW = molecular weight B = branched molecule L = linear molecule min = minutes BC = before coupling Unless otherwise stated the ratios and percentages referred to in the Experiments are by weight.

Experiments 11,12 and 13 relate to copolymers having small polystyrene blocks at one or two ends ofthe molecule.

Experiments 14to 15C relate to copolymers having a branched structure.

Experiments 17 to 24 relate to copolymers having different proportions of bound styrene in the main chain ofthe molecule.

Experiments 26,27 and 28 relate to copolymers subsequently converted to branched form by coupling using DEAP.

There now follows descriptions of the production of the copolymers.

EXPERIMENT 11 A styrene-butadiene copolymer was produced using the ingredients and conditions referred to in Table 1A below. The procedure used was as follows.

A 10 litres stainless steel reactor was charged with 4000 grams of cyclohexane. Thereafter were added a first (1) batchwise charge of monomer and the ODMB, and the contents were brought to a temperature of 60"C. Next, the impurities in the solvent/monomer blend, capable of reacting with sec.

butyllithium were deactivated by titration with a sec.

butyllithium solution in cyclohexane (conc. 100 mmol/l). After a temperature rise of 0.5"C was observed, indicating that all impurities capable of interfering with the polymerisation reaction had been deactivated, the sec. butyllithium solution (33.3 ml of a 100 mmol/l solution in cyclohexane) was added to initiate the polymerisation.

After 10 minutes the second (2) batchwise charge of monomers was added, immediately followed by the first continuous addition of monomer over a period of 25 minutes. The temperature of the reactor contents was kept at 60"C by cooling.

Immediately after the first continuous addition of monomers had been completed the second continuous addition of monomers was pumped into the reactor during 25 minutes. Thereafter, the polymerisation reaction was allowed to proceed for a further 60 minutes by which time a monomer conversion of more than 99.9% had been obtained. Then 0.5 gram of methanol was added to kill the polymerisation reaction. 0.5 gram of 2,6 di-tert. butylparacresol was added as a stabiliser. The polymer was recovered from the solution by coagulation with steam.

EXPERIMENT 12 The procedure of Experiment 11 was followed except that there was only one batchwise charge of monomers, which immediately after initiation of the polymerisation was followed by three successive, continuous additions of monomer(s) over a period of 55 min.

EXPERIMENT 13 The procedure of Experiment 11 was followed, except that there was a third continuous addition, immediately after the second continuous addition had been completed.

EXPERIMENTS 14, 14A, 15, 16, 16B, 16Cand 16D Seven branched styrene-butadiene copolymers were produced using the ingredients and conditions referred to in Table IIA. The procedure used was that described in Experiment 12 except that there were only two continuous additions and, in Experiments 14, 14A, 15, 16B, 16C and 16D, there was added to the reactor after 90 or 120 minutes a coupling agent as indicated in Table IIA. Use of the coupling agent (0.08 mmol) DEAP or SnC14) resulted in the copolymer chains being partly (about 50%) coupled to give a polymer with a branched structure having a fourfold molecular weight.

In experiment 16 coupling or branching was obtained by use of DVB in a third addition stage.

After the coupling methanol and 2.6 di-tert. butylparacresol were added as described in Experiment 11.

EXPERIMENTS 17 to 24 Further styrene-butadiene copolymers were produced using the ingredients and conditions referred to in Tables IIIA and IVA. The procedure was that described in Experiment 11 except that there was only one batchwise monomer charge and only one continuous monomer charge.

EXPERIMENTS 26, 27 and 28 Three further styrene-butadiene copolymers were produced using the ingredients and conditions referred to in Table VA. The procedures used were as follows: Experiment 26 Begin tail Amount: 5% of total polymer: styrene/butadiene ratio 90/10 Main chain Amount: 95% of total polymer: styrene/butadiene ratio 19.5/80.5 A stainless steel reactor of 10 litres capacity was charged with 4000 grams of cyclohexane, 45 grams of styrene, 5 grams of butadiene and 2.25 grams of ODMB. The contents were brought to a temperature of 60"C by external heating of the reactor. Next the polymerisation of the monomers was initiated by addition of 66.6 ml of a solution of S-BuLi in cyclohexane of a concentration of 100 mmol/litre.After a polymerisation time of 30 minutes during which virtually all of the monomers were converted into a styrene-butadiene copolymer, a blend of 185 grams of styrene and 765 grams of butadiene was added in 20 minutes. During this addition the temperature of the reactor contents was kept at 60"C. After the monomers had been added the polymerisation reaction was prolonged for another 60 minutes at 60"C after which 0.30 grams of DEAP were added in order to couple the living linear polymer chains into a polymer of radial structure. Next 0.5 grams of 2.6 di-tertiary butyl-paracresol were added as a stabiliser, after which the copolymerwas recovered from the solution by steam coagulation and drying. Characteristics of the polymer are shown in Table VB.

Experiment 28 Begin tail Amount: 2% of total polymer; styrene/butadiene ratio 46/60 Main chain Amount: 98% of total polymer; styrene/butadiene ratio 22.7/77.3 The reactor of Experiment 26 was charged with 4000 grams of cyclohexane, 8 grams of styrene, 12 grams of butadiene and 2.25 grams of ODMB. The monomers were polymerized at 600C by addition of 66.6 ml of a S.BuLi solution in cyclohexane of 100 mmol/litre. After 30 minutes a blend of 222 grams of styrene and 758 grams of butadiene were fed into the reaction zone in 20 minutes. Thereafter the same procedure was followed as in Experiment 26.

Characteristics of the polymer are likewise shown in Table VB.

Experiment 27 Begin tail Amount: 5% of total polymer; styrene/butadiene ratio 40/60 Main chain Amount: 95% of total polymer; styrene/butadiene ratio 40/60 and ratio 22.1/77.9.

The reactor of Experiment 26 was charged with 4000 grams of cyclohexane, 20 grams of styrene, 30 grams of butadiene and 2.25 grams of ODMB.

Polymerisation was initiated by addition of 66.6 ml of a S-BuLi solution in cyclohexane of 100 mmol/litre. After 30 minutes a blend of 210 grams of styrene and 740 grams of butadiene were fed into the reaction zone in 20 minutes. Thereafter the same procedure was followed as in Experiment 26.

Characteristics of the polymer are shown in Table VB.

TABLE IA Batchwise Charged Continuously Charged Exp. Sample Monomer Modifier, BuLi 1st 2nd 3rd Polymer No. No. ppm on m.mol Addition Addition Addition isation total conditions intake 11 WB223/224 1)16.7gS 450 ODMB 3.33 435gS/B 500gS/B - 120min.

after 10 min. ratio 11.7/ ratio 31/69 60 C 2) 48.39 S/B 88.3 in 25 in 25 min.

ratio 11.7/883 min 12 WB225/226 48.3g S/B 450 ODMB 3.33 435gS/B 500gS/B 16.79S 120 mien.

ratio 11.7/88.3 ratio 11 7/ ratio 31/69 in 60 C.

88. in 25 in 25 min. 5 min.

rnin.

13 WB 231/232 1)16.7gS 450 ODMB 3.33 420g S/B 500gS/B 16.795 120 min.

after 10 min. ratio 8.4/ ratio 31/ in 60 C 2) 46.69 S/B 91.6 in 69 in 25 5 min.

ratio 8.4/91.6 25 min. main.

Polymerisation Data Solvent: Cyclohexane,4000 g Amount of Polymer: 1000 g Final Solids Content: 20% w TABLE POL YMERISA TION DA TA Solvent: cyclohexane, 4000 9 Final Solids content: 20% w Amount of polymer: 1000 9 Batchwise Charged Continuously Charged Exp. Monomer modifier BuLi Ist 2nd 3rd Polymer- Coupling No. ppm on mmol Addition Addition Addition isation Agent total conditions mmol intake 14 150 9 S/B 450 ODMB 3.33 350gS/B 500 9 S/B 90min.600C DEAP ratio 151 ratio ratio31/69 85 15/85 in in 25 min.

25 min.

14A 150gS/B 150 3.5 350gS/B 500 9 S/B 90 min.50 C DEAP ratio Diglyme ratio 15185 ratio 31/69 15/85 in 25 min. in 25 min.

15 150gS/B 450ODMB 5.5 350gS/B 500gS/B 90min.60 C SnCl4 ratio ratio 15/85 ratio 31/69 15/85 in 25 min. in 25 min.

16 150 9 S/B 450 ODMB 3.33 350 9 S/B 500 9 S/B 3.33mmol 120min.60 ratio 15/85 ratio 15/85 ratio 31/69 DVB in 25 min. in 25 min.

16B 23 9 S 450 ODMB 8.0 900 9 S/B 77gB 120 min.60 C DEAP ratio 23/77 in 20 min in 80 min.

16C 94.3 9 S 150 6.7 After 5 min. 70gB l00min.50 C DEAP 5.0 9 B Diglyme 830 9 S/B in 35 min.

ratio 16.3/ 83.7 in 40 min.

16D 23.0gS 450ODMB 6.7 After1 min. 73.2gB 120min.600C DEAP gB3.8 900gS/B in 30 min.

ratio 23/77 in 80 min.

TABLE IIIA Batchwise Charged Continuously Charged Exp. Sample Monomer modifier BuLi 1st 2nd 3rd Polymer No. No. ppm on m.mol Addition Addition Addition isation total conditions intake 17 D250/251 900 grams 450 ODMB 3.33 After 15 - - 120 min.

S/B ratio min 100 60"C 22.1/77.9 grams S/B ratio 31/69 in 15 min.

18 D252/253 900grams 4500DMB 3.33 after 15 - - 120min.

S/B ratio min 100 60 C 16.6/83.4 grams S/B ratio 31/69 in 15 min.

19 D255/256 900 grams 450 ODMB 3.33 after 15 - - 120 min.

S/B ratio min 100 60 C 6.6/93.4 grams S/B ratio 31/69 in 15 min.

20 D257/259 900grams 4500DMB 3.33 after 15 - - 120min.

butadiene min 100 60 C grams S/B ratio 31/69 in 15 min.

POL YMERISA TION DA TA Solvent: Cyclohexane, 4000 9. Amount of Polymer: 1000 9 Final Solids content: 20% w.

TABLE VIVA Batchwise Charged Continuously Charged Exp. Sample Monomer Modifier BuLi tst 2nd Polymer No. No. ppm on m.mol Addition Addition isation total conditions intake 21 WB236/237 100 grams 450 3.33 After1 main. After21 min. 120min.

S/B ratio 400 grams S/B 500 grams S/B 60 C 15/85 ratio 15/85 ratio 31/69 in 20 main. in 25 min.

22 WB242/243 100grams 450 3.33 After 1 min. After21 min. 120 min.

S/B ratio 400 grams S/B 500 grams 60"C 10/90 ratio 10/90 S/B ratio in 20 min. 36/64 in 25 min.

23 WB244/245 100 grams 450 3.33 After1 min. After21 min. 120min.

S/B ratio 400 grams S/B 500 grams S/B 60 C 5/95 ratio 5/95 in ratio 41/59 20 min. in 25 min.

24 WB246/247 100 grams 450 3.33 After1 min. After21 min. 120 min.

S/B ratio 400 grams 500 grams 60 C 0/100 S/B ratio S/B ratio 0/100 in 46/54 in 20 min. 25 min.

POL YMERISA TION DA TA Solvent: cyclohexane 4000 g Final Solids Content: 20% w Amount of Polymer: 1000 g TABLE VA Batchwise Charged 1st step 2nd step Continuously Polymerisation conditions ODMB ODMB charged in the Monomer (modifier) (modifier) second step time min. femp. "C Exp. No. Styrene Butadiene ppm sec. ppm Styrene Buta- 1st Znd 1st 2nd & Sample grams grams on total BuLi on grams diene st. st. st st.

intake mmol total grams intake 26 (D292/ 293) 49.5 5.5 450 7.33 450 204 842 30 60 50 50 27 (PS241) 8.8 13.2 450 7.33 450 244 847 30 60 50 50 28 (PS242) 22 33 450 7.33 450 231 814 30 60 50 50 POL YMERISA TION DA TA Solvent 1 st step: Cyclohexane 2.340 kg Solvent 2nd step: Cyclohexane 2.340 kg Final Solids Content: 19%w Amount of Polymer: 1100 grams Salient data concerning the structure of the copolymers of the Experiments, is set out in Tables IB, IIB, IIIB, IVB and VB shown below.

In the tables: the Mooney values are ML 1 + 4100 C; the IR analysis values are in weight % Q=Mw/Mn TABLE GPCAnalysis IR Analysis Butadiene part Exp. Sample Peak appMwx 103 Mw/Mn vinyl CIS trans Styrene Mooney No. No. Mw Mn 11 WB223/224 450 453 252 1.80 46 19 35 24.6 106 12 WB225/226 390 377 220 1.72 46 19 35 23.9 90 13 WB231/232 400 383 248 1.55 49 15 36 24.3 98 TABLE RIB CPC analysis IR analysis on AppMwx 103 butadiene part Exp. Sample Structure Chain Peak Mw Mn Mwl Coupling Vinyl Cis Trans Styrene Mooney No. No.Modifier Structure BC Mn 14 GM38/40 ODMB B/DEAP 510 695 379 1.83 19 46 19 35 23.7 108 /41 14A GM43/46 Diglyme B/DEAP 450 736 210 3.5 30 70 10 20 22.8 114 147 15 Ps210/211 ODMB B/SnC14 324 612 256 2.38 35 50 17 33 24.1 94 16 PS208/209 ODMB BIDVB 460 558 252 2.21 10 50 17 33 24.8 119 copol 168 WRC5801 ODMB B/DEAP 190 441 147 3.01 56 47 18 35 24.9 56 16C PS12141 Diglyme BIDEAP 320 626 139 4.5 42 66 11 23 22.9 78 216 16D WRC5802 ODMB B/DEAP 183 432 278 1.55 63 47 17 36 24.8 47 TABLE fflB GPC Analysis IR Analysis AppMwx 1 Butadiene part Exp.

No. Sample No. Peak Mw Mn Mw/Mn Vinyl ClS Trans Styrene Mooney 17 D250/251 410 403 258 1.56 48 17 35 24.7 98 18 D252/253 470 522 314 1.66 48 17 35 20.1 100 19 D255/256 460 458 313 1.47 49 17 34 10.2 102 20 D257/259 490 503 308 1.63 51 17 32 3.4 110 TABLE I VB GPC Analysis IR Analysis Exp. APPMwx 103 ~ ~ Butadiene part No. Sample No. Peak Mw Mn Mw/Mn Vinyl CIS Trans Styrene Mooney 21 WB236-237 500 503.3 323.2 1.56 48 17 35 25.4 107 22 WB242/243 440 433.3 282.7 1.53 47 17 36 26.1 114 23 WB244-245 470 473.9 285.0 1.66 48 17 35 24.7 110 24 WB246-247 500 498.5 318.0 1.57 47 19 34 24.6 104 TABLE VB Initial part ofpolymer ("Begin tail") Main Chain Exp. No.Amount on Styrenelbutadiene Amount on Styrenel Overall ( & Sample total polymer % ratio WIW total butadiene Styrene No.J polymer % ratio WIW content % (IR AnalysisJ 26 (D292/ 293) 5 90/10 95 19.5/80.5 23.5 27 (PS241) 5 40/60 95 22.1/77.9 23.2 28 (PS242) 2 40/60 98 22.7/77.3 23.1 TABLE VB (CONTINUEDJ CHARACTERISTICS OF SBR POL YMERS (CONTINUEDJ Structure of butadiene GPCAnalysis portion ofcopolymer App. peak Exp.No. 1,2 cis 1,4 Trans MW x 103 ~ Mooney ( & Sample 1,4 BC CE Mw MN Mw/Mn Viscosity No.) 26 (D292/ 293) 50.9 13.6 35.5 230 76 535 271 1.97 74 27 (PS241) 50.6 15.7 33.7 219 66 524 262 2.00 66 28(PS242) 52.6 13.6 33.9 211 70 555 262 2.12 65 By use of the reaction kinetics of the various processes of the Experiments 11 et seq there have been produced graphs showing a plot of % differential styrene in the copolymer part of the molecule against % monomer conversion (which corresponds to % molecular size of the polymer molecules). Thus, for example, Prints 11-13 do not show the polystyrene block present at an end of the molecule.

These graphs are included in the accompanying drawings and bear numbers corresponding to the numbers of the Experiments; for example "Exp.

11/12" refers to the structure of the products of Experiments 11 and 12 and "Exp. 13" refers to the product of Experiment 13.

Referring to print"Exp. 17" of the accompanying drawings by way of example, it will be seen that the molecule of the polymer of Experiment 17 can be regarded as being composed of two portions: a first or main chain portion comprising about 90% of the molecule, over which the styrene content rises from 17% at one end (the "begin" end) of the molecule to 30% at the end of said first portion, and a second or end portion comprising the remaining 10% orthereabouts of the molecule. This last-mentioned portion has a styrene-rich component or portion in which the styrene content rises rapidly from about 30% at the 90% monomer conversion mark towards 100%, and the styrene content at the outer end of the end portion (or later end in terms of the production of the polymer) is 97%.

The various values for the sizes of portions of the molecule and the differential styrene contents thereof can be obtained by reading off the respective print.

On each of the prints there has been marked a first (threshold) value (T) at one end of a portion of the graph corresponding to 5% of the monomer conversion, and a second (maximum) value (M) within said 5% portion. In all the prints except those numbered 1 6C, 26,27 and 28 said 5% portion extends from 0% to 5% or from 95% to 100% monomer conversion; in Print 1 6C said 5% portion extends from 3 to 8% monomer conversion; in Print 26 it extends from 2Wa to 7Wa; and in Print 28 it extends from Sto SWa.

In Print 27 only a 2Wa portion of the monomer conversion is shown. In all prints except Print 27 the respective terminal 5% portion of monomer conversion has been bisected to divide said portion into two 2Wa zones of monmer conversion, which illustrates the change in differential styrene content over those two zones. In all instances (including Print 27) the styrene content rises by more than 14 percentage points.

It is to be noted that the prints show differential styrene content of copolymer only; where there is a polystyrene component it is not shown on the print.

In Print 1 6C the horizontal line at 94% styrene content refers to copolymer.

As is made clear from the prints, reference above to certain percentage proportions of the "length of the molecule" are derived from the corresponding figures for monomer conversion, and it is not to be inferred that the molecules of any given polymer are all of the same length.

It will be seen from the tables that the vinyl content (that is the amount of the butadiene component of the polymer that has polymerised at the 1,2positions) is in each case 30% or more and that most of the values lie in the range 40 to 50%, especially 45 to 50%. The styrene content of most of the polymers is in the range 20 to 30%, especially 25 to 30%, though some values are about 10% or less and some values above 50%. In general, most values are at least 30%.

Each of the copolymers has been formulated in an elastomer composition having in each case the following constitution.

Parts by weight Copolymer 100 Sulphur 1.75 Accelerator - CBS (cyclo hexylbenzthiazyl sulphenamide) 1 Carbon black N 375 50 Antioxidant BLE 75 2 Zinc oxide 3 Stearic acid 1 The elastomeric compositions were vulcanised at 140"C for 60 minutes in a steam autoclave, using a 15 minute rise to temperature.

The elastomeric compositions have been tested to assess their wet grip properties on a road surface. Each of those compositions was used as the tread compound of model tyres of size 2.25-8 (dimensions in inches).

These model tyres were subjected to the following tests to determine wet grip. Grip on a Delugrip road surface (Delugrip is a Registered Trade Mark) was measured using the variable speed internal drum machine (VSIDM) described in a paper by G. Lees and A. R. Williams in Journal of the Institute of the Rubber Industry. Vol.8, No. June 1974. Measurements of the wet grip were made for locked wheel sliding friction, Rolling resistance was measured on the rotary powerloss machine.

The results obtained are shown in Table D below.

TABLED Experiment Wet grip - Rolling No. (sliding resistance friction) (Calculated according to Equation 3.1) Comparison S-1502 100 100 Invention 11 118 99 12 118 99 13 112 100 14 14A 115 102 15 101 98 16 100 99 16B 108 101 16C 110 103 17 116 99 18 106 97 19 101 98 20 94 97 21 119 98 22 119 100 23 117 99 24 113 98 26 113 95 27 111 93 28 110 96 It will be seen from Table D that the tyres tested have a good combination of wet grip and rolling resistance. It is believed that the begin or tail end portion or portions of the polymer molecule having the styrene-rich component makes a major contribution to the good wet grip and that the remainder of the molecule having a substantial vinyl content and free from a styrene-rich component makes a major contribution to the good rolling resistance.

The invention is further illustrated by the following Experiments 29 to 34, in which, unless stated otherwise, the proportion of ingredients in the compositions is given in parts by weight.

The compositions of Experiments 29 to 31 include as their polymer ingredient solution styrene-butadiene copolymers referred to below as copolymers J and K, respectively. The bound styrene content, vinyl content (expressed as a percentage by weight of the butadiene content of the copolymer) and molecular weight are shown in the following table, and both copolymers are polymers which have a linear structure.

Copolymer Styrene Vinyl Molecular Content Content Weight (% it) (% it) x103 J 23 33 450 K 24.8 30 429 That portion of the copolymers in which the butadiene has reacted by "head-to-tail 1,4 polymerisation has been found largely to have a trans configuration. The molecular weights referred to in the table are peak molecular weights obtained by gell permeation chrotatography using polystyrene as a comparative standard.

Elastomer compositions of the invention, suitable for use as tyre treads, have been obtained by blending together the following ingredients and vulcanising for 40 minutes at 140"C in a steam autoclave.

Ingredients Experiment No.

29 30 31 Copolymers 100.00 CopolymerK 100.00 72.50 Dutrex 729 Oil 27.50 Sulphur 1.75 1.75 1.75 Stearic acid 1.00 1.00 1.00 Zinc Oxide 3.00 3.00 3.00 75% BLE 2.00 2.00 2.00 C.B.S. accelerator 1.00 1.00 M.B.S. accelerator - - 1.00 N 375 carbon black 50.00 50.00 50.00 The compositions of Experiments 32 to 34 each contains as its polymer ingredient a solution styrene-butadiene copolymer referred to below as copolymer A, B and C respectively. The bound styrene content, vinyl content (expressed as a percentage by weight of the butadiene content of the copolymer) and molecular weight are shown in the following Table, and they are all polymers having a linear structure.

Copolymer Styrene Vinyl Molecular Content Content Weight (% IR) (IR) x 103 A 22 67 480 B 25 52 480 C 24.7 47 427 That portion of the copolymers in which the butadiene has reacted by "head-to-tail" 1,4 polymerisation has been found largely to have a trans configuration. The molecular weights referred to in the table are peak molecular weights obtained by gel permeation chromatography using polystyrene as a comparative standard.

Three elastomer compositions of the invention, suitable for use as tyre treads, have been obtained by blending together the following ingredients and vulcanising for 40 minutes at 140"C, in a steam autoclave.

Ingredients Experiment No.

32 33 34 CopolymerA 100.00 - - Copolymer B 100.00 Copolymer - - 100.00 Sulphur 1.75 1.75 1.75 Stearic acid 1.00 1.00 1.00 Zinc oxide 3.00 3.00 3.00 75% BLE 2.00 2.00 2.00 C.B.S. accelerator 1.00 1.00 M.B.S. accelerator - - 1.00 N 375 carbon black 50.00 50.00 50.00 The compositions of Experiments 29 and 34 have been found to have a desirable combination of wet grip and rolling resistance properties as shown in Table E.

TABLE E Experiment Wet grip- Rolling resis No. sliding tance (calculated friction according to Equation 3.1) S-1502 100 100 (comparison) 29 110 93 30 111 96 31 123 110 32 110 100 33 118 99 34 116 99 EXPERIMENTS 35, 36 AND 37 Several other copolymers have been produced according to the conditions referred to in Table VIA, and their properties are given in Table VIB. In Experiments 35 and 36 isoprene is used in a way such that it polymerises substantially entirely in the "tail" portion of the copolymer -- an example of the use with butadiene of a second conjugated diene hydrocarbon.

The copolymers of Experiments 35 and 36 are begin tail polymers and the nature of the tail is determined largely by the batchwise charge. In Experiment 35 the diene component in the batchwise charge is half butadiene and half isoprene leading to a tail having a substantial isoprene content; in Experiment 36 the batchwise diene component is wholly isoprene leading to a tail substantially without butadiene.

In Experiment 37, as in Experiments 35 and 36, addition of monomers after the start of the reaction is carried out continuously.

TABLE VIA Solution SBR - Polymerisation procedure Polymer batch size: Polymerisation temp: 40.0 kg Solvent: 55 C-60 C Coupling cyclohexane Final agent: dimethyl solids content: adipate Coupling 20.0 %w Ortho time: 15 min.

dimethoxybenzene: Total polymerisation 450 ppm s-Buli: 0.267 time: 120 min.

mol (kinetic MW, be- Reactor - R 260/3501 fore coupling: 150.000) nominal capacity

Sample number 10802 10803 10804 35 1 36 37 Condltlons .

Batchwise charged Butadiene, kg D.077 i - I 0.45 Isoprene, kg 0.077 0.154 Styrene, kg 0.92 0.92 2.76 1st Continuous addition Butadiene, kg 27.57 27.27 Isoprene, kg - 6.89 Styrene, kg 8.28 2.07 6.44 time of addition, min. 80 20 80 2nd Continuous addition Butadiene, kg 3.08 20.68 3.08 Isoprene, kg - - Styrene, kg - 6.21 time of addition, min. 20 60 20 3rd Continuous addition Butadiene, kg, in 20 min. 3.08 TABLE WB

GPC - analysis Infrared Analysis Butadiene portion Exp. Sample app. MW x 10 3 CE, Mw X 10-3 Mn x 10-3 Q Styrene, 1.2 1.4 1 A Mooney No. No. peak AC % % % trans cis 35 10802 220 585 57 402 215 1.88 23.9 47.7 29.7 22.6 44 36 10803 370 1100 51 506 260 1.96 24.2 40.5 26.4 33.2 73 37 10804 235 600 56 420 250 1.68 25.4 48.8 23.0 60 The copolymers of Experiments 35-37 have been used, as the tread portion of full size tyres (155 SR13 SP4), and their wet grip and rolling resistance has been determined as follows: Wet grip: using the internal drum testing facility at University of Karlsruhe Rolling resistance: using the rotary power loss machine referred to above.

The results obtained were as shown in Table F below.

TABLE F Exp. Compound Wet Grip Rating Rolling No. Polymer Peak Cornering Mean Resistance {ca/uculated according to Equation 3.1) Comparisons: - S-1502 100 100 100 100 - S-1712 108 104 106 103 - OEP/LTP 107 100 103 115 35 10802 113 107 110 101 36 10803 120 116 118 99 37 10804 117 120 118 98

Claims (34)

1. An elastomeric copolymer of an aromatic vinyl compound and a conjugated diene, suitable for use in the tread portion of a pneumatic tyre, said copolymer having a vinyl content (as herein defined) of at least 30% by weight and having a differential content of the aromatic vinyl compound such that in at least one of its end portions said differential content shows a sharp and substantial increase in the direction of the outer extremity of said end portion.

2. A copolymer according to Claim 1, in which said aromatic vinyl compound is styrene.

3. A copolymer according to Claim 1 or 2, in which said diene is butadiene.

4. A copolymer according to Claim 1 or 2, in which said diene is isoprene.

5. A copolymer according to Claim 1, in which the copolymer is a styrene-butadiene copolymer.

6. An elastomeric styrene-butadiene copolymer, suitable for use in the tread portion of a pneumatic tyre, whose percentage differential styrene content changes in a portion of not more than 5% of the copolymer chain (as determined by monomer conversion) from a first value to a second value, said second value being at least 25 percentage points greater than said first value; and said portion lies within a terminal 10% portion of the copolymer chain (as determined by monomer conversion).

7. A copolymer according to Claim 6, in which said 5% portion is a terminal portion of the copolymer chain.

8. A copolymer according to Claim 6 or 7, in which the percentage differential styrene content changes in a zone of not more than 2Wa of the copolymer chain (as determined by monomer conversion) from a first value to a second value, said second value being at least 14 percentage points greater than said first value; and said zone lies within a terminal 10% portion of the copolymer chain (as determined by monomer conversion).

9. A copolymer according to Claim 7 or 8, in which said second value is at 100% monomer conversion.

10. A copolymer according to Claim 7 or 8, in which said second value is at 0% monomer conversion.

11. A copolymer according to any of the preceding claims, which consists substantially wholly of styrene and butadiene.

12. Acopolymer according to any of the preceding claims, in which the bound styrene content of the copolymer is at least 10% by weight.

13. A copolymer according to any of the preceding claims, in which said vinyl content (as herein defined) is in the range from 30% to 60% by weight

14. A copolymer according to any of the preceding claims, which is a linear compound.

15. A copolymer obtained by coupling together two or more copolymers as claimed in any of the preceding claims.

16. A copolymer according to Claim 15, in which said coupling is effected using a tetra-functional coupling agent.

17. A copolymer according to Claim 1 or Claim 6, substantially as described herein with reference to the accompanying drawings.

18. Astyrene-butadiene copolymer substantially as described herein and having a differential styrene content substantially as shown in any one of Prints 1, 2,3,4, 6,7, 8,9 and 10 of the accompanying draw- ings.

19. A styrene-butadiene copolymer substantially as described herein and having a differential styrene content substantially as shown in any one of Prints 11/12,13, 14/14A/15/16, 16B, 16C, 16D, 17to24,2627 and 28 of the accompanying drawings.

20. A styrene-butadiene copolymer substantially as described herein and having a differential styrene content substantially as shown in Print 35/36 or 37 of the accompanying drawings.

21. A process for the production of an elastomeric copolymer as claimed in Claim 1 or Claim 6, in which a mixture of the aromatic vinyl compound and the conjugated diene is subjected to solution polymerisation conditions in the presence of an initiator, using means to ensure that an enhanced or increasing proportion of the aromatic vinyl compound is formed at an opening or closing stage of the copolymerisation reaction.

22. A process according to Claim 21, in which said proportion is formed at a closing stage of the copolymerisation reaction and the reaction is carried to substantially complete conversion.

23. A process according to Claim 21 our22, in which said means comprises use of a structure modifier favouring 1,2 polymerisation of the conjugated diene at the expense of 1,4 or other alpha-omega polymerisation.

24. A process according to Claim 21, 22 or 23, in which said means comprises the introduction into the reaction zone of an excess amount of the aromatic vinyl compound in an opening or closing stage of the copolymerisation reaction.

25. A process according to Claim 24, in which said introduction is carried out continuously.

26. A process according to Claim 24, in which said excess amount is introduced prior to the start of said reaction.

27. A process according to Claim 21, substantially as described herein.

28. A process for the production of an elastomeric composition as claimed in Claim 1 orClaim 6, substantially as disclosed in any one of Experi ments1,2,3,4,6,7,8,9and10.

29. Aprocessforthe production of an elastomeric composition as claimed in Claim 1 or Claim 6, substantially as disclosed in any one of Experi ments 11,12,13,14, 14A, 15,16, 16B, 16C, 16D, 17to 24, 26, 27 or 28.

30. A processforthe production of an elastomeric composition as claimed in Claim 1 or Claim 6, substantially as disclosed in Experiments 35, 36 or 37.

31. An elastomericcopolymerobtained bythe process of any of Claims 21 to 30.

32. An unvulcanised elastomeric composition comprising a blend of an elastomeric copolymer as claimed in any of Claims 1 to 20 and 31 with vulcanisation compounding ingredients.

33. Atyre whose tread has been formed by full canising an elastomeric composition as claimed in Claim 32.

34. Atyre whose tread comprises an elastomeric copolymer as claimed in any of Claims 1 to 20 and 31, aftervulcanisation.