GB2118952A

GB2118952A - Anionic polymerisation of unsaturated monomers

Info

Publication number: GB2118952A
Application number: GB08310457A
Authority: GB
Inventors: David Hugh Richards; Malcolm John Stewart
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1982-04-26
Filing date: 1983-04-18
Publication date: 1983-11-09
Also published as: GB8310457D0; GB2118952B

Abstract

An ionically terminated polymer of general formula I <IMAGE> wherein (M)n- is a polyalkene or a poly-1,3-diene, X is H or <IMAGE> Y is a group III element, R is an alkyl or an aryl group and A is an alkali or an alkaline earth metal. In a preferred embodiment M is isoprene or butadiene, Y is boron, R is C1-C4 alkyl, phenyl or monosubstituted phenyl and A is lithium or sodium. If the ionic polymer is to be used as a thermoplastic elastomer then X is <IMAGE> and -(M)n- is a poly-1,3-diene with a molecular weight between 2000 and 10,000 and a high 1,4 content. Suitable monomers are styrene, vinyl naphthalenes, vinyl pyridines, vinyl quindines, an acrylic acid ester and an acrylic acid nitrile.

Description

SPECIFICATION Hydrocarbon polymers with ionic terminal groups The present invention relates to hydrocarbon polymers with ionic terminal groups and, in particular, to such polymers when prepared in an anionically initiated polymerisation terminated by the addition of a non-sterically hindered Lewis acid.

Block copolymers have been defined as "polymers composed of molecules in which two or more polymeric segments of different chwmical composition are attached end to end".

The synthesis of these polymers has been greatly facilitated by the advent of anionic polymerisation processes. Generally, these processes have three stages, (a) initiation, in which the monomeric material is contacted with a monofunctional or difunctional anionic initiator (in most cases an organo alkali metal compound), (b) propagation, in which the monomer is allowed to polmerise to form living polymer chains with negatively charged ends, and (c) termination, in which the living polymer chains are treated with a suitable reagent to quench the polymerisation and, in most cases, to form monofunctionally or difunctionally terminated polymers.

The controi that could be exercised over the polymerisation of olefins and dienes in an anionically initiated reaction led directly to the discovery and development of ABA thermoplastic elastomers. In this type of copolymer, A represents a polymeric segment of high Tg (plastic component) whilst B represents a central polymeric segment of low Tg (elastomeric component). Because the entropy of mixing of the polymers is low, the components are generally mutually incompatible and form a two phase system in which the plastic micelles are embedded in a rubber matrix.

At ambient temperatures, since the elastomeric chain ends are locked to the plastic micelles, which act in place of chemical cross links, the material exhibits elastomeric properties resembling those of vulcanised rubber. As the temperature is raised through the Tg of the plastic component, however, the material exhibits thermoplastic properies and eventually melts.

This means that, unlike normal rubber, a device made from these materials may be remoulded as required.

The first ABA thermoplastic polymer developed, and still the most important polymer of this type, was derived from styrene and butadiene (SBS polymers). These polymers are generally prepared by the consecutive anionic polymerisation of styrene, butadiene and styrene.

They generally consist of polystyrene segments (MWt 10-20,000) flanking a polybutadiene segment (MWt 50-100,000).

This type of thermoplastic elastomer suffers from two major disadvantages. First, its molecular weight between cross links is very high, far higher than that (about 2,000 to 10,000) associated with chemically crosslinked elastomers of normal strength. Second since its strength is lost at its Tg and above, and this transition is only second order, irreversible changes, such as creep, can occur under stress at temperatures considerably below (ca 400C) this value. Its working range is consequently very limited.

It is one object of the present invention to provide ionically-terminated polymeric materials that may be used as thermoplastic elastomers, and that have a low molecular weight between crosslinks and a narrow solid-liquid transition range (hereinafter referred to as the "melting" range).

It will be seen however that the present invention is not limited in its scope to materials that are useful as thermoplastic elastomers, it also encompasses ionically terminated polymeric materials of low, medium or high molecular weight that may be used as, for example, detergents or emulsifying agents.

Other aims, objects and advantages of the present invention will become apparent from the following description thereof.

According to the present invention there is provided an ionically terminated hydrocarbon polymer of general formula I (-) X(M)nY(R)3 A wherein -(M)- comprises a hydrocarbon polymer, as hereinafter defined, X is H or If) (+) -Y-(R)3 A Y is a Group Ill element, R is an alkyl or an aryl group and A is an alkali or an alkaline earth metal.

--(M),,- comprises a polymer, with a hydrocarbon backbone, that is derived from an alkene, or a 1 ,3-diene. The monomerlM) will be chosen from the alkenes or conjugated dienes that can be polymerised anionically (i.e. in a reaction initiated by an organo alkali or an organo alkaline earth metal).If the ionically terminated hydrocarbon polymer of the present invention is to be used as a thermoplastic elastomer then the polymer will be derived from a conjugated 1 ,3-diene, especially a diene containing from 4 to 8 carbon atoms, it will have a high 1,4 content and its molecular weight will be between 2-10,000, especially 3-7000. Suitable alkenes include the optionally substituted styrenes, vinylnaphthalenes, vinylpyridines, vinylquinolines, acrylic acid esters and acrylic acid nitriles, whilst suitable 1,3-dienes include the optionally alkyl substituted butadienes, especially isoprene and butadiene itself.

Preferably Y is boron or aluminium, with boron being particularly preferred, R is a C1-C4 alkyl, phenyl or monosubstituted phenyl group and A is lithium or sodium. If the present ionic polymer is to be used as a thermoplastic elastomer then X is (-) (+) -Y-(R)3A.

It will be seen that the present materials comprise a non-polar hydrocarbon chain and either one or two ionic terminal groups. The effect of this structure is that the terminal groups reject the hydrocarbon environment of the polymer chain and migrate clusters. If the polymer is difunctional (i.e. has ionic terminal groups on both of its ends) this association leads to a "crosslinked" network of polymer chains held together by ionic interactions. On the other hand, if the polymer is monofunctional (i.e. has an ionic terminal group on only one of its ends) then this association results in a dramatic apparent molecular weight increase of the polymer.

It is the presence of both a non-polar hydrocarbon chain and one or two ionic terminal groups in the same molecule that gives the present materials their advantageous properties. In the case of a difunctional polydiene a cross linked network of polymer chains held together by ionic forces is obtained. If the polydiene has a high 1,4content then it will exhibit elastomeric properties at ambient temperature, even if its molecular weight (between crosslinks) is as low as 2000.

(Preferably its molecular weight (between crosslinks) is between about 2000 and 10,000, especially about 3000 and 7000). In the case of butadiene this represents chain lengths of between about 35 and 185, especially about 55 and 130. Further, since the cross linked network is held together by ionic interactions the elastomeric material will have a narrow melting range, above which it will flow. It follows that a difunctional polydiene according to this invention will not only function as a thermoplastic elastomer but will also have a lower molecular weight between cross links and a narrower melting point range than an SBS thermoplastic elastomer of equivalent quality. As a result a difunctional polydiene of the present type will be stronger than an equivalent SBS elastomer and will retain its strength up to a point just below its melting point.

This latter property is a result of the narrow melting range discussed above and should be compared with the significant loss of strength observed in SBS elastomers at temperatures well below their transition temperatures.

The present materials, having a lipophilic (hydrocarbon) portion and a hydrophilic (ionic) portion may also be useful as surface active agents, for example as detergents or emulsifying agents. In this case the polymer may be monofunctional or difunctional, and --(M),,- may be derived from an alkene or a diene and have a molecular weight up to 100,000. Preferably however the hydrocarbon polymer will have a relatively low molecular weight of between about 2,000 and 10,000, especially about 3,000 and 7,000. (It should be noted that these molecular weights refer to a single ionically terminated hydrocarbon polymer, not to a cluster of these polymers held together by ionic interactions).

In order to facilitate the manufacture of the present materials there is provided, in a further aspect of the present invention, a process for the preparation of an ionically terminated hydrocarbon polymer of general formula I, (+) X(M)nY(R)3 A I wherein-(M)- comprises a hydrocarbon polymer, X is H or (-) (+) Y-(R)3A Y is a Group III element, R is an alkyl or an aryl group and A is an alkali or an alkaline earth metal comprising contacting an alkene or a conjugated 1,3-diene with an anionic initiator in a solvent, allowing the alkene or 1,3-diene to polymerise and terminating the polymerisation by the addition of a non-sterically hindered Lewis acid.

The lexis acid will have the general formula YR3, wherein Y is a Group III element and R is either an alkyl or an aryl group. It must be nonsterically hindered, so that the Group Ill element is readily accessible to attack by the propagating anion in the termination step of the above process. In other words it must react quickly with the propagating anion to form an ionic polymer of general formula I. In preferred embodiments of the present process Y is either boron or aluminium, with boron being particularly preferred, and R is a C1-04 alkyl, phenyl or monosubstituted phenyl group. It should be noted that the three organic substituents bound to Y in the present Lewis acids may be the same or different groups. Preferably however all three organic substituents are the same group (e.g.

ethyl, butyl or phenyl).

The alkene or 13-diene will be chosen from the group of unsaturated organic compounds that can be polymerised anionically (i.e. in a reaction initiated by an organo alkali or an organo alkaline earth metal). If the ionically terminated hydrocarbon polymer formed by the present process is to be used as a thermoplastic elastomer then the polymer will be derived from a 1 ,3-diene, especially a diene containing from 4 to 8 carbon atoms, it will have a high 1,4-content and its molecular weight will be between 2,000 and 10,000, especially 3,000 and 7,000 (i.e. for butadiene, n will be between about 35 and 185, especially about 55 and 130). Suitable alkenes include the optionally substituted styrenes, vinyl naphthalenes, vinylpyridines, vinylquinolines, acrylic acid esters and acrylic acid nitriles, whilst suitable 1 ,3-dienes include the optionally alkyl substituted butadienes, especially isoprene and butadiene itself.

The anionic initiator may be of either the mono functional or the difunctional type. If the ionically terminated hydrocarbon polymer formed by the present process is to be used as a thermoplastic elastomer then the initiator will be difunctional.

Preferably the anionic initiator is an organo alkali or alkaline earth metal initiator, especially an alkyl or an aryl derivative of either lithium or sodium.

Suitable monofunctional initiators include monoalkyl lithium salts such as n-butyl and sec-butyl lithium, whilst suitable difunctional initiators include sodium derivatives of aryl compounds, such as the disodium tetramer of a-methylstyrene (4 Na2) or the electron transfer reagent sodium naphthalene, and alkyl lithium compounds such as dilithiobutane.

The solvent in which the present process is conducted will be determined by the choice of monomer and initiator and by the polymer structure required. Generally the solvent should dissolve both the monomer and the initiator. In the case of the anionic polymerisation of an alkene by a monofunctional initiator, the solvent will generally be an ether (either alkyl or cycloalkyl) or, which is preferred, a hydrocarbon.

Similarly, the anionic polymerisation of a 1,3diene by a monofunctional initiator may also be performed in either an ether or a hydrocarbon solvent. However, if a polymer with good elastomeric properties (i.e. with a high 1,4content) is required a hydrocarbon solvent should be used.

Most difunctional initiators are soluble in ethers (alkyl or cycloalkyl) but insoluble in hydrocarbons. Exceptions to this rule include the dilithio derivatives of long chain alkyl or aralkyl compounds such as the product of the reaction between metadiisopropenyl benzene and secbutyl lithium. In these exceptional cases the polymerisation of 13-dienes by difunctional initiators may be conducted in a hydrocarbon solvent. Under these conditions a polymerised diene of general formula I (+) (X=-Y-(R)3- A) with a high 1 4-content and good elastomeric properties will generally be obtained.

However, in most cases, the insolubility of the difunctional initiator in a hydrocarbon prevents the use of this type of solvent. In these circumstances a polar solvent, especially an ether such as tetrahydrofuran, must be employed.

Unfortunately the polymerisation of 1 ,3-dienes in such polar solvents is known to lead to products with low 1,4-content and poor elastomeric properties. This disadvantage may be somewhat alleviated by contacting the 1 ,3-diene in a polar solvent with both the difunctional initiator and a sterically hindered Lewis acid in the manner described in our co-pending UK patent application No. 8212033 (Agent's Ref: JX16224/02).

Preferably the stericaliy hindered Lewis acid will be a triaryl derivative of a Group III element in which the aryl group is substituted by one or more substituents. However in certain cases, where the aryl derivative itself is large the Lewis acid may be an unsubstituted triaryl derivative of a Group III element. Preferably the Group Ill element is either boron or aluminium, with boron being particularly preferred. Examples of suitable sterically hindered Lewis acids are trimesityl boron (tri (2,4,6 trimethylphenyl) boron) and tri (2,6-dimethyl phenyl) boron. An example of a Lewis acid that is unsuitable for this step, its Group Ill element being insufficiently sterically hindered, is triphenyl boron.

The degree of 1,4-enhancement is determined by both the choice of the sterically hindered Lewis acid and by the other reaction conditions, such as the temperature, the molar ratio of reactants and the choice of initiator. For example, when the polymerisation of butadiene, initiated by a Na2, is conducted in tetrahydrofuran in the absence of a sterically hindered Lewis acid, the 1 ,4-content of the polymer obtained is about 13%. The addition of trimesityl boron to the above reaction mixture, in a 1 to 1 and 3 to 1 molar ratio (acid to initiator), increases the 1 ,4-content of the product to, respectively, 25 to 51%. For further details of this method of 1,4-enhancement reference should be made to the above mentioned UK patent application No. 8212033 (Agent's Ref: JX16224/02), the disclosure of which is hereby incorporated by reference.

The present polymerisation reaction is preferably carried out at a temperature in the range between 0 and 1000C, especially between 100 and 800C. The maximum temperature that may be employed in a given system will depend primarily on the solvent chosen, for example in cyclohexane the maximum temperature (at ambient pressure) is 81 OC, whilst in tetrahydrofuran the maximum temperature is 660C. In the case of 1,3-diene polymerisation by difunctional initiators, reaction temperatures towards the upper end of these ranges are particularly preferred since, in most cases the 1,4content of the polydiene increases as the temperature increases. In all other cases reaction temperatures close to the ambient temperature (00 to 400) are particularly preferred.

The amount of initiator added to the reaction mixture will be determined by the structure of the monomer to be polymerised and by the desired molecular weight of the polymer. Typically for the production of a polymer with a molecular weight between about 2,000 and 10,000, between 10 and 50 mmoles, especially 1 5 and 35 mmoles of initiator is used for each mole of monomer.

The amount of non-sterically hindered Lewis acid added to the reaction mixture after the polymerisation step must be sufficient to completely terminate the reaction. Generally a 2 to 3 molar excess of acid over initiator is preferred.

Finally, the amount of sterically hindered Lewis acid added to a 1,3-diene polymerisation mixture to enhance the 1,4-content is determined by the level of 1,4-polymerisation that is required.

Generally a molar ratio (acid to initiator) of between 2 to 1 and 10 to 1 is preferred.

The ionically terminated hydrocarbon polymers of the present invention and processes for their manufacture will now be described by way of example only.

Example 1 In a 500 ml 3 necked R/B flask, equipped with a serum cap, nitrogen inlet/outlet and magnetic follower, was placed 250 ml of redistilled hexane.

This was cooled two 7800 and, with stirring, 100 ml of dried butadiene was slowly distilled into the system. The solution was placed under an inert nitrogen atmosphere and allowed to warm to OOC when 25 ml of n-butyl lithium solution (1.6 M in hexane) was injected into the mixture to initiate polymerisation. The solution was slowly allowed to warm to ambient temperature and to polymerise overnight. The reaction was then terminated by the addition of a two fold molar excess of triethyl boron, injected as the etherate.

A cloudy suspension of lithium [(polybutadienyl) tris (ethyl)] boron resulted that was evaporated to dryness and then dried under vacuum.

Example 2 The procedure of Example 1 was repeated except that the polymerisation was terminated by the addition of tri-n-butyl boron.

Example 3 The procedure of Example 1 was repeated except that the polymersiation was terminated by the addition of triphenyl boron.

Example 4 The procedure of Example 1 was repeated except that the polymerisation was terminated by the addition of triethyl aluminium.

Example 5 In a 500 ml 3 necked R/B flask, equipped with a serum cap, nitrogen inlet/outlet and magnetic follower, was placed 100 ml of redistilled tetrahydrofuran (THF). The solution was placed under an inert nitrogen atmosphere and cooled to 0 C.

25 mi of n-butyl lithium solution (1.6M in hexane) was then injected into the mixture and this was followed by the dropwise addition, with vigorous stirring, of styrene (125 g) in THF (150 ml). The polymerisation was allowed to continue for 0.5 hr at 0--1 OOC. The reaction was then terminated by the addition of a two fold molar excess of triphenyl boron. A cloudy suspension of lithium [(polystyryl) tris (phenyl)] borane resulted that was evaporated to dryness and then dried under vacuum.

Example 6 A 500 ml, 3 necked R/B flask equipped with a magnetic follower, serum cap, bubbler for butadiene and a nitrogen inlet/outlet was used as the reaction vessel. THF (200 ml) together with butadiene (10 ml out of 80 ml stored in an attached graduated flask) was introduced to the vessel. a4Na2 (0.005 g) in THF was then added and the monomer in solution was allowed to polymerise for 5 mins. A solution of trimesityl boron (11.0 g) in THF (20 ml) was then added to the reaction mixture and this was followed by the remaining butadiene. The polymerisation was allowed to proceed at 200C for 30 mins. The reaction mixture was then cooled to 1500 and the reaction was terminated by the addition of a two fold molar excess of triphenyl boron. A cloudy suspension of sodium [(polybutadienyl) tris (phenyl)] borane resulted that was evaporated to dryness and then dried under vacuum. The polydiene was found to have a 1,4-content of 51%.

Claims

1. An ionically terminated hydrocarbon polymer of general formula I ( ) (+) X(M)nY(R)3 A wherein --(M),,- comprises a hydrocarbon polymer (as hereinbefore defined), X is H or (+) -Y-(R)3 A, Y is a Group Ill element, R is an alkyl or an aryl group and A is an alkali or an alkaline earth metal.

2. A polymer according to claim 1 wherein Y is boron.

3. A polymer according to claim 1 wherein Y is aluminium.

4. A polymer according to any one of claims 1 to 3 wherein R is 01-O4alkyl, phenyl or monosubstituted phenyl.

5. A polymer according to any one of claims 1 to 4 wherein A is lithium or sodium.

6. A polymer according to any one of claims 1 to 5 wherein --(M),,- comprises a polymerised 1,3-diene.

7. A polymer according to claim 6 wherein --(M),,- comprises polymerised 1,3-diene derived from a 1,3-diene containing from 4 to 8 carbon atoms.

8. A polymer according to claim 7 wherein (M) comprises a polyisoprene or a polybutadiene.

9. A polymer according to any one of claims 6 to 8 wherein (M) has a molecular weight between 2000 and 10000.

10. A polymer according to any one of claims 1 to 5 wherein (M) comprises a polymerised alkene derived from a styrene, a vinylnaphthalene, a vinylpyridine, a vinylquinoline, an acrylic acid ester or an acrylic acid nitrile.

11. An ionically terminated hydrocarbon polymer substantially as hereinbefore described with particular reference to any one of Examples 1 to 6.

1 2. A thermoplastic elastomer comprising a polymer according to claim 9 wherein X is (+) -Y-(R)3A, and (M) has a high 1,4-content.

1 3. A thermoplastic elastomer according to claim 12 substantially as hereinbefore described with particular reference to Example 6.

14. A process for the preparation of an ionically terminated hydrocarbon polymer of general formula I (+) X(M)nY(R)3 A I wherein (M) comprises a hydrocarbon polymer, X is H or (+) -Y-(R)3 A, Y is a Group Ill element, R is an alkyl or an aryl group and A is an alkali or an alkaline earth metal, comprising contacting an alkene or a conjugated 13-diene with an anionic initiator in a solvent, allowing the alkene or 1,3-diene to polymerise and terminating the polymerisation by the addition of a non-sterically hindered Lewis acid of general formula YR3 wherein Y is a Group Ill element and R is an alkyl or an aryl group.

1 5. A process according to claim 14 wherein Y is boron.

16. A process according to claim 14 wherein Y is aluminium.

1 7. A process according to any one of claims 14 to 1 6 wherein R is C1-C4 alkyl, phenyl or monosubstituted phenyl.

1 8. A process according to any one of claims 14 to 1 7 wherein the anionic initiator comprises an organo alkali or alkaline earth metal initiator.

1 9. A process according to claim 18 wherein the anionic initiator is difunctional.

20. A process according to claim 1 9 wherein the anionic initiator comprises the disodium tetramer of a-methylstyrene, sodium naphthalene or dilithio butane.

21. A process according to claim 18 wherein the anionic initiator is monofunctional and comprises n-butyl lithium or sec-butyl lithium.

22. A process according to any one of claims 14 to 21 wherein the 1,3-diene contains from 4 to 8 carbon atoms.

23. A process according to claim 22 wherein the 1,3-diene comprises isoprene or butadiene.

24. A process according to any one of claims 14 to 23 wherein the 1,3-diene is allowed to polymerise until the molecular weight of --(M),,- is between 2000 and 10000.

25. Aprncess according to any one of claims 14 to 21 wherein the alkene comprises a styrene, a vinylnaphthalene, a vinylpyridine, a vinylquinoline, an acrylic acid ester or an acrylic acid nitrile.

26. A process according to any one of claims 14 to 20 and 22 to 24 wherein the conjugated 1,3-diene is contacted with a difunctional initiator in a polar solvent in the presence of a sterically hindered Lewis acid.

27. A process according to claim 26 wherein the sterically hindered Lewis acid comprises a sterically hindered triaryl derivative of a Group Ill element.

28. A process according to claim 27 wherein the Group Ill element is boron.

29. A process according to claim 28 wherein the sterically hindered Lewis acid comprises trimesityl boron or tri (2,6-dimethylphenyl) boron.

30. A process according to any one of claims 14 to 18 and 21 to 25 wherein the anionic initiator is monofunctional and the solvent comprises a hydrocarbon.

31. A process according to any one of claims 14 to 20 and 22 to 29 wherein the anionic initiator is difunctional and the solvent comprises an alkyl ether or a cycloalkyl ether.

32. A process according to claim 31 wherein the solvent comprises tetrahydrofuran.

33. A process for the preparation of an ionically terminated hydrocarbon polymer substantially as hereinbefore described with particular reference to any one of Examples 1 to 6.

34. An ionically terminated hydrocarbon polymer whenever prepared by a process according to any one of claims 14 to 33.

35. An ionically terminated hydrocarbon polymer whenever prepared by a process according to any one of claims 26 to 29.

36. A thermoplastic elastomer comprising a polymer according to claim 35 wherein --(M),,- has a molecular weight between 2000 and 10000 and a high 1,4-content.