GB1571669A - Branched block copolymers and their manufacture - Google Patents

Description

(54) BRANCHED BLOCK COPOLYMERS AND THEIR MANUFACTURE (71) We, BASF AKTIENGESELLSHAFT, a German Joint Stock Company of 6700 Ludwigshafen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to selectively hydrogenated, non-elastomeric branched block copolymers which prior to hydrogenation comprise a predominant proportion of units of a monovinylaromatic compound and a lesser proportion of units of a conjugated diene, and which possess high transparency and clarity, as well as good mechanical properties, in particular high impact strength.

The manufacture by polymerization of styrene and butadiene with lithium-hydrocarbons as initiators, of block copolymers in which one or more non-elastomeric polymer blocks are bonded to one or more elastomeric polymer blocks, has been disclosed.

Depending on the content of polymer blocks in the total polymer, these thermoplastic block copolymers exhibit non-elastomeric or elastomeric properties. Successive polymerization of the monomers gives block copolymers having a linear structure. If such linear block copolymers are coupled to one another by means of polyfunctional reactive compounds, branched block copolymers, having a star-shaped structure result. Such branched block copolymers, described, for example, in British Patent 985,614, have a symmetrical structure and in general exhibit better processability than the linear block copolymers.

It has also been disclosed that styrenebutadiene block copolymers having a high styrene content are dear thermoplastic products having a high impact strength. Even though the block copolymers of this type hitherto developed and proposed have satisfactory properties in some respects, they nevertheless fail to fulfil many practical requirements. In particular, their physical and mechanical properties, such as their impact strength, leave much to be desired, or the products do not have the transparency desirable for many applications.

German Laid-Open Application DOS 1,959,922 discloses branched block copolymers having a star-shaped structure, comprising a predominant proportion of styrene and a lesser proportion of a conjugated diene, which are stated to combine good impact strength, clarity, good processability and resistance to external factors in one and the same polymer. These branched block copolymers are obtained by coupling styrene diene two-block copolymers in which the terminal polystyrene blocks have different lengths. It is true that they exhibit better mechanical properties than branched block copolymers in which the polymer blocks of the branches all have the same structure, but they are not entirely satisfactory, especially in respect of their tensile strength.

The present invention seeks to improve the mechanical properties of styrene-diene block copolymers which contain a predominant proportion of styrene, and in particular to produce products having improved tensile strength. In addition, the products should be transparent and as glass-clear as possible, and should be readily processable and very stable.

We have found, surprisingly, that the mechanical properties of non-elastomeric, branched block copolymers comprising a predominant proportion of a monovinylaromatic compound and a lesser proportion of a conjugated diene can be improved if such branched block copolymers, which have a quite specific composition and structure of the blocks at the branch points, are hydrogenated selectively.

Accordingly, the present invention provides a selectively hydrogenated, non-elastomeric branched block copolymer of the general formula (AB)nX where A is a non-elastomeric polymer segment exhibiting polymodal distribution and based on a monovinyl-aromatic compound, B is a hydrogenated elastomeric polymer segment based on a conjugated diene of 4 to 8 carbon atoms and having a crystallinity of less than 5%, X is the n-valent radical of a polyfunctional coupling agent by means of which the polymer blocks (A-B) which form the branches are chemically bonded to one another from the polymer segments B, and n is an integer not less than 3, the proportion of units of the monovinylaromatic compounds in the branched block copolymer being from 60 to 95% by weight and the content of olefinic double bonds in the branched block copolymer having been reduced by selective hydrogenation to a residual content of less than 5%.

It is true that German Laid-Open Application DOS 2,125,344 has already proposed the selective hydrogenation of unsymmetric- ally branched block copolymers of styrene and a conjugated diene which possess a homopolymer block in at least one branch.

This is intended not only to increase the maximum use temperature of the polymers obtained but also to improve their thermal and oxidative stability and to modify their solubility properties. However, the products disclosed in German Laid-Open Application DOS 2,125,344, which, according to the Example given, contain a predominant proportion of the conjugated diene, do not exhibit the desired combination of proper.

ties, i.e. high transparency coupled with good mechanical properties, especially high tensile strength. U.S. Patents 3,595,942 and 3,700,633 also disclose the hydrogenation of elastomeric, rubbery, branched block copolymers.

Here again, the hydrogenation only improves the oxidation resistance and processability of the branched rubbery block copolymers Accordingly, it was entirely surprising and unforeseeable that the selective hydrogenation of non-elastomeric, branched block copolymers comprising a predominant proportion of a monovinyl-aromatic compound and a lesser proportion of a conjugated diene, which have a quite specific composition and structure of the blocks, gives products which are not only transparent and substantially glass-clear, and are easily processable, but which also exhibit increased tensile strength.

Examp]es of monovinyl-aromatic compounds which may be used to synthesize the branched block copolymers of the invention are styrene, styrenes which are alkylated in the side chain, e.g. a-methylstyrene, and nuclear-substituted styrenes, e.g. vinyltoluene or ethylvinyl-benzene. The monovinyl-aromatic compounds may be employed individually or as mixtures with one another.

However, the use of styrene alone is preferred. Examples of conjugated dienes which, according to the invention, may be used individually, or as mixtures with one another, for the manufacture of the branched block copolymers are butadiene, isoprene and 2,3-dimethyl-butadiene. Butadiene or isoprene give particularly advantageous results, and of the two butadiene is preferred.

The non-elastomeric, branched block copolymers of the invention contain a predominant proportion of units of the monovinyl-aromatic compound and may be manufactured by successive polymerization of the monovinyl-aromatic compound and of the conjugated diene in solution in the presence of a monolithium-hydrocarbon as the initiator, monomer and initiator being added stepwise, followed by coupling of the resulting living linear block copolymers with a polyfunctional, reactive compound as the coupling agent, and in turn followed by selective hydrogenation of the resulting branched block copolymers. Prior to hydrogenation, the block copolymers of the invention comprise from 60 to 95% by weight, especially from 70 to 90% by weight, of units of the monovinyl-aromatic compound and from 40 to 5% by weight, preferably from 30 to 10% by weight, of the units of the conjugated diene. The intrinsic viscosity of the selectively hydrogenated, non-elastomeric, branched block copolymers (which is a measure of the molecular weight) is as a rule from 60 to 180 cm3/g and preferably from 70 to 140 cmg/g. It is determined by measuring the viscosity in an 0.5% strength by weight solution in toluene at 25"C.

The manufacture of the non-hydrogenated branched block copolymers, which exhibit a polymodal distribution in the non-elastomeric polymer segments A made up of the monovinyl-aromatic compounds, is essentially known and is, for example, largely described in German Laid-Open Application DOS 1,959,922. Advantageously, the branched block copolymers are manufactured as follows: First, the non-elastomeric polymer segments A exhibiting a polymodal distribution are manufactured from the monovinyl-aromatic compounds. For this purpose, a substantial portion of the total amount of the monovinyl-aromatic compound is polymerized in a first process stage, under conventional conditions, by means of a relatively small amount of the monolithium-hydrocarbon initiator, in an inert solvent. From 50 to 80% by weight, preferably from 60 to 75% by weight, of the total amount of the monovinyl-aromatic compound employed, overall, for the manufacture of the branched block copolymers should be used in this first process stage. The total amount of monovinyl-aromatic compound used for the manufacture of the branched block copolymers is from 60 to 95% by weight, especially from 70 to 90% by weight, of the total monomers used for the manufacture of the polymer.

The amount of initiator employed in the first process stage depends above all on the desired molecular weight of the polymer and is in general from 0.1 to 5 mmoles per mole of the monovinyl-aromatic compounds employed in this first process stage. Preferably, the polymerization in the first process stage is carried out with from 0.4 to 2 mmoles of initiator per mole of the monovinyl-aromatic compounds employed in this stage. The conventional monolithium-hydrocarbons of the general formula RLi, where R is an aliphatic, cycloaliphatic, aromatic or mixed aliphatic-aromatic hydrocarbon radical, are used as initiators. The hydrocarbon radical may have from 1 to about 12 carbon atoms. Examples of the lithiumhydrocarbon initiators to be employed according to the invention are methyl-lithium, ethyl-lithium, n-, sec.- or tert.-butyl-lithium, isopropyl-lithium, cyclohexyl-lithium, phenyllithium or p-tolyl-lithium. Preferably, the monolithium-alkyl compounds where alkyl is of 2 to 6 carbon atoms are employed, n-butyl-lithium and sec.-butyl-lithium being particularly preferred.

The polymerization of the monovinylaromatic compounds is carried out in solution in a inert organic hydrocarbon solvent.

Suitable hydrocarbon solvents are aliphatic, cycloaliphatic or aromatic hydrocarbons which are liquid under the reaction conditions and preferably contain from 4 to 12 carbon atoms. Examples of suitable solvents are isobutane, n-pentane, iso-octane, cyclopentane, cyclohexane, cycloheptane, benzene, toluene, dixylenes and others. Mixtures of these solvents may also be employed.

Furthermore, the polymerization can be carried out in the presence of small amounts, in general from 10-3 to 5% by weight, based on the total solvent, of ethers, e.g. tetrahydrofuran, dimethoxyethane and anisole, thereby influencing the rate of polymerization, and the configuration of the diene polymer segment B, in the conventional manner. The concentration of the monomers in the reaction solution is not critical and can be so chosen that any desired apparatus can be used for the polymerization. In general, the polymerization is carried out in a solution of from 10 to 30% strength by weight in the inert solvent.

The polymerization is carried out under the conventional conditions for anionic polymerization with lithium-organic compounds, for example in an inert gas atmosphere, with exclusion of air and moisture. The polymerization temperature may be from 0 to 120"C and is preferably kept at from 40 to 80"C.

In this first process stage, the polymerization is taken to virtually complete conversion of the monovinyl-aromatic compounds employed. This gives a solution of nonelastomeric, living polymers of the monovinyl-aromatic compounds, with active ter minal lithium-hydrocarbon bonds, to which further monomers can add.

In a second process stage, the remainder of the monovinyl-aromatic compound, i.e.

from 20 to 50% by weight, preferably from 25 to 40% by weight, of the total monovinyl-aromatic compound used for the manufacture of the branched block copolymer is added, in one or more portions, to the above reaction solution of the non-elastomeric living polymers based on the monovinyl-aromatic compounds with lithium-terminated chain ends capable of undergoining further polymerization; each addition of a portion of the monovinyl-aromatic compound is accompanied by the addition of a further amount of initiator. The amount of fresh initiator added to the reaction solution in the second process stage whenever adding a portion of the monovinyl-aromatic compound should be as great or greater than the original amount of initiator employed in the first process stage of the polymerization. Preferably, the amount of fresh initiator added with the addition of each portion of the monovinyl-aromatic compound in the second process stage is from 1 to 15 times, and especially from 1 to 10 times, the amount of initiator originally employed. It is particularly advantageous if this factor is from 1 to 5, especially if, as described in more detail below, trifunctional or tetrafunctional coupling agents are employed in the subsequent coupling reaction. Suitable initiators are the monolithium-hydrocarbons which can also be used in the first process stage; preferably, the initiator used is the same as in the first process stage. It is advantageous to introduce the additional fresh initiator into the reaction solution in each case prior to adding a further portion of the monovinyl-aromatic compound.

Though it is possible, in the second process stage, to add the remaining portion of the monovinyl-aromatic compound to the reaction solution in as many portions as desired, it is preferred to add it to the polymerization solution in 1 or 2 portions, and adding it in one portion has proved particularly advantageous. In the second process stage, the same polymerization conditions as in the first process stage are maintained, and after each further addition of monomer and initiator sufficient time is allowed to elapse to enable the freshly added monomer to polymerize virtually completely.

The monomers added in the second process stage undergo addition to the active, lithium-terminated chain ends of the pre viously formed polymers, and also form new chains of living polymers, as a result of the fresh initiator added with each portion of monomer. Accordingly, after complete polymerization of the monomers in the second process stage, the solution obtained contains polymers of the monovinyl-aromatic compound with different average chain lengths, i.e. the non-elastomeric polymer segments A formed from the monovinyl-aromatic compounds have a polymodal distribution.

The polymodality of the polymer segments A corresponds to the total number of additions of initiator and monomer and is thus preferably 2 or 3, a bimodal distribution being particularly advantageous. After completion of the polymerization of the monovinyl-aromatic compounds in the second process stage, the chain ends of the polymodal non-elastomeric polymer segments A carry active, reactive lithium-carbon bonds to which further monomers can add. The active living polymer segments are hereinafter referred to as A-Li.

In a third process stage, the polymer segments B are then polymerized onto the active chain ends of the non-elastomeric polymer segments A-Li, to form the polymer blocks (A-B) which form the branches of the block copolymer of the invention.

For this purpose, the total amount of the conjugated diene monomer is added to the fully polymerized reaction solution from the second process stage. The amount of conjugated diene is from 5 to 40% by weight, preferably from 10 to 30% by weight, of the total monomers employed to manufacture the branched block copolymers of the invention. The conjugated dienes are polymerized under the same polymerization conditions as in the first two process stages, and again the reaction is taken to virtually complete conversion of the monomers. When polymerizing the conjugated dienes, it is necessary to ensure that the number of alkyl side chains of the linear diene polymer segments is sufficiently great that, after hydrogenation, the polymer segments B have a crystallinity of less than 5%. If only branched dienes, e.g. isoprene or 2,3-dimethylbutadiene, are employed, this condition is as a rule observed without taking any special measures. If, however, butadiene, by itself or as the predominant constituent, is employed as the conjugated diene, the polymerization in the third process stage must be carried out in the presence of small amounts of ethers to give a sufficiently high 1,2-vinyl content of the butadiene polymer segments. If the ether, which is in general employed in amounts of from 10-3 to 5% by weight and preferably from 10-2 to 2% by weight, based on total solvent, is not already present from the first process stage, it can be introduced into the reaction solution, in the third process stage, together with the butadiene. If the conjugated diene employed is butadiene, the 1,2-vinyl content of the butadiene polymer segments should be from 25 to 50% by weight and preferably from 32 to 40% by weight, if products having satisfactory properties are ultimately to result.

As explained, elastomeric polymer segments B based on the conjugated dienes are polymerized onto the polymodal, non-elasto meric polymer segments A, produced from the monovinyl-aromatic compounds, in the third process stage. After completion of the polymerization of the monomers in the third process stage, the reaction solution thus contains living, linear block copolymers comprising a non-elastomeric polymer segment produced from the monovinyl-aromatic compounds and an elastomeric diene polymer segment, and these linear block copolymers have a polymodal distribution and possess an active, reactive lithium-carbon bond at each free end of the elastomeric diene polymer segment.

These active linear block polymers are then reacted in a further process stage, by adding a polyfunctional reactive compound as a coupling agent. The polyfunctional coupling agent used should be at least trifunctional, i.e. it should be capable of reacting with at least 3 of the active living block copolymer chains, at their terminal lithiumcarbon bonds, to form a chemical bond, so that a single, coupled and accordingly branched block copolymer is formed. The coupling of lithium-terminated living polymers with polyfunctional coupling agents is known in the art and disclosed, for example, in the publications cited in the introductory section, especially in British Patent 985 614.

Examples of suitable coupling agents for the manufacture of the branched block copolymers of the invention are polyepoxides, epoxidized linseed oil, polyisocyanates, e.g.

benzo- 1 ,2,4-triisocyanate, polyketones, polyanhydrides, e.g. pyromellitic dianhydride, or polyhalides. Dicarboxylic acid esters, e.g.

diethyl adipate or the like, may also be used as coupling agents. A further preferred group of coupling agents comprises the silicon halides, especially silicon tetrachloride, silicon tetrabromide, trichloroethylsilane or 1 ,2-bis-(methyldichlorosilyl)-ethane. Polyvinyl-aromatics, especially divinyl-benzene, may also be used as coupling agents, as disclosed, for example, in U.S. Patent 3,280,084.

In that case, a few divinylbenzene units undergo addition, with crosslinking, to form a branching center, by means of which the preformed polymer blocks are bonded to one another.

The nature of the polyfunctional coupling agent employed is not critical, provided it does not significantly impair the desired properties of the end product. The use of a trifunctional or tetrafunctional coupling agent of the type described, or of divinylbenzene, is preferred. In general, the polyfunctional coupling agent is added to the reaction solution in amounts equivalent to the total amount of the living polymer blocks, i.e. equivalent to the number of the active lithium-carbon bonds in the previously formed linear copolymer blocks. The reaction of the active, linear block copolymers with the coupling agent is preferably carried out under the same reaction conditions as the preceding polymerization of the monomers.

Following the coupling reaction and, advantageously, prior to isolating the reaction product from the reaction solution, the olefinic double bonds of the branched block copolymers obtained are hydrogenated selectively. The selective hydrogenation can be carried out in the conventional manner, using molecular hydrogen and catalysts based on metals, or salts of metals, of group 8 of the periodic table, as described, for example, in U.S. Patent 3,113,986, German Published Application DAS 1,222,260, Ger man Laid-Open Application DOS 1,013,263 or U.S. Patent 3,700,633. According to these publications, the selective hydrogenation of the olefinic double bonds of the branched block copolymer is preferably carried out in a homogeneous phase, using catalysts based on salts, especially carboxylates, eno lates or alkoxides, of nickel, cobalt or iron, which have been reduced with metal-alkyls, especially aluminum-alkyls, at hydrogen pressures of from 1 to 100 bars and at from 25 to 1500C. The selective hydrogenation is continued until the content of olefinic double bonds in the branched block copolymer has been reduced to less than 5% and preferably less than 2%. This residual content is determined by a Wijs titration or by analysis by IR spectroscopy. In particular, the hydrogenation is continued until the olefinic double bonds have been reduced virtually completely. Preferably, the hydro genation is carried out under conditions such that the aromatic double bonds of the branched block copolymer are not attacked.

The process of manufacture determines the composition and structure of the selectively hydrogenated, non-elastomeric branched block copolymers of the invention. The most probable structure of these corresponds to the general formula (AB)nX where A is a non-elastomeric polymer segment, exhibiting polymodal distribution, based on the monovinyl-aromatic compounds, B is an elastomeric polymer segment based on the conjugated dienes, the contact of olefinic double bonds in the segment having been reduced to less than 5%, and preferably less than 2 ,b, by selective hydrogenation, X is the radical of the polyfunctional coupling agent and n is an integer not less than 3, in general from 3 to 10 and preferably 3 or 4.

The non-elastomeric polymer segments A are, more particularly, homopolystyrene segments. Their molecular weight and polymodality depend primarily on the intended end use of the product. The molecular weight is decided, in the conventional manner, by the amount of initiator employed per mole of monomer; the polymodality depends, as already described, on the frequency of addition of initiator and is preferably 2 or 3 and especially 2. The elastomeric polymer segment B based on the conjugated dienes should have a crystallinity of less than 5.% and preferably of less than 2% after hydrogenation. The crystallinity of the polymer segments B is determined by differential calorimetry, using a Perkin-Elmer DSC calorimeter.

The selectively hydrogenated, non-elastomeric branched block copolymers of the invention generally possess not only high transparency and clarity, but also good aging resistance and weathering resistance, and are easily processable. They are in general distinguished by their good mechanical properties and are in particular superior in tensile strength to conventional products, as described in German Laid Open Application DOS 1,959,922. The branched block copolymers of the invention can readily be processed by conventional methods used for thermoplastics, e.g. extrusion, deep-drawing or injection molding, and are above all suitable for the manufaeture of moldings and packaging materials. They may be employed by themselves or as mixtures with other thermoplastics.

The Examples which follow illustrate the invention. Parts and percentages are by weight, unless stated otherwise. The intrinsic viscosity, measured in 0.5% by weight strength solution in toluene at 250cm is given as a measure of the molecular weight.

The tensile strength Z was determined on a compression-molded half-dumb-bell according to DIN 53,455.

EXAMPLE I 4.3 kg of cyclohexane and 680 g of styrene are titrated with sec. -butyl-lithium in a 10-1 pressure kettle, under an inert gas atmosphere and whilst excluding moisture, until the polvmerization starts. 17 mmoles of sec.butyl-lithium are then added and the polymerization is carried out at 50"C for about 1.5 hours, by which time conversion is vir- tually complete. The polystyrene formed has an instrinsic viscosity of 28.7 :fcm3/g]. A further 17 mmoles of sec.-butyl-lithium are then added, followed by 340 g of styrene, and the polymerization is continued for 1 hour at 500C. The polystyrene then has an intrinsic viscosity of 30.3 fcm3 /gj. 11 g of THF and 340 g of butadiene are then added and the mixture is completely polymerized in the course of 3 hours at 600 C. The product has an intrinsic viscosity of 47.9 fcm3/gl.

It is subjected to coupling with 8.5 mmoles of SiC14. The instrinsic viscosity of the product is 81.8 fcm3/gl.

The product is then hydrogenated for 6 hours at from 75 to 800C under a hydrogen pressure of 10 atmospheres gauge, using 100 ml of a nickel hydrogenation catalyst The finished product contains less than 1 % of olefinic double bonds (determined by a Wijs titration) and has a glass transition temperature of --69"C. The tensile strength Z is found to be 333 kp/cm2.

EXAMPLE II 2.7 kg of cyclohexane and 600 g of styrene were titrated with sec.-butyl-lithium in a 6 1 pressure kettle under an inert gas atmosphere, and were then polymerized with 0.33 g of sec.-butyl-lithium for 30 minutes. The temperature was initially 54"C. 0.22 kg of cyclohexane, 0.9 g of sec.-butyl-lithium and 225 g of styrene were added to the reaction solution at 71 0C and polymerization was carried out for one hour; 10 g of THF and 250 g of butadiene are then polymerized onto the product in the course of one hour at about 74 C. Coupling was carried out with 10 ml of Epoxol 9-5 (as marketed by Swift Chemical Corp. EPOXOL is a Registered Trade Mark) in 150 ml of toluene. The intrinsic viscosity of the product was 91.9 m3/g].

The product was then hydogenated for 6 hours at from 75 to 80"C under a hydrogen pressure of 10 atmospheres gauge, using 100 ml of a nickel hydrogenation catalyst.

The end product contained less than 1 % of olefinic double bonds (determined by a Wijs titration) and had a glass transition temperature of - 63 0C. The tensile strength Z was 320 kp/cm2.

COMPARATIVE EXAMPLE The procedure followed was as described in Example II, with the sole difference that after coupling the styrene-butadiene block copolymer was not hydrogenated. The tensile strength of this product was about 190 kp/cm2.

WHAT WE CLAIM IS: 1. A selectively hydrogenated, non-elastomeric branched block copolymer of the general formula (A--B)nn-X where A is a non-elastomeric polymer segment exhibiting polymodal distribution and based on a monovinyl-aromatic compound B is a hydrogenated elastomeric 'polymer segment based on a conjugated diene of 4 to 8 carbon atoms and having a crystallinity of less than 5%, X is the n-valent radical of a polyfunctional coupling agent by means of which the polymer blocks (A-B) which form the branches are chemically bonded to one another from the polymer segments B, and n is an integer not less than 3, the proportion of units of the monovinylaromatic compound in the branched block copolymer being from 60 to 95% by weight and the content of olefinic double bonds in the branched block copolymer having been reduced by selective hydrogenation to a residual content of less than %.

2. A selectively hydrogenated, nonelastomeric branched block copolymer as claimed in claim 1, in which the elastomeric polymer segment B, prior to hydrogenation, is a polybutadiene segment having a 1,2vinyl content of from 25 to 50% by weight.

3. A copol

Claims (15)

**WARNING** start of CLMS field may overlap end of DESC **. tually complete. The polystyrene formed has an instrinsic viscosity of 28.7 :fcm3/g]. A further 17 mmoles of sec.-butyl-lithium are then added, followed by 340 g of styrene, and the polymerization is continued for 1 hour at 500C. The polystyrene then has an intrinsic viscosity of 30.3 fcm3 /gj. 11 g of THF and 340 g of butadiene are then added and the mixture is completely polymerized in the course of 3 hours at 600 C. The product has an intrinsic viscosity of 47.9 fcm3/gl. It is subjected to coupling with 8.5 mmoles of SiC14. The instrinsic viscosity of the product is 81.8 fcm3/gl. The product is then hydrogenated for 6 hours at from 75 to 800C under a hydrogen pressure of 10 atmospheres gauge, using 100 ml of a nickel hydrogenation catalyst The finished product contains less than 1 % of olefinic double bonds (determined by a Wijs titration) and has a glass transition temperature of --69"C. The tensile strength Z is found to be 333 kp/cm2. EXAMPLE II 2.7 kg of cyclohexane and 600 g of styrene were titrated with sec.-butyl-lithium in a 6 1 pressure kettle under an inert gas atmosphere, and were then polymerized with 0.33 g of sec.-butyl-lithium for 30 minutes. The temperature was initially 54"C. 0.22 kg of cyclohexane, 0.9 g of sec.-butyl-lithium and 225 g of styrene were added to the reaction solution at 71 0C and polymerization was carried out for one hour; 10 g of THF and 250 g of butadiene are then polymerized onto the product in the course of one hour at about 74 C. Coupling was carried out with 10 ml of Epoxol 9-5 (as marketed by Swift Chemical Corp. EPOXOL is a Registered Trade Mark) in 150 ml of toluene. The intrinsic viscosity of the product was 91.9 m3/g]. The product was then hydogenated for 6 hours at from 75 to 80"C under a hydrogen pressure of 10 atmospheres gauge, using 100 ml of a nickel hydrogenation catalyst. The end product contained less than 1 % of olefinic double bonds (determined by a Wijs titration) and had a glass transition temperature of - 63 0C. The tensile strength Z was 320 kp/cm2. COMPARATIVE EXAMPLE The procedure followed was as described in Example II, with the sole difference that after coupling the styrene-butadiene block copolymer was not hydrogenated. The tensile strength of this product was about 190 kp/cm2. WHAT WE CLAIM IS:

1. A selectively hydrogenated, non-elastomeric branched block copolymer of the general formula (A--B)nn-X where A is a non-elastomeric polymer segment exhibiting polymodal distribution and based on a monovinyl-aromatic compound B is a hydrogenated elastomeric 'polymer segment based on a conjugated diene of 4 to 8 carbon atoms and having a crystallinity of less than 5%, X is the n-valent radical of a polyfunctional coupling agent by means of which the polymer blocks (A-B) which form the branches are chemically bonded to one another from the polymer segments B, and n is an integer not less than 3, the proportion of units of the monovinylaromatic compound in the branched block copolymer being from 60 to 95% by weight and the content of olefinic double bonds in the branched block copolymer having been reduced by selective hydrogenation to a residual content of less than %.

2. A selectively hydrogenated, nonelastomeric branched block copolymer as claimed in claim 1, in which the elastomeric polymer segment B, prior to hydrogenation, is a polybutadiene segment having a 1,2vinyl content of from 25 to 50% by weight.

3. A copolymer as claimed in claim 1 or 2 wherein the polymer segment A is essentially polystyrene.

4. A copolymer as claimed in any of claims 1 to 3 wherein from 70 to 90% by weight of the branched block copolymer is provided by segments A.

5. A process for the manufacture of a branched block copolymer as claimed in claim 1, by successive polymerization of from 60 to 95% by weight of a monovinylaromatic compound and from 40 to 5% by weight of a conjugated diene of 4 to 8 carbon atoms in an inert solvent in the presence of a monolithium-hydrocarbon as the initiator, subsequent coupling, and hydrogenation of the resulting polymers, in which, in a first process stage, from 50 to 80% by weight of the total amount of the monovinyl-aromatic compound is polymerized in the presence of a monolithium-hydrocarbon initiator, until the conversion is virtually complete, thereupon, in a second process stage, the remaining portion of the total monovinyl-aromatic compound employed is added, in one or more portions, to the reaction solution, in each case with addition of further amounts of initiator equal to or greater than the amounts of initiator originally employed, and the polymerization is again taken to virtually complete conversion of the monomers, after which, in a third process stage, the total amount of the conjugated diene is added to the reaction solution and is polymerized under conditions such that at least 25% by weight of the

copolymerized conjugated diene units possess alkyl branches after copolymerization, and finally, when virtually complete conversion of the monomers has been reached, the linear block copolymers obtained, possessing active, terminal lithium-carbon bonds, are coupled to one another, by addition of a polyfunctional coupling agent having a functionality of at least 3, to form a branched block copolymer, the olefinic double bonds of which are then reduced, by selective hydrogenation, to a residual content of less than 5%.

6. A process as claimed in claim 5 wherein the amount of initiator employed in the first process stage is from 0.1 to 5 mmoles per mole of mono-vinyl aromatic compound employed in the said stage.

7. A process as claimed in claim 5 or 6 wherein n-butyl-lithium or sec-butyl-lithium is used as the initiator.

8. A process as claimed in any of claims 5 to 7 carried out in the presence of from 10-3 to 5% by weight of an ether to influence the polymerization rate.

9. A process as claimed in any of claims 5 to 8 wherein the amount of fresh initiator added with the addition of each portion of the monovinyl-aromatic compound in the second process is from 1 to 15 times the amount of initiator originally employed.

10. A process as claimed in any of claims 5 to 9 wherein the remainder of the monovinyl-aromatic compound is added in a single portion in the second process stage.

11. A process as claimed in any of claims 5 to 10 wherein butadiene, by itself or as the predominant constituent, is employed as the conjugated diene and the third process stage is carried out in the presence of from 10-3 to 5% by weight, based on total solvent, of an ether so that the 1,2-vinyl content of the butadiene polymer segments is from 25 to 50% by weight.

12. A process as claimed in any of claims 5 to 11 wherein the coupling agent used is a polyepoxide, epoxidized linseed oil, a polyisocyanate, a polyketone, a polyhalide, a polyanhydride or a silicon halide.

13. A process for the manufacture of a selectively hydrogenated, non-elastomeric branched block copolymer as claimed in claim 1 carried out substantially as described in either of the foregoing Examples I and IL

14. A selectively hydrogenated, nonelastomeric branched block copolymer when manufactured by a process as claimed in any of claims 5 to 13.

15. Moldings and packaging materials made from a block copolymer as claimed in any of claims 1 to 4 or 14. - -