CA1087793A - Branched block copolymers and their manufacture - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE: Branched block copolymers of from 60 to 95% by weight of a monovinyl-aromatic compound and from 40 to 5% by weight of a conjugated diene, of which the olefinic double bonds are selectively hydrogenated. The copolymers have a structure of the general formula (A-13)n-X, where A is a polymer segment, exhibiting polymodal distribution, based on the monovinyl-aromatic compound, B
is a hydrogenated polymer segment based on the conjugated diene, having a crystallinity of less then 5%, X is the radical Or an at least trifunctional coupling agent and n is an integer not less than 3. The copolymers may be used for the manufacture of highly trans-parent shaped articles, especially packaging materials.

Description

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O.Z. 31,89 BRANCHED BLOCK COPOLYMERS AND THEIR MANUFACTURE

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 com-pound and a lesser proportion of units of a conjugated diene, and which possess high transparency and clarity, as well as ~ood mechanical pro-perties, 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 con-tent 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 cou~led to one another by means of polyfunctional reactive compounds, branched block copolymers having a star-shaped structure resu7t. 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 t~lat styrene-butadiene block copoly-- 1 - ~

~sm3 o.%. 31,~n~

mers ~laving a high styrene content are clear 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 pro-ducts 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, comprisin~ a pre-dominant 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.
It is an object of the present invention to improve the mechani-cal properties of styrene-diene block copolymers which contain a pre-dominant proportion of styrene, and in particular to produce products having improved tensile stren~th. In addition, the products should he transparent and as glass-clear as possible, and should be readilv processable and very stable.
We have found, surprisingly, that this object is achieved and that the mechanical properties of non-elastomeric, branched block co-polymers comprising a predominant proportion of a monovinyl-aromatic compound and a lesser proportion of a conju~ated diene can be improved if such branched block copolymers, which have a quite specific com-position and structure of the blocks at the hranch points, are hydro-genated selectively.
Accordingly, the present invention relates to selectively hydro-1 ~ ~ 3 0-~ 3l,8~6 genated, non-elastomeric branched block copolymers of the general formula (A-B)n-X
where A is a non-elastomeric polymer segment, exhibiting polymodal distribution, based on a monovinyl-aromatic compound, B is a hydrogenated elastomeric polymer segment based on a con-jugated diene of 4 to 8 carbon atoms and having a crystallinity of less then 5%, X is the radical of a poly~unctional coupling agent by means of which the polymer blocks (A-B) which form the branches are chemically bonded to one another at the polymer segments B and n is an integer not less than 3, the proportion of the monovinyl-aromatic 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 o~ unsymmetrically 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 modi~y their solubility properties i~o~ever, the products dis-closed in German ~aid-Open Application DOS 2,125,344, which, according to the Example given, contain a predominant proportion Or the con-jugated diene, do not exhibit the desired combination o~ properties, i.e. high transparency coupled with good mechanical properties, especially high tensile strength~ U.S Patents 3,5~,9~2 and 3,700,633 also disclose the hydrogenation o~ elastos~eric, rubbery, branched block copolymers. Here again, the hydrogenation only improves the oxidation resistance and processa~ility of the hranched 10~93 ~ . . 31,896 rubbery block copolymers. Accordingly, it was entirely sur~rising and unforeseeable that the selective hydrogenation Or 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.
Examples 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. ~-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. ~owever, the use of st~rene 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 pre-ferred.
The non-elastomeric, branched block copolymers of the invention comprise a predominant proportion of the monovinyl-aromatic compound and are 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 com-pound 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 should comprise from 65 to 95~0 by weight, especially from 70 to 90% by weight, of the - monovinyl-aromatic compound and from 40 to 5% by weight, preferably from 30 to 10~ by weight, of the conjugated diene. ~he intrinsic 1~ _ 7. 31,8~6 1 ~ ~ 3 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 cm3/g. It is determined by measuring the viscosity in an 0.5~, strength by weight solution in toluene at 25C.
The manufacture of the non-hydrogenated branched block copoly-mers 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. Specifically, the branched block copolymers are manufactured as follows:
First, the non-elastomeric polymer segments A exhibiting a poly-modal distribution are manufactured from the monovinyl-aromatic com-pounds. 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 ~ mmoles per mole of the monovinyl-aromatic compounds employed in this first process stage. Preferably, the poly-merization 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 conv~ntional monolithium-hydrocarbons of the general formula RLi, where R is an aliphatic, cycloaliphatic, aromatic or mixed aliphatic-aromatic hydrocarbon radical, are used 10~779;~ o.z. 31,896 as initiators. The hydrocarbon radical may have rrom 1 to about 12 carbon atoms. Examples Or the lithium-hydrocarbon initiators to be employed according to the invention are methyl-lithium, ethyl-lithium, n-, sec.- or tert.-butyl-lithium, isopropyl-lithium, cyclohexyl-lithium, phenyl-lithium or p-tolyl-lithium. Preferably, the mono-lithium-al~yl 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 monovinyl-aromatic compounds is carried out in solution in a inert organic hydrocarbon solvent. Suitable hydrocarbon solvents are aliphatic, cycloaliphatic or aromatic hydro-carbons 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, cyclo-heptane, 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. tetrahydro-furan, dimethoxyethane and anisole, thereby influencing the rate Or polymerization, and the configuration of the diene polymer segment 8, 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 con-ditions 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 ~rom 0 to 120C and is preferably kept at from 40 to 80C.
In this first process stage, the polymerization is taken to virtually complete conversion of the monovinyl-aromatic compounds employed. This gives a solution of non-elastomeric, living polymers i~ ~ 93 O.Z. 31,8~6 of the monovinyl-aromatic compounds, with active terminal lithium-hydrocarbon bonds, to which further monomers can add.
In a second process stage, the remainder Or the monovinyl-aro-matic 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 Or the branched block copolymer is added, in one or more portions, to the above reaction solution of tbe non-elastomeric living polymers based on the mono~inyl-aromatic compounds with lithium-terminated chain ends capable Or undergoing further polymeri-zation; each addition of a portion Or the monovinyl-aromatic compound is accompanied by the addition of a further amount Or initiator. The amount of fresh initiator added to the reaction solution in the second process stage whenever adding a portion Or the monovinyl-aro-matic compound should be as great or greater than the original amount Or initiator employed in the first process stage Or the polymeri-zation. Preferably, the amount of fresh initiator added with the addition of each portion Or the monovinyl-aromatic compound in the second process stage is from 1 to 15 times, and especially from 1 to 10 times, the amount Or initiator originally employed. It is particu-larly advantageous if this factor is from 1 to 5, especially if, asdescribed in more detail ~elow, 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 firs~ 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 com-pound.
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 1087793 o.z. 31,896 stage, the same polymerization conditions as in the first process stage are maintained, and after each further addition Or 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 Or the previously formed polymers, and also form new chains Or 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-aro-matic compound with different average chain lengths, i.e. the non-elastomeric polymer segments A formed from the monovinyl-aromatic com-pounds have a polymodal distribution. The polymodality of the polymer segments A corresponds to the total number Or additions of initiator and monomer and is thus preferably 2 or 3, a bimodal distribution being particularly advantageousO After completion of the polymeri-zation Or the monovinyl-aromatic compounds in the second process stage, the chain ends Or the polymodal non-elastomeric polymer seg-ments A carry active, reactive lithium-carbon bonds to which further r 20 monomers can add. The active living polymer segments are hereinafter referred to as A-Li.
In a third proces B stage, the polymer segments B are then poly-merized onto the active chain ends of the non-elastomeric polymer segments A-Li, to form the polymer bloc~s (A-B) which form the branches of the block copolymer of the invention. For this purpose, the total amount Or 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 manu-facture the branched block copolymers of the invention. The conjugateddienes are polymerized under the same polymerization conditions as ~ in the first two process stages, and again the reaction is taken to 911~. ~r~mr~ nn~ q~nn n~ 1-.hP mnnr~mPt~q WhPn r~lvmerizin~

1~8~93 o.z. 31,8~6 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.~. isoprene or 2,3-dimethylbutadiene, are employed~this condition is as a rule observed without taking any special measures. I~, however, butadiene, by itself or as the predominant constituent, is employed as the con-jugated 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.
I~ the ether, which is in general employed in amounts of rrom 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 ~irst process stage, it can be introduced into the reaction solution, in the third process stage, together with the butadieneO 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 con-; jugated dienes are polymerized onto the polymodal, non-elastomeric polymer segments A, produced from the monovinyl-aromatic compounds, in the third process stageO A~ter completion of the polymerization o~
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 Or the elastomeric diene polymer segment.
These active linear block polymers are then reacted in a ~urther - process stage, by adding a polyfunctional reactive compound as a coupling agent. The polyfunctional couplin~ agent used should be at _ q _ 1~8~93 o.z. 31,896 least trifunctional, i.e. it should be capable of reacting with at least 3 of the active living block copolymer chains, at their terminal lithium-carbon 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 lntroductory section, especially in British Patent 985,614.
Examples of suitable coupling agents for the manufacture Or the branched block copolymers Or the invention are polyepoxides, epoxi-dized linseed oil, polyisocyanates, e~g. benzo-1,2,4-triisocyanate, polyketones, polyanhydrides, eOg. pyromellitic dianhydride, or poly-halides. Dicarboxylic acid esters, e.g. diethyl adipate or the like, may also be used as coupling agents. A further preferred group Or 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 pro-perties of the end product. The use of a trifunctional or tetra-functional coupling agent of the type described, or Or divinylbenzene, is preferredO In general, the polyfunctional coupling agent is added to the reaction solution in amounts equivalent to the total amount Or the living polymer blocks, i.e. equivalent to the number Or the active lithium-carbon bonds in the previously formed linear copolymer blocks. The reaction of the active, linear bloc~ copolymers with the coupling agent is preferably carried out under the same reaction con-ditions as the preceding polymerization of the monomers.

1087793 o.z. 3l,8g~

Following the coupling reaction and, advantageously, prior to isolating the reaction product from the reaction solution, the ole-finic double bonds of the branched block copolymers obtained are hydrogenated selectivelyO The selective hydrogenation can be carried out in the conventional manner, using molecular hydrogen and catalysts based on meta~.s, or salts of metals, of group 8 of the periodic table, as described, for example, in U S. Patent 3,113,986, Oerman Published Application DAS 1,222,260, German Laid-Open Application DOS 1,013,263 or U.S. Patent 3,700,6330 According to these publications, the selec-~10 tive 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, enolates 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 150Co The selective hydrogenation is cont;nued until the content of olerinic double bonds in the branched block copolymer has been reduced to less than 5% and pre-ferably less than 2%. This residual content is determined by a Wijs titration or by analysis by IR spectroscopy. In particular, the hydro-- 20 genation is continued until the olefinic double bonds have been reduced virtually completely. Preferably, the hydrogenation is carried out under conditions such that the aromatic double bonds o~ the branched block copolymer are not attacked. ~;
~ he process of manufacture determines the composition and structure of the selectively hydrogenated, non-elastomeric branched block copolymers of the invention~ The most probable structure Or these corresponds to the general formula (A-B)n-X

where A is a non-elastomeric polymer segment, exhibiting polymodal distribution, based on the monovinyl-aromatic compounds, ~ is an elastomeric polymer segment based on the conjugated O Z. 31,8~6 ~Q8779;~
dienes, the content of olefinic double bonds in the segment having been reduced to less than 5%, and preferably less than 2%, by selec-tive 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 2D 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 Or the polymer segments B is determined by differential calorimetry, using a Perkin-Elmer DSC calorimeter.
The selectively hydrogenated, non-elastomeric branched block co-polymers of the invention possess not only high transparency and 20 clarity, but also good aging resistance and weathering resistance, and are easily processable~ They are distinguished by their good mechanical properties and are in particular superior in tensile strength to the conventional products, as described in German Laid-Open Application DOS 1,959,922. The branched block copolymers Or the invention can readily be processed by conventional methods used for thermoplastics, e.gO extrusion, deep-drawing or injection molding, and are above all suitable for the manufacture of m~ldings and packaging materials. They may be employed by themselves or as mixtures with other thermoplastics.
- 30 The Examples which follow illustrate the invention. Parts and percentages are by weight, unless stated otherwise. The intrinsic viscosity, measured in 0.5% strength solution in toluene at 25C, is given as a measure of the molecular weight. The tensile strength Z

OOZ. 31,86 was determined on a compression-molded half-dumb-bell according to DIN 53,455.
EXAI/!PLE
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 E~as atmosphere and whilst excluding moisture, until the polymerization startsO 17 mmoles of secO-butyl-lithium are then added and the poly-merization is carried out at 50C for about 1 5 hours, by which time conversion is virtually completeO The polystyrene formed has an intrinsic viscosity of 28.7 E~m3/~. A further 17 mmoles Or sec.-10 butyl-lithium are then added, followed by 340 g of styrene, and the polymerization is continued for 1 hour at 50C. The polystyrene then has an intrinsic viscosity of 30.3 ~cm3~g] 0 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 60Co The product has an intrinsic vis-cosity Or 47.9 ~cm3~g~0 It is subjected to coupling with 8.5 mmoles of SiCl4. The intrinsic viscosity Or the product is 81.8 ~cm3/g~.
The product is then hydrogenated for 6 hours at from 75 to 80C
under a hydrogen pressure of 10 atmospheres gauge, using 100 ml of a nickel hydrogenation catalystD The finished product contains less 20 than 1% of olefinic double bonds (determined by a Wijs titration) and has a glass transition temperature of -69C. The tensile strength Z is found to be 333 kp~cm .
EXAMPLE II
2.7 kg of cyclohexane and 600 g of styrene were titrated with sec~-butyl-lithium in a 6 l 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 54C. 0.22 kg of cyclohexane, 0.9 g of sec.-butyl-lithium and 225 g of styrene were added to the reaction solution at 71C and polymerization was carried out for one hour; 10 g of THF and 250 g of butadiene are then poly-30 merized onto the product in the course of one hour at about 74C.Coupling was carried out with 10 ml of Epoxol 9-5 ~as marketed by 1087793 o~z. 31,~g~

Swift Chemical Corp.) in 150 ml of toluene. The intrinsic viscosity Or the product was 91.9 ~cm3/ ~ O
The product was then hydrogenated for 6 hours at from 75 to 80C
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 -63C. The tensile stren~th Z was 320 kp/cm2.
COMPARATIVE EXAMP~E
The procedure followed was as described in Example 2, with the sole difrerence that after coupling the styrene-butadiene block co-polymer was not hydrogenated. The tensile strength Or this product was about 190 kpJcm20

Claims (3)

WE CLAIM:

1. A selectively hydrogenated, non-elastomeric branched block copolymer of the general formula (A-B)n-X

where A is a non-elastomeric polymer segment exhibiting polymodal dis-tribution and based on a monovinyl-aromatic compound, B is a hydrogenated elastomeric polymer segment based on a con-jugated 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 the monovinyl-aromatic 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 then 5%.

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

3. 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 monovinyl-aromatic 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%.