FR2640979A1 - Block polymer compositions and process for their preparation - Google Patents

Abstract

New polymer compositions comprising a tetrablock linear copolymer and a diblock linear copolymer. These two copolymers consist of polydiene blocks and of polyvinylaromatic blocks and the tetrablock copolymer additionally comprises a portion consisting of diene and vinylaromatic monomer units linked statistically into a chain. The diblock copolymer may also contain a portion of this type. The polymer compositions can be obtained either by mixing the individual block polymer components or by means of original novel synthesis processes which make it possible to obtain the two block copolymers simultaneously. The polymer composition may be mixed with polystyrene. The polymer compositions of the invention are suitable for all uses in which good impact strength, transparency and processability properties are necessary.

Description

SEQUENCE POLYMER COMPOSITIONS AND THEIR METHOD
OF PREPARATION. -
The present invention relates to new polymer compositions comprising two block copolymers.

 Also falling within the scope of the present invention are the methods of obtaining simultaneously, during the same synthesis process, the two components of a composition of the invention.

 These polymer compositions are endowed with very good impact resistance and workability characteristics, as well as with high transparency, properties which make them suitable for all the uses reserved for transparent and impact resistant materials.

 In past years, block copolymers obtained by block copolymerization of monomers dienes conjugated with vinyl aromatic monomers, such as for example block copolymers of polybutadiene and polystyrene and block copolymers of polyisoprene and polystyrene, have experienced a development particular. These copolymers can be used as such, that is to say in the very form in which they are produced by anionic copolymerization giving “living” polymers, as well as after their partial or total hydrogenation.

 This is the case of linear block copolymers, consisting of alternating polydien and polyvinylarene blocks, having a particular structure and distribution of the individual blocks, and having a structural formula of type (I) or (II); these copolymers have an extremely favorable balance of their physical and mechanical properties.

These block copolymers, as well as their derivatives, have been described by the Applicant in the Patent Application
French N '87/15504 and in a Patent Application during the procedure.

 In the prior art, there is also known the mixture of two block copolymers with one another, and with a polystyrene resistant or not to impact, to obtain resins endowed with improved properties.

It has also been described, for example in patents
GB N 1,130,770 and 1,335,077, and in US patent N 4,086,298, compositions and / or block copolymers characterized by the use of structures suitable for all uses for which transparent and impact-resistant polystyrene has been used in the past or is still in use. However, the copolymers claimed in these references of the prior art do not have an optimal balance of rheological properties, impact resistance and transparency.

 Such a balance of properties is further modified by the possible use of polystyrene which, if it improves on the one hand the optical characteristics, on the other hand reduces the impact resistance of the compositions thus obtained.

 The drawbacks of the prior art are overcome in the polymer compositions according to the present invention, which consist of mixtures of two block copolymers, corresponding respectively to the general formulas (I) or (II) and (III) or (IV), as we mentioned it above.

Consequently, and according to a first of its aspects, the present invention relates to new polymeric compositions comprising
(a) from 40 to 90% by weight of a linear copolymer, consisting of four alternating blocks, which can be represented by the formula (I) or (II)
Bi -T-A1 -B2 -A2 (I)
or Bi -Ai -B2 -T-A2 (II) in which Ai, A2, B1, B2 and T respectively represent two vinyl-aromatic sequences, two diene sequences and a portion of random copolymer formed from diene and vinyl-aromatic monomer units and
(b) from 10 to 60% by weight of a linear copolymer consisting of two blocks, which can be represented by the formula (III) or (IV):
B3 -T-A3 (III)
or
B3 -A3 (IV) in which A3 represents a poly (vinylaromatic) sequence which may or may not be identical to the sequence Ai or A2, Ba represents a polydiene block which may or may not be identical to the sequence B1 or B2, and T represents the statistical copolymer mentioned above above.

 In the polymer compositions in accordance with the present invention, the weight-average molecular weight of the tetra-block copolymer of formula (I) or (II) is in the range from 50,000 to 300,000, while the weight-average molecular weight of the bi-block copolymer of formula (III) or (IV) is in the range from 20,000 to 150,000.

 In these compositions, the weight percentage of the random copolymer block T is in the range from 5 to 50%, preferably from 10 to 25%, relative to the total weight of the two copolymers which constitute the mixture.

 In these compositions, the total proportion of vinyl-aromatic monomer units is in the range from 60 to 90% by weight, the complement to 1008 being constituted by diene monomer units.

 In the definitions above, the sequences Ai, As, As and Bi, B2 and B3 are practically pure sequences, that is to say almost entirely constituted respectively of vinyl-aromatic units and of diene units.

In preferred embodiments, the sequences Ai, As and As are polystyrene, Bi,
B2 and B3 are polybutadiene blocks and T is a styrene-butadiene random copolymer.

 In another preferred embodiment, the total proportion of diene units in the polymer composition is in the range of 20 to 30% by weight, and the weight average molecular weight of the tetra-block copolymer is in the range from 100,000 to 200,000; and the weight average molecular weight of the biblock copolymer is in the range of 40,000 to 100,000. To said compositions may be added another polystyrene resin, for example polystyrene or impact resistant polystyrene, in an amount which does not adversely affect the combination of properties of the binary composition.

 The polymer compositions in accordance with the present invention constitute progress from the point of view of their properties, in comparison with the block copolymers of formula (I) or (II), considered individually.

 One can obtain the linear block copolymer consisting of four alternating blocks and corresponding to the general formula (I) or (II), according to the present invention, by polymerization, operating in an aliphatic or cycloaliphatic organic solvent, to which may have been added suitable amounts of a polar aliphatic or cycloaliphatic compound chosen from ethers and amines, at temperatures ranging from 30 to 150 ° C., and under a pressure equal to or greater than atmospheric pressure, in the presence alkyl-metal or aryl-metal initiators, usually used in the synthesis of polymers according to an anionic polymerization process giving living polymers.

 This synthetic route is described in French Patent Application N 87/15504, concerning these copolymers and a process for synthesizing them.

 On the contrary, it is possible to synthesize the two-block copolymers corresponding to the formulas (III) or (IV) above, according to the methods known in the prior art, as well as by the same method of synthesis as that described in the patent application mentioned above. , the reaction being limited to the first step.

More particularly, in order to synthesize copolymers of formula (III), metered amounts of butadiene and styrene are mixed in a state with one another, and the solution polymerization is carried out. in a hydrocarbon, with an initiator suitable for anionic polymerization giving living polymers, until the conversion of the monomers is complete or practically complete; in this way, a living bisquenced copolymer B3 -T-As is formed which is made up of non-pure blocks, that is to say blocks linked to each other by a copolymer chain made up of monomer units of butadiene and styrene, linked in a statistical manner. The polymer formed is recovered after prior blocking of the living active centers, by introduction of a compound having an acidic character.

The recovery of the polymer is carried out according to the usual methods, for example by entraining the solvent with steam and drying the polymer.

 The copolymer of formula (IV) is obtained by anionic polymerization, with sequential addition of the monomers, in accordance with the prior art.

 The constituents of the polymer compositions in accordance with the present invention can be mixed and formulated according to the methods and methods well known in the prior art. For example, the ingredients can be mixed in the molten state and extruded, or mixed in mixers of the Banbury type, or they can be mixed in the state of solutions.

 The Applicant has also developed, and this constitutes another object of the present invention, new and original methods which make it possible to produce the two components simultaneously, by a single method of synthesis.

According to one of these methods, the synthesis is carried out according to the following steps
(1) during the first step, a mixture, composed of a vinyl-aromatic monomer and of a conjugated diene monomer, is copolymerized, in the presence of an initiator, until the conversion of the monomers is practically complete , in an apolar solvent chosen from solvents in which polystyrene having a weight average molecular weight in the range from 30,000 to 120,000 is soluble at concentrations ranging from 5 to 20% by weight.

An alkyl lithium compound is used as the initiator, in order to generate a living polymer system. The solvent system can contain polar compounds (ethers, amines, etc.), in a maximum concentration of 0.1% by weight1 relative to the solvent.

 The proportion by weight of the monomers introduced as a mixture in the first step of the reaction according to the process of the present invention is in the range from 30 to 60% by weight, relative to the total of the monomers introduced into the reaction. During the reaction described above, a living copolymer of the type ~ Bi -T-Ai is formed.

 (2) In a second step, a certain proportion, situated between 10 and 60%, of the living active centers generated during the first reaction step is blocked, by adding to the reaction system compounds containing hydrogen atoms of character acid in their chemical structure (water, alcohols, etc.).

 The amount of these active center blocking compounds added in the second step of this process is in the range from 10 to 50 mol%, relative to the number of moles of initiator introduced in the first step.

 (3) In a third step, a conjugated diene is introduced into the living polymer system from the second step, and this conjugated diene undergoes complete conversion.

 (4) In a fourth step, a vinyl-aromatic monomer is introduced which is almost completely converted.

 (5) In a fifth step, the living active centers are all blocked by the addition of an acidic compound.

In this way, a mixture of two copolymers is obtained
B1-T-As-B2-A2 + B1-T-Al.

According to a variant, one operates thus
- in a first step, a dosed amount of one
conjugated diene is polymerized by polymerization
anionic giving living polymers up to
complete or practically complete conversion of the monomer;
- in a second step, a dosed amount of one
vinyl aromatic monomer is added to the mass
reaction resulting from the first stage, and polymerized
until complete or nearly complete conversion
vinyl aromatic monomer introduced
- In a third step, the partial blocking of a certain proportion, situated in the interval ranging from 10 to 60% of the active centers produced in the preceding steps, is carried out by means of the addition of compounds containing atoms of acidic hydrogen
- In a fourth step, a mixture consisting of metered amounts of a diene monomer and a vinyl-aromatic monomer is introduced, and the polymerization is continued until the complete or practically complete conversion of these monomers
the mixture of polymers produced from the fourth stage is recovered after prior blocking of the living active centers by means of a compound which contains hydrogen atoms of an acidic nature, so that the mixture consisting of polymers
Bs-As-B2-T-A2 + B1-At
According to another variant, one operates thus
- In a first step, dosed quantities of a conjugated diene are polymerized by anionic polymerization giving living polymers, until the complete or practically complete conversion of the monomer
- in a second step, a dosed quantity of a vinyl-aromatic monomer is added to the reaction mass resulting from the first step and polymerized until the complete or practically complete conversion of the vinyl-aromatic monomer introduced
- in a third step, an additional aliquot portion of anionic polymerization initiator is added
- in a fourth step, dosed quantities of a diene and of a vinyl-aromatic monomer, in the state of mixture with one another, and representing from 30 to 608 of the total of the monomers used in the process , are added and the polymerization is continued until complete or practically complete conversion of the monomers
the copolymer is recovered from the polymerization products resulting from the fourth step, after prior blocking of the living active centers by means of the addition of a compound which contains hydrogen atoms of an acidic nature, so that the the mixture formed by the two polymers is obtained
Bs-Al-B2-T-A2 + B2-T-A2
According to yet another variant, one operates thus
- In a first step, dosed quantities of a conjugated diene and of a vinyl-aromatic monomer, in mixture with each other, are polymerized by anionic polymerization giving living polymers, until complete or practically conversion full of monomers
- in a second step, an additional aliquot fraction of anionic polymerization initiator giving living polymers is added
- In a third step, a metered amount of a conjugated diene is added, and the polymerization is continued until complete or practically complete conversion of this monomer
- In a fourth step, a metered amount of a vinyl-aromatic monomer is introduced, and the polymerization is continued until complete conversion of this monomer
the mixture of polymers produced and resulting from the fourth stage is recovered, after prior blocking of all living active centers by means of a compound containing hydrogen atoms of an acidic nature, so that a mixture consisting of by the two polymers
Bl-TA-B2-A2 + B2-A2
In the various variants indicated above, the polymerization is carried out in an organic, aliphatic or cycloaliphatic solvent, at temperatures in the range from 30 to 150 μl and under a pressure equal to or greater than atmospheric pressure, in the presence an alkyl metal or an aryl metal
In addition, the polymerization can be carried out in the presence of at least one linear or cyclic polar compound, chosen from ethers and amines, present in amounts ranging from 0.01 to 0.1 parts by weight. relative to the solvent.

 Preferably, the solvent is cyclohexane and the reaction temperature is in the range from 50 to 100-C, the initiator is an alkyl lithium having from 3 to 7 carbon atoms in the alkyl group, and this initiator is used in proportions ranging from 0.025 to 0.20 parts by weight per 100 parts by weight of vinyl aromatic monomer.

 Preferably, the diene is butadiene and the vinyl aromatic compound is styrene.

 Another of the variants constituting alternative routes for the process described above comprises, at the end of the first stage of polymerization of the mixture of monomers, a second stage during which a second aliquot fraction of initiator is introduced.

This variant leads to the formation of an additional number of active centers s1 as a result of which pure block copolymers, described above by the formula, are formed.
B3 -T-A3.

In this case, the various successive steps are as follows
(1) synthesis of a pure diene sequence
(2) synthesis of a pure vinyl-aromatic sequence,
linked to the diene sequence
(3) introduction of an aliquot fraction
additional initiator
(4) introduction of a mixture consisting of
diene monomer and a vinyl monomer
aromatic; the proportion of monomers
introduced in the state of mixing one with
the other is in the range of 30
at 60%, relative to the total of the monomers
introduced into the reaction
(5) blocking of living active centers by means of
compounds containing hydrogen atoms to
acidic character.

 The preferred initiators for the intended purpose are those belonging to the group consisting of linear or branched alkyllithium compounds, and preferred among these is n-butyl lithium and s-butyl lithium. In the various stages of the process, these initiators are usually used in proportions ranging from 0.025 to 0.2 parts by weight per 100 parts of monomers subjected to the polymerization.

 The solvents suitable for the intended purpose are nonpolar solvents, and among these, those in which polyvinylaromatic sequences having a weight average molecular weight in the range from 50,000 to 200,000 are soluble; among the most suitable of them, mention may be made of cyclohexane and benzene.

 These solvents may contain polar compounds (for example ethers, amines, etc.), the presence of which also causes, in addition to an increase in the speed of the polymerization reaction in the various stages, an increase in the weight of the part of random copolymer T; among these compounds, tetrahydrofuran is preferred.

 The following experimental examples are only given by way of illustration and do not limit the scope of the present invention.

Benole 1
600 g of anhydrous cyclohexane, 55 g of styrene (99.9% pure) and 13 g of butadiene (99.85% pure) are introduced into a 1000 cm 3 reactor. The temperature of the reaction mass is raised to 50 ° C., and 0.055 g of s-butyl lithium (in solution in n-hexane) is introduced.

 25 minutes later, the reaction mass reaches the temperature of 75 ° C., and the conversion of the monomers is practically complete. 0.009 g of pure methanol is then added to the system, followed by 7 g of butadiene.

After 10 minutes, the reaction mass reaches a temperature of 85 ° C., and the conversion of butadiene is practically complete.

 Finally, 25 g of styrene are added, and 15 minutes later. the conversion is practically complete, the temperature of the reaction mass having reached the value of 90 ° C.

 The blocking of living active centers is carried out by adding 0.5 cm 3 of H 2 O to the polymer solution.

 In addition, 1.0 g of triphenyl-nonyl-phosphite and 0.2 g of (pentaerythrityltetraalkyl- (3,5-di-t-butyl-4-hydroxyphenyl-propionate)) are added to the polymer solution.

 The polymer mixture is recovered by entrainment in the vapor of the reaction solvent, and subsequent drying in a vacuum oven at 60 ° C. for 24 hours.

 The physical characteristics of the two components of the polymer blend are given in Table 1.

The optical and mechanical properties of samples molded by compression at 180C are collated in Table 2.

In these tables, as well as in the following Xes "IFC" denotes the melt flow index, and "Mp" denotes the weight average molecular weight.

Table 1
Example- MpxlO-3 Mpx10-3 Total IFC Sequence (4)
ple (B1 TA1 B2A2) (B1 TA1) styrene to styrene (g / 10) - (1)%. ()% .. (3 - (3) minutes)
1,140 80 80 68 4.5
Analysis
GPC (1)
Ik analysis (9!
decomposition by OsO4 (3)
200 C, i kg (4)
Table 2 - Transparency% 92 - Tensile strength MPa '22.5 - Elongation at break% 35 - Module MPa 784 - Impact resistance IZOD on test piece
notched, at 23-C kg x cm / cm 2.5
EXAMPLE 2
850 g of anhydrous cyclohexane, 37.5 g of styrene and 12.5 g of butadiene are introduced into a 2000 cm 3 reactor; the temperature of the system is raised to 55 ° C., and 0.040 g of n-butyllithium (in solution in n-hexane) are introduced.

 45 minutes later, the reaction mass reaches a temperature of 69 ° C. and the conversion of the monomers is practically complete.

 Then added to the reaction solution 0.032 g of n-butyl lithium, then 37.5 g of butadiene; after 20 minutes, the reaction mass reaches the temperature of 78 ° C. and the conversion is practically complete.

 112.5 g of styrene are then added to the polymer solution, and 60 minutes after, the conversion of the monomers is complete.

 The reaction is then stopped by adding 2 g of methanol to the solution containing the mixture of polymers.

The addition of an antioxidant, the recovery and the drying of the polymer mixture are carried out in the same way as in Example 1.

 The physical characteristics of the two components of the polymer blend are given in Table 3.

 The mechanical and optical properties of samples molded by compression at 180-C are collated in Table 4.

Table 3
Example- Mpx10-3 Mpx10-3 Total IFC sequence ple (BsTAsB2A2) (B2A2) styrene styrene (g / tO)
N (1) (1)% (2) 8 (3) minuta6}
2,170 85 75 65 3.5
Table 4 - Transparency% 86 - Tensile strength MPa 16.2 - Elongation at break% 100 - Module MPa 686 - Impact resistance IZOD on test piece
notched, at 23'C kg x cm / cm 3.2
EXAMPLE 3
in a 1.5 liter capacity reactor, 1.2 kg of cyclohexane, 0.3 g of tetrahydrofuran, 100 g of styrene and 45 g of butadiene are introduced, the temperature of the mass is raised to 50 ° C., and cn introduce 0.12 g of n-butyl lithium in solution in n nexane.

 30 minutes later, the reaction mass reaches the temperature of 80 ° C., and the conversion of the monomers is practically complete.

 0.012 g of H2O, then 15 g of butadiene, are then added to the reaction system. 10 minutes later, the reaction mass reaches the temperature of 90-C and the conversion of butadiene is complete.

 Finally, 40 g of styrene are added, and after 15 minutes of reaction, the conversion is complete.

 Before recovering the solid polymer, the active active centers are blocked by adding 2 g of isopropyl alcohol to the system.

 After the addition of antioxidant as in Example 1, the recovery of the solid polymer is carried out by entrainment of the solvent with steam and subsequent drying of the solid residue at 65 ° C. for 24 hours.

 The physico-chemical characteristics of the product obtained are given in Table 5.

Table 5
Example- Total Sequence Mpxl0-3 Mpx10-3 IFC ple styrene styrene (B1TArB2A2) (BiTAs) (g / 10
N '%% ~~~~~~~~~~~ ~~~~~~ minutes) 3 70 50 130 80 8.5
Example 4
5 kg of the mixture described in Example 3 are mixed with 5 kg of commercial crystalline polystyrene (Mp (GPC): 250,000). The mixture obtained is passed through a twin screw extruder equipped with a heating jacket.

This operation is repeated twice to obtain an optimal mixture. The material is then transformed into 0.5 cm long granules.

 The properties of the compound thus prepared are determined on samples molded by compression at 180 ° C., and are given in Table 6.

Table 6 - IFC gilo minutes 8.2 - Transparency% 90 - Tensile strength MPa 18.1 - Elongation at break% 100 - Module MPa 1176 - Impact resistance IZOD on test piece
Notch, at 23 C kg x cm / cm 3.5
EXAMPLE 5
600 g of anhydrous cyclohexane and 30 g of butadiene (99.84% pure) are introduced into a reactor with a capacity of 1 liter. The whole is heated to r0 C, and 0.09 g of s-butyl lithium is added.

 20 minutes later, the polymerization of butadiene is complete and the temperature of the reaction mixture is approximately 60 ° C.

 70 g of styrene (99.9% pure) are then added to the system, and the reaction is completed in 20 minutes; during this period, the reaction mass reaches the temperature of 82 ° C.

 0.058 g of initiator (n-butyl lithium) is then added to the reaction mass, then a mixture composed of 70 g of styrene and 30 g of butadiene. The reaction is carried out in 25 minutes, and the final temperature of the reaction mixture is 102-C.

 At the end of the process, 3 g of methyl alcohol are added to the system to block the active centers.

 Then added to the polymer solution 0.8 g of TNP (triphenyl-nnyl-phosphite) and 0.15 g of pentaerythrityl-tetraalkyl- (3,5-di-t-butyl-4-hydroxyphenyl) -propionate.

 The polymer mixture is recovered by entrainment of the solvent with steam. and the recovered product is then dried in an oven at 60 ° C for 36 hours.

 The physicochemical and mechanical properties of the polymer mixture are given in Tables 7 and 8.

Table 7
Example- Total Sequence Mpxl0-3 Mpx10-3 IFC ple styrene styrene (B1TA1B2TA2) (BlTA2) (g / 10
N%%% minutes)
5 70 56 125 75 10
Table 6 - Transparency% 85 - Tensile strength e 125 - Elongation at break MPa 0.11 - Module MPs 666.4 - Impact resistance IZOD on test piece
notched, at 23iC kg x cm / cm 5.0

Claims (14)

 1. Polymer composition, characterized in that it comprises  (a) from 40 to 90% by weight of a linear copolymer consisting of four alternating blocks, which can be represented by the formula  B1-T-A1-B2-A2 (I)  or  Bs-Ao -B2 -T-As (II) in which  Ai and As are polyvinylaromatic sequences  B1 and B2 are polydien sequences ;;  T is a part of a copolymer chain, consisting of diene and vinyl-aromatic monomer units linked in a statistical fashion  (b) from 10 to 60% by weight of a linear copolymer consisting of two blocks, which can be represented by the formula  B3-T-A3 (III) or by the formula  B3-A3 (IV) in which  As represents a polyvinyl-aromatic sequence, identical or not to the sequence Ai or As  B3 represents a polydiene sequence, identical or not to the sequence B1 or B2  T has the meaning given above.  2. Polymer composition according to claim 1, characterized in that the linear tetra-block polymer (I) or (II) has a weight-average molecular weight in the range from 50,000 to 300,000.  3. A polymer composition according to claims 1 and 2, characterized in that the average molecular weight of the polymers represented by formulas (III) or (IV) is in the range from 20,000 to 150,000.  4. Polymer composition according to the preceding claims, characterized in that, in this composition, the total proportion of vinyl-aromatic monomer units is in the range from 60 to 90% by weight, the balance to 100% consisting of diene monomer units.  5. A polymer composition according to the preceding claims, characterized in that the polydiene blocks are polybutadien blocks and the polyvinyl-aromatic blocks are polystyrene blocks.  6. A polymer composition according to the preceding claims, characterized in that the individual components are mixed in order to constitute a single composition.  7. Method for preparing a polymer composition according to the preceding claims, characterized in that  - in a first step, dosed quantities of a diene and of a vinyl-aromatic monomer are polymerized, in the state of mixture with each other, by anionic polymerization giving living polymers, until conversion complete or practically complete of the monomers;  - in a second step, a certain proportion, situated in the range from 10 to 60%, of the active active centers produced in the first reaction step are blocked by the addition of compounds containing hydrogen atoms of an acidic nature  - a dosed quantity of diene is added to the mass resulting from the second stage, and polymerized, in a third stage, by anionic polymerization giving living polymers, until the conversion of the introduced diene is complete or practically complete  - a metered quantity of vinyl-aromatic monomer is added to the product resulting from the third stage and polymerized, in a fourth stage, until the conversion of the vinyl-aromatic monomer introduced is complete or practically complete  the mixture of polymers is recovered, from the polymerization products resulting from the fourth step, after the prior blocking of all living active centers by means of a compound which contains hydrogen atoms of an acidic nature, so that that we obtain the mixture constituted by the two polymers  B1-T-Al-B2-A2 + B-T-Az.  8. A process for the preparation of a polymer composition according to claims 1 to 5, characterized in that  - In a first step, a metered quantity of a conjugated diene is polymerized by anionic polymerization giving living polymers, until complete or practically complete conversion of the monomer  - in a second step, a metered quantity of a vinyl-aromatic monomer is added to the reaction mass resulting from the first step, and polymerized until the complete or practically complete conversion of the vinyl-aromatic monomer introduced  - In a third step, partial blocking is carried out of a certain proportion, situated in the interval ranging from 10 to 60%, of the active centers produced in the preceding steps, by means of the addition of compounds containing atoms of acidic hydrogen  - In a fourth step, a mixture consisting of metered amounts of a diene monomer and a vinyl-aromatic monomer is introduced, and the polymerization is continued until the complete or practically complete conversion of these monomers  the mixture of polymers produced from the fourth stage is recovered after prior blocking of the living active centers by means of a compound which contains hydrogen atoms of an acidic nature, so that the mixture consisting of polymers  B1-A1-B2-T-A2 + B1-Au.  9. Process for the preparation of a polymer composition according to claims 1 to 5, characterized in that  - In a first step, dosed quantities of a conjugated diene are polymerized by anionic polymerization giving living polymers, until the complete or practically complete conversion of the monomer  - in a second step, a dosed quantity of a vinyl-aromatic monomer is added to the reaction mass resulting from the first step and polymerized until the complete or practically complete conversion of the vinyl-aromatic monomer introduced  - in a third step, an additional aliquot portion of anionic polymerization initiator is added  - in a fourth step, dosed quantities of a diene and of a vinyl-aromatic monomer, in the state of mixture with one another, and representing from 30 to 60% of the total of the monomers used in the process, are added and polymerization is continued until complete or almost complete conversion of the monomers  the copolymer is recovered from the polymerization products resulting from the fourth step, after prior blocking of the living active centers by means of the addition of a compound which contains hydrogen atoms of an acidic nature, so that the the mixture formed by the two polymers is obtained  Bs-A1-Bz-T-A2 + H2 -T-As.  10. Process for the preparation of a polymer composition according to claims 1 to 5, characterized in that  - In a first step, dosed quantities of a conjugated diene and of a vinyl-aromatic monomer, in mixture with one another, are polymerized by anionic polymerization giving living polymers, until complete or practically conversion full of monomers  - in a second step, an additional aliquot fraction of anionic polymerization initiator giving living polymers is added  - In a third step, a metered amount of a conjugated diene is added, and the polymerization is continued until complete or practically complete conversion of this monomer  - In a fourth step, a metered amount of a vinyl-aromatic monomer is introduced, and the polymerization is continued until complete conversion of this monomer  - the mixture of polymers produced and resulting from the fourth stage is recovered, after prior blocking of all living active centers by means of a compound containing hydrogen atoms of an acidic nature, so that a mixture consisting of by the two polymers  Bl-T-A1-B2-Aw + H2 -As.  11. Method according to claims 7 to 10, characterized in that the polymerization is carried out in an organic, aliphatic or cycloaliphatic solvent, at temperatures in the range from 30 to 150 ° C and under a pressure equal to or greater than atmospheric pressure, in the presence of an alkyl metal or an aryl metal.  12. Method according to claim 11, characterized in that the polymerization can be carried out in the presence of at least one linear or cyclic polar compound, chosen from ethers and amines, present in amounts ranging from 0.01 to 0.1 parts by weight relative to the solvent.  13. Method according to claims 7 to 10, characterized in that the solvent is cyclohexane and in that the reaction temperature is in the range from 50 to 100 ° C, the initiator is an alkyl lithium comprising from 3 to 7 carbon atoms in the alkyl group, said initiator being used in proportions ranging from 0.025 to 0.20 parts by weight per 100 parts by weight of vinyl aromatic monomer.  14. Method according to claims 7 to 10, characterized in that the diene is butadiene and the vinyl-aromatic compound is styrene.