CA2005285A1

CA2005285A1 - Polymeric compositions and method for preparing them

Info

Publication number: CA2005285A1
Application number: CA002005285A
Authority: CA
Inventors: Sergio Custro; Alessandro Zazzetta
Original assignee: Enichem Elastomeri SpA
Current assignee: Enichem Elastomeri SpA
Priority date: 1988-12-23
Filing date: 1989-12-12
Publication date: 1990-06-23
Also published as: NO895089D0; BR8906764A; IT1227901B; DK623289A; MX169895B; GB2226322A; NL194674C; NL194674B; CN1158866A; DE3942668A1; CN1043512A; NL8903136A; GB8927958D0; CN1036139C; DK623289D0; BE1003108A3; NO177599B; FR2640979A1; SE8904152L; ES2021927A6

Abstract

1.

"POLYMERIC COMPOSITIONS AND METHOD FOR PREPARING THEM"
Abstract New polymeric compositions comprising:
(a) a linear copolymer constituted by four alternating blocks, which can be represented by the formula (I) or (II) B1-T-A1-B2-A2 (I) or B1 A1-B2-T-A2 (II) wherein:
B1 and B2 are polydienic blocks, preferably polybutadienic blocks;
A1 and A2 are polyvinylaromatic blocks and preferably polystyrenic blocks;
T is a portion of random copolymer formed by dienic and vinylaromatic monomer units;
(b) a linear copolymer constituted by two blocks, which can be represented by the formulae (III) or (IV) B3-T-A3 (III) or B3-A3 (IV) wherein:
B3 is a polydienic block;
A3 is a polyvinylaromatic block;

2.
and T has the above seen meaning.
The polymeric composition, which can be blended with polystyrene, can be obtained both by blending the individual components, and by means of novel and original synthesis processes which make it possible both (A) and (B) components to be obtained simultaneously.
The polymeric compositions according to the instant invention are suitable for all of those uses in which characteristics of impact strength, transparence and processability are required.

Description

~9~35~r~3 1 .

'POLYMERIC COMPOSITIONS AND METHOD FOR PREPARING THEM' The present invention relates to polymeric compositions comprising:
a linear copolymer constituted by four alternating blocks which can be represented by the formula (I) or tlI):
Bl-T-A1-B2-A2 (I) or Bl-A~-B2-T-A2 (II) wherein A1, A2 , Bl, 82 and T respectively are 2 vinylaromatic blocks, 2 dienic bLocks and a portion of random copolymer formed by dienic and vinylaromatic monomer units;
and a copolymer constituted by two blocks, which can be represented by the formulae ~III) or (IV) B3-T-A3 tIII) or B3~A3 (IV) wherein:
A3 is a polyvinylaromatic block different from, or equal to, Al block or A2 block; B3 is a polydienic block different from, or equal to, Bl block or Bz block, and T
represents the hereinabove seen randorn copolymer~
Also the processes for obtaining both oF the above said components of said blend simultaneously and during the same process of synthesis fall within the scope of the present invention.
The polymeric compositions are endowed w;th very good characteristics of impact strenyth and processability, combined with a hiyh transparence, which make them suitable for all of the uses provided for the 20()5285 transparent, impact resistant materials.
During the past years~ the block copolymers obtained from the block-copolymerization of conjugated dienic monomers with vinylaromatic monomers, such as, e.g., the block copolymers of polybutadiene and polystyrene, and the block copolymers of polyisoprene with polystyrene, have shown a particular development. Such copolymers can be used as such, i.e., in the same form as they are produced by the living anionic copo1ymerization, as well as after their partial or total hydrogenation.
This is the case of the linear block copolymers const;tuted by alternating blocks of polydienes and polyvinylarenes, with a particular structure and distribution of the ;ndiv;dual blocks, and having a structural formula of (I) or (II) type, which show an extremely favourable balance of physical and mechanical characteristics. Such block copolymers, as well as their derivatives, are disclosed by the present Applicant in the Canadian Patent Application N.551.131 and in a co-pending patent application.
The blending of block copolymers both with oneanother, and with impact-resistant and non-impact-resistant polystyrene in order to achieve resins endowed with improved properties, is known from the prior art as well.
Compositions and/or block copolymers characterized by the use of polymeric structures suitable for all of those uses for which transparent impact-resistant polystyrene has been used in the past, or is still being used at present, are reported, e.g., in BP patent No.
1,130,770; BP patent No. 1,335,077; and U.S. patent No.

201~5X~35 3.

4,086,298.
However, the copolymers claimed in these references do not show an optimum balance of rheoLogic properties, impact strength and transparency.
Such a balance of properties is further modified by the possible use of polystyrene which, if, on the one hand, enhances the optical characteristics, on the other hand decreases the impact strength of the so-obta;ned compositions.
The drawbacks which affect the prior art are overcome by the polymeric compositions according to the instant invention, constituted by blends of the two block copolymers respectively complying with the general formulae tI) or (II) and (III) or (IV), as hereinabove seen.
In accordance therew;th, and according to a first aspect thereof, the present invention relates to new polymeric compositions comprising:
(a) from 40 to 90% by weight of a block copolymer of general formula (I) or (II);
(b) from 10 to 60~ by weight of a block copolymer of general formula (III) or (IV), wherein the symbols Al, Az, A3, Bl, Bz, B3 and T have the hereinabove seen meaning.
The polymeric compositions according to the present invention are characterized in that the weight average molecular weight of the four-block copolymer of formula (I) or (II) is comprised within the range of from 50,000 to 300,000, whilst the weight aver3ge molecular weight of the two-block copolymer is comprised within the range of from 20,000 to 150,000.

zoos~as In the same compositions, the percentage by weight of the random block T is comprised within the range of from 5 to 50%, preferably of from 10 to 25%, as referred to the total of the two copolymers which constitute the blend.
In the same compositions, the total amount of vinylaromatic monomeric units is comprised w;thin the range of from 60 to 90~ by weight, with the balance to 100% being constituted by the dienic monomeric units.
In the above definitions, the blocks Al, Az, A3 and Bl, Bz and B3 are practically pure blocks; i.e., they are nearly totally constituted by vinylarenic and dienic units.
In the preferred form of practical embodiment, the blocks Bl, Bz and B3 are polystyrenic blocks and T is a random styrene-butadiene copolymer.
Still in the preferred form of practicaL embodiment, the total content of dienic units in the polymeric composition is comprised within the range of from 20 to 30U/o by weight, and the weight average molecular weight of the 4-block copolymer is comprised within the range of from 100,00û to 200,000; and the weight average molecular weight of the 2-block copolymer is comprised within the range of from 40,000 to 100,000. To said compositions, another polystyrenic resin, such as, e.g., polystyrene or impact-resistant polystyrene, can be added in such an amount as not to endanger the combination of properties of the binary composition.
Polymeric compositions according to the present invention constitute a progress from the viewpoint of the properties, as compared to the block copolymer of formula 5.

(I) or (II), when considered on an individual basis.
The linear block copolymer constituted by 4 alternating blocks complying with the general formula tI) or (II) according to the present invention can be obtained by polymerization, by operating in an organic solvent to ~hich suitable amounts are optionally added of an aliphatic or cycloal;phatic polar compound selected from among ethers or amines, at temperatures comprised within the range of from 300C to 1500C, and under pressures equal to, or higher than, atmospheric pressure, in the presence of said alkyl-metaL or aryl-metaL
initiators, as customarily used in the synthesis of polymers according to the living anionic polymerization method.
Such a synthesis route is disclosed in the Canadian Patent Applicatiorl N . 551.131 relating to the disclosure of such copolymers and to the method for synthetizing them. The two-block copolymers complying with the above formulae (III) or (IV) can be synthetized, on the contrary, according to methods as known from the prior art, as well as by means of the same process of synthesis as disclosed in the above said patent application, with the reaction being limited to the first step. More particularly, for the synthesis of the copolymers of formula '.(III), metered amounts of butadiene and styrene are fed to~a reactor as a mixture with one another, and the polymerization is carried out in a solution in a hydrocarbon, with a suitable initiator for the living anionic polymerization, until monomer conversion is complete or substantially complete; in this way, a living two-block copolymer ~005X~35 6.

is formed, which is constituted by non-pure blocks, i.e., by blocks linked to each other by a copolymer;c portion of chain, constituted by randomly linked monomeric units of butadiene and styrene.
The polymer formed is recovered after the preliminary quenching of the living active centres, by feeding a compound having an acidic character.
The recovery of the polymeric material is carried out according to the customary methods, such as, e.g., by steam evaporating the solvent and drying the polymer.
The copoLymer of formula tIV) is obtained by anionic polymerization, with the sequential addition of the monomers according to the prior art.
The constituents of the polymeric compositions according to the instant invention can be blended and compounded according to the modalit;es and routes as weLl-known from the prior art. So, e.g., the ingredients can be blended in the molten state and extruded or mixed in Banbury type mixers, or they can be blended as solutions. The present Applicant has also developed --and they constitute a further purpose of the present invention -- novel and original processes which make it possible the 2 components to be produced s;multaneously and by means of one single synthesis process.
According to one of such processes, the synthesis is carried out according to the following steps:
(1) a first step during which a mixture is copolymerized in the presence of initiators, until the conversion of the monomers is nearly complete, which mixture is composed by a vinylaromatic monomer and a coniugated ~005285 7.

dienic monomer in an apolar solvent selected from among the solvents in which poLystyrene having a weight average molecular weight comprised within the range of from 30,000 to 120,000 is soluble at S concentrations of from 5 to 20% by weight. The initiators are alkyl-lithium compounds in order to originate a living polymeric system. The solvent system can contain polar compounds (such as ethers, amines, and so forth), at a maximum concentration of 0.1% by weight relatively to the solvent.
The percentage by weight of the monomers fed in mixture to the first reaction step accord;ng to the process of the present invention, is compr;sed within the range of from 30 to 60% by weight, relatively to all of the monomers fed to the reaction. During the above d;sclosed react;on, a l;v;ng copolymer of Bl-T-A
type is formed.
t2) A second step, ;n which a percentage of the living active centres generated during the first reaction step are quenched by means of the addition to the reaction system of compounds characterized in that they contain acidic hydrogen atoms (H20, alcohols, and so forth ) in their chemical structure.
The amount of active centre quenchers added to the second step of such a process is comprised within the range of from lO to 50~ by mol, relative~y to the moles of initiator fed to the first step.
(3) A third step, in which a conjugated diene is fed to the living system coming from the second step and undergoes a complete conversion.

t4) A fourth step in which a vinylaromatic monomer is fed and is nearly totally converted ~5) A fifth step in wh;ch the living active centres are totally quenched by feeding a compound with an acidic S reactivity.
In such a way, the blend of the two copolymers B1-T-Al-B2-A2 ~ B1-T-A
is obtained.
This process is carried out under the above indicated general conditions of temperature and pressure, i.e., at temperature values comprised within the range of from 300c to 1500C and under atmospheric, or superatmospher;c pressure.
One of the variants constituting alternative routes for the above said process compr;ses, at the end of the first step of polymerization of the mixture of monomers, a second step in which a second aliquot of initiator is fed.
Such a route leads to the formation of an additional number of active centres, on which pure block copolymers, previously described by means of the formula are formed.
In that case, the various sequential steps are:
(1) Synthesis of a pure dienic block;

(2) Synthesis of a pure vinylaromatic block in sequence to the dienic block;

(3) Feed of a further aliquot of initiator;

(4) Feed of a mixture constituted by a dienic monomer and a vinylaromatic monomer. The amount of monomers fed in mixture with each other is comprised within the 9.

range of from 30 to 60% relatively to all of the monomers fed to the reaction.

(5) Quench;ng of the l;v;ng act;ve centres by means of a compounds which contains acidic hydrogen atoms.
By means of processes analogous to those as hereinabove disclosed, two polymer blends can be respectively obtained:
Bl-Al-B2-T-A2 + B1-A
or Bl-T-A1-B2-Az ~ Bz-A2.
The initiators preferred for the intended purpose are those belong;ng to the group consisting of both linear and branched alkyl-lithium compounds, and, among them, n-butyl-lithium and sec.-butyl-lithium.
These initiators are customar;ly used in the various steps of the process in amounts comprised within the range of from 0.025 to 0.2 parts by weight per each 100 parts of monomer submitted to polymer;zation.
The solvents su;table for the ;ntended purpose are the apolar solvents and, among them, those ;n which the polyvinylaromatic blocks with a weight average molecular weight comprised with;n the range of from 50,000 to 200,000 are soluble; among the most suitable of them cyclohexane and benzene can be c;ted.
Said solvents can contain polar compounds (such as ethers, am;nes, and so forth), whose presence also causes, bes;des an ;ncrease in the speed of reaction of polymerization which takes in the various steps, an increase in the weight of the random copolymeric portion T; among them, tetrahydrofuran is preferred.
The following experimental examples are illustrative Z0~)5~85 1 0 .

and non limitative of the purview of the present invention.
Exam el _1 600 9 of anhydrous cyclohexane, 55 9 of (99.9~ pure) styrene and 13 9 of (99.~5% pure) butadiene are fed to a stirred reactor of 1000 cm3; the temperature of the mass is increased up to 500C, and 0.055 9 of sec.-butyl-lithium (in solution in n~hexane) is fed.
25 minutes later, the react;on mass reaches the temperature of 75OC and the conversion of the monomers is practically complete. Then, to the system 0.009 9 of pure methanol and, in sequence, 7 9 of butadiene are added.
After 10 minutes, the reaction mass reaches the temperature of 85OC and the conversion of butadiene is practically complete.
Finally, 25 9 of styrene is added, and 15 minutes later the conversion is practically complete, with the temperature of the mass having reached the value of 900C.
The quenching of the l;ving active centres is carried out by adding 0.5 cc of H20 to the polymeric solution.
1.0 9 of triphenyl-nonyl-phosphite and 0.2 9 of Cpentaerythrityl-tetraalkyl-(3,5-di-tert.-butyl-4-hydroxyphenyl-propionate)~ are added to the polymeric solution.
The recovery of the polymeric blend is carried out by means of the steam distillation of the reaction solvent and subsequent drying in a vacuum oven at 60OC
for 2L hours.
The physical characteristics of the two components of the polymeric blend are reported in Table No. 1. The ~g~5;~
1 1 .

mechanical and optical propert;es of specimens compression-moulded at 180C are summarized in Table No.
2.
I - -b 1- - N o Mw x 10-3 Mw x 10-3 Total Block MFI (4) Example (BlTA B2A2) (BlTAl) styrene styrene (gl10 No. ~ (1) % (2) % (3) _ minutes) 1 140 80 80 684.5 Ana1Y_1_ GPC (1) I.R. analysis (2) Demolition with OSO4 (3) 2000C, 5 kg (4) Table_No _2 - Transparency % 92 - Tensile strength kg/cmZ 23û
- Elongation at break 'X 35 - Modulus kg/cm~ ~000 - IZOD with notch at 23OC kg x cm/cm 2.5 E__mpl_ 2 850 9 of anhydrous cyclohexane, 37.5 9 of styrene and 12~5 9 of butadiene are fed to a stirred reactor of 2000 cm3; l:he temperature of the system is increased up to 55OC, and 0.040 9 of n-butyL-lith;um (in solution in n-hexane) is fed.
minutes later, the reaction mass reaches -the temperature of 68OC and the conversion of ~he monomers is practically complete.
Then, to the reaction solution 0.032 ~ of n-butyl-lithium and, in sequence, 37.5 9 of butadiene are added;after 20 minutes, the reaction mass reaches the zaos~s~

temperature of 780C and the conversion is practically complete.
112.5 9 of styrene is then added to the polymeric solution and, 60 minutes later, the conversion of the monomer is complete.
The reaction is then quenched by adding 2 g of methanol to the solution containing the polymeric blend.
The add;tion of the antioxidant, and the recovery and drying of the polymer are carried out ;n the same way as in Example 1.
The physical characteristics of the two components of the polymeric blend are reported in Table No. 3.
The mechanical and optical properties of specimens compress;on-moulded at 180C are summarized in Table No.
154~
T_ble_NQ _3 Total Block MFI
Example Mw x 10-3 Mw x 10-3 styrene styrene ~g/10 NQ ____ tBlTA1B2Az) (B2~)____ ___%__- ___.%___ m1_ut__) 2û 2 170 85 75 653.5 T3ble_NQ _~
- Transparency % 86 - Tensile strength kg/cm2 165 - Elongation at break ~ 100 - Modulus kg/cm2 7,000 - IZOD with notch at 230C kg x cm/cm 3.2 E_3m~ol__3 1.2 kg of cyclohexane, 0.3 9 of tetr3hydrofuran, 100 g of styrene and 45 9 of butadiene are fed to a reactor of 1.5 litres of capacity; the temperature of the mass is increased up to 500C, and 0.12 g of n-butyl-13.

lithium (in solution in n-hexane) is fed.
30 minutes later, the re3ct;0n mass reaches the temperature of 80C and the conversion of the monomers is practicalLy comple-te.
Then, 0.012 9 of H20 and, in sequence, 15 9 of butadiene are added to the system.
10 minutes later, the reaction mass reaches the temperature of 90OC and the conversion of butadiene is complete.
1û Finally, 40 9 of styrene is added and after 15 minutes of reaction the conversion is complete.
Before recovering the solid polymer, the Living active centres are quenched by adding 2 9 of isopropyl alcohol to $he system.
After the addition of antioxidant as in Example 1, the recovery of the solid polymer is carried out by steam distillation of the solvent, and subsequent drying of the solid residue at 65C for 24 hours.

The physical-chemical characteristics are reported in Table 5.
Table Nc~ 5 Block MFI
Example styrene styrene Mw x 10-3 Mw x 10-3 lg/10 No ____ ___~___ ___%___ (~TAlB2A2) (BlTA2)____ minutes) 3 70 50 130 80 8.5 ___mole 4 5 kg of the blend disclosed in Ex3mple 3 is mixed with 5 kg of commercial crystal polystyrer,e cMwrGpc) 250 x 103]. Said mass is fed to a twin-screw extruder equipped hlith a heated jacket. This operatior is repeated ~ r~ 3 1 ~, .

twice in order to enable the optimal mixing to be obtained; the material is then transformed into granules of 0.5 cm of lensth.
The properties of the so prepared compound are determined on specimens compression-moulded at 180C and are reported in Table No. 6.
Table No 6 - MfI 9/10 minutes 8.2 - Transparency % 90 - Tensile strength kg/cm2 185 - Elongation at break % 100 - Modulus kg/cm212,000 - IZOD with notch kg x cm/cm 3.5 Exam,ol_ 5 60û 9 of anhydrous cyclohexane and 30 9 of t99.84%
pure) butadiene are fed to a reactor of 1 litre of capacity; the mass is heated up to 600C, and 0.09 9 of sec.-butyl-lithium is added.
20 minules later, the polymerization of butadiene is complete and the reaction temperature is of about 600C.
70 9 of (99.9~ pure) styrene is therl added to the system~ and the reaction is complete within a time of 20 minutes; during said time period, the reaction mass reaches the temperature of 820C.
To the reaction mass 0.058 9 of initiator (n-butyl-lithium) and, in sequence, a reaction mixture composed by 70 9 of styrene and 30 9 of butadiene are added~
The reaction takes place within a ZS-minutes time and the end reaction temperature is of 1020C.

Z00~5 At the end of the process, 3 9 of methyl alcohol is added to the system in order to quench the active centres.
To the polymeric solution 0.8 9 of TNPP (triphenyl-nonyl-phosph;te) and 0,15 9 of pentaerythrityl-tetraalkyl-t3,5-di-tert.-butyl-4-hydroxyphenyl)-propionate are added to the polymeric solution.
The polymer is recovered by means of the steam distillation of the solvent, and the recovered polymer is subsequently dried in a vacuum-oven at 60OC for 36 hours.
The physical-chem;cal characteristics and the mechanical characteristics of the polymeric blend are reported in Tables No. 7 and 8.
T3bl__No _7 Total Block MFI
Example styrene styrene Mw x 10-3 Mw x 10-3 (9/10 No ____ ___~/~___ ___Z___ tE;lAl~TA2) (B1TA2)____ mi_ut_s) _3ble_No _8 - Transparency % 85 - Elongation at break % 125 - Tensile strength kg/cm2 1.10 - Modulus kg/cm2 6,800 - IZOD with notch at 230C kg x cm/cm 5.0

Claims

1. Polymeric composition comprising:
(a) from 40 to 90% by weight of a linear copolymer of four alternating blocks, which can be represented by the formula B1-T-A1-B2-A2 (I) or B1-A1-B2-T-A2 (II) wherein:
A1 and A2 are polyvinylaromatic blocks;
B1 and B2 are polydienic blocks;
T is a copolymeric chain portion formed by randomly linked dienic and vinylaromatic monomer units;
(b) from 10 to 60 parts by weight of a linear copolymer of two blocks, which can be represented either by the formula:
B3-T-A3 (III) wherein:
A3 is a polyvinylaromatic block either equal to, or different from, the Al block or the A2 block;
B3 is a polydienic block either either to, or different from, to B1 block or to B2 block;
T has the above seen meaning;
or, alternatively, by the formula:
B3-A3 (IV)

2. Polymeric composition according to claim 1, characterized in that the linear 4-block polymer (I) or (II) has a weight average molecular weight comprised within the range of from 50,000 to 300,000.

17.

3. Polymeric composition according to claims 1 and 2, characterized in that the average molecular weight of the polymers identified by the formulae (III) or (IV) is comprised within the range of from 20,000 to 150,000.

4. Polymeric composition according to the preceding claims, characterized in that in said composition the total amount of vinylaromatic monomeric units is comprised within the range of from 60 to 90% by weight, with the balance to 100% being constituted by dienic monomeric units.

5. Polymeric composition according to the preceding claims, characterized in that the dienic blocks are polybutadiene blocks and the polyvinylaromatic blocks are polystyrenic blocks.

6. Polymeric composition according to the preceding claims, characterized in that the individual components are blended in order to constitute the same composition

7. Process for preparing a polymeric composition according to the preceding claims, characterized in that:
- in a first step metered amounts of a diene and of a vinylaromatic monomer are polymerized in mixture with each other by living anionic polymerization, up to complete, or nearly complete, monomer conversion;
- in a second step a percentage comprised within the range of from 10 to 60% of the active living centers produced by the first reaction step are partially quenched by means of the addition of compounds containing acidic hydrogen atoms;
- a metered amount of diene is added to the mass coming from the second step, and is polymerized in a third step by means of the living anionic polymerization 18.

route until the conversion of fed diene is complete or substantially complete;
- a metered amount of vinylaromatic monomer is added to the product coming from the third step and is polymerized, in a fourth step, until the conversion of - the vinylaromatic monomer fed is complete or substantially complete;
the blend of polymers is recovered from the polymerization products from the fourth step, after the preliminary complete quenching of the living active centres by means of a compound which contains acidic hydrogen atoms, in that way the polymeric blend constituted by the two polymers B1-T-A1-B2-A2 + B1-T-A1 being obtained.

8. Process for preparing a polymeric composition according to claims from 1 to 5, characterized in that:
- in a first step metered amounts of a conjugated diene are polymerized by living anionic polymerization, up to complete, or nearly complete, monomer conversion;
- in a second step, a metered amount of a vinylaromatic monomer is added to the reaction mass coming from the first step and is polymerized until the conversion of the vinylaromatic monomer fed is complete or substantially complete;
- in a third step, the partial quenching is carried out of a percentage comprised within the range of from 10 to 60% of the active centers produced in the previous steps by means of the addition of compounds containing acidic hydrogen atoms;
- in a fourth step a mixture constituted by metered 19.

amounts of a diene and of a vinylaromatic monomer is fed, and the polymerization is continued until the conversion of said monomers is complete, or nearly complete;
the produced blend of polymer coming from the fourth step is recovered after the preliminary quenching of - the living active centres by means of a compound which contains acidic hydrogen atoms, in that way the polymeric blend constituted by the polymers B1-A1-B2-T-A2 + B1-A1 being obtained.

9. Process for preparing a polymeric composition according to claims from 1 to 5, characterized in that:
- in a first step metered amounts of a conjugated diene are polymerized by living anionic polymerization, up to complete, or nearly complete, monomer conversion;
- in a second step, a metered amount of a vinylaromatic monomer is added to the reaction mass coming from the first step and is polymerized until the conversion of the vinylaromatic monomer fed is complete or substantially complete;
- in a third step, a further aliquot of initiator of anionic polymerization is added;
- in a fourth step metered amounts of a diene and of a vinylaromatic monomer, in mixture with each other, and in amounts comprised within the range of from 30 to 60%
relatively to the monomers fed to the process are added and the polymerization is continued until the complete, or nearly complete, conversion of the monomers is obtained;
- the copolymer is recovered from among the 20.

polymerization products coming from the fourth step, after the preliminary quenching of the living active centres by means of the addition of a compound which contains acidic hydrogen atoms, in that way the polymeric blend constituted by the two polymers B1-A1-Bz-T-A2 + B2-T-A2 being obtained.

10. Process for preparing a polymeric composition according to claims from 1 to 5, characterized in that:
- in a first step metered amounts of a conjugated diene and of a vinylaromatic monomer, in mixture with each other, are polymerized by living anionic polymerization, up to complete, or nearly complete, monomer conversion;
- in a second step, a further aliquot of initiator of living anionic polymerization is added;
- in a third step, a metered amount of a conjugated diene is added and the polymerization is continued until the conversion of the monomer is complete or substantially complete;
- in a fourth step a metered amount of a vinylaromatic monomer is fed and the polymerization is continued until the complete conversion of the monomer is obtained;
- the mixture of polymers produced and coming from the fourth step is recovered after the preliminary complete quenching of the living active centres by means of a compound which contains acidic hydrogen atoms, in that way a polymeric blend constituted by the two polymers B1-T-A1-B2-A2 + B2-A2 being obtained.

21.

11. Process according to claims 7, 8, 9, 10, characterized in that the polymerization is carried out by operating in an organic, aliphatic or cycloaliphatic solvent at temperatures comprised within the range of from 30 to 150°C and under a pressure equal to, or higher than, atmospheric pressure, in the presence of an alkyl-metal or of an aryl-metal.

12. Process 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 selected from among ethers or amines, in amounts comprised within the range of from 0.01 to 0.1 parts by weight, relatively to the solvent.

13. Process according to claims 7, 8, 9, 10, characterized in that the solvent is cyclohexane and the reaction temperature is comprised within the range of from 50 to 100°C, the initiator is an alkyl-lithium containing from 3 to 7 carbon atoms in the alkyl group, and said initiator is used in amounts comprised within the range of from 0.025 to 0.20 parts by weight per each 100 parts by weight of vinyl-aromatic monomers.

14. Process according to claims 7, 8, 9, 10, characterized in that the diene is butadiene and the vinylaromatic compound is styrene.