Thermoplastic elastomeric block copolymers and process for the preparation thereof

This invention relates to a thermoplastic elastomeric block copolymer and process for the preparation thereof. According to this invention, the block copolymer obtained through block copolymerization process at higher temperature and adding butyllithium in twice during copolymerization are mixed block copolymer of A.sub.1 -B-A.sub.2 /B-A.sub.2 and B-A.sub.2 /B-A.sub.2 (where A represents monovinyl aromatic monomer; B represents conjugated diene monomer) having different blocks, higher elongation and appropriate melt index. The process of this invention is simple and efficient. The thermoplastic elastomeric block copolymer of this invention are particularly useful for various applications of typical styrene butadiene block copolymer, especially plastic modifier.

FIELD OF THE INVENTION 
The present invention relates to a thermoplastic elastomeric block 
copolymer of C.sub.4 -C.sub.6 conjugated diene/monovinyl aromatic monomer 
having two kinds of structures, the properties of which, such as 
elongation, tensile strength at break, melt index and hardness can be 
conveniently regulated according to the application requirements so that 
it can be suitable for use in various applications of conventional styrene 
butadiene block copolymers, and to a process for the preparation thereof. 
BACKGROUND OF THE INVENTION 
The tri-block copolymer having poly-monovinyl aromatic monomer (typically 
styrene) block, poly-conjugated diene(typically butadiene) block and 
poly-monovinyl aromatic monomer block (abbreviated as A-B-A) is a kind of 
thermoplastic elastomer and has been widely used. The known processes for 
the preparation thereof include: two-step process which comprises first 
charging the initiator and styrene, and then charging styrene and 
butadiene after completion of the first polymerization step; coupling 
process which comprises first charging the initiator and styrene, charging 
butadiene after completion of the first polymerization step, and then 
charging a coupling agent after completion of the second polymerization 
step; or three-step process which comprises first charging the initiator 
and styrene, charging butadiene after completion of the first 
polymerization step, and then charging styrene after completion of the 
second polymerization step. 
The results of application show the A-B-A copolymers still have some 
drawbacks in their properties, and therefore many attempts have been made 
to improve their properties by regulating their structures. For example, 
U.S. Pat. No. 4,104,330 disclosed a tetra-block copolymer having monovinyl 
aromatic monomer homopolymer block, conjugated diene homopolymer block, 
tapered monovinyl aromatic monomer/conjugated diene copolymer block and 
monovinyl aromatic monomer homopolymer block; U.S. Pat. No. 4,600,749 
disclosed a multi-block copolymer having at least four blocks, that is 
tapered monovinyl aromatic monomer/conjugated diene copolymer block, 
monovinyl aromatic monomer homopolymer block, conjugated diene homopolymer 
block and monovinyl aromatic monomer homopolymer block. These copolymers 
are characterized in that in their molecular chains, there is further 
incorporated tapered monovinyl aromatic monomer/conjugated diene copolymer 
block in addition to monovinyl aromatic monomer homopolymer block, 
conjugated diene homopolymer block and monovinyl aromatic monomer 
homopolymer block, thereby forming multi-block copolymers having more than 
four blocks. As thermoplastic elastomer, many properties of these 
tetra-block or multi-block copolymers have been improved, but some 
properties are still not sufficiently desirable, more particularly, when 
the content of monovinyl aromatic monomer is higher, the elongation is 
lower, the hardness tends to be larger or the melt index and the strength 
are inappropriate, and therefore fail to meet the requirements of some 
application fields. Moreover, the procedures for preparing these block 
copolymers are complicated, and the polymerization lasts too long, which 
leads to a low production efficiency. 
DISCLOSURE OF THE INVENTION 
It is an object of the present invention to provide a thermoplastic 
elastomeric block copolymer comprising of C.sub.4 -C.sub.6 conjugated 
diene/monovinyl aromatic monomer having two kinds of structures, the 
properties of which can be conveniently regulated according to the 
application requirements, wherein the constituents of said composition can 
be controlled by conveniently controlling the process conditions for the 
preparation thereof, and thus their properties can be controlled, which 
renders them suitable for use in various applications, for example plastic 
modifier, asphalt modifier, materials for shoe manufacturing and other 
non-tyre industrial articles of universal rubber as well as adhesives etc. 
Another object of the present invention is to provide a thermoplastic 
elastomeric block copolymer C.sub.4 -C.sub.6 conjugated diene/monovinyl 
aromatic monomer block copolymers, which has high elongation and 
appropriate melt index, and is especially suitable for use as plastic 
modifier. 
A further object of the present invention is to provide a convenient 
process for the preparation of the thermoplastic elastomeric block 
copolymer mentioned above, wherein the constituents of the resultant 
copolymer can be controlled by altering the process conditions, thereby 
controlling its properties, said process can be operated in a convenient 
manner, and the polymerization time can be greatly shortened, which leads 
to a high production efficiency. When containing a copolymer of formula 
B-A.sub.2 /B-A.sub.2 (II) having only one monovinyl aromatic monomer 
homopolymer block, the properties of a tetra-block copolymer of formula 
A.sub.1 -B-A.sub.2 /B-A.sub.2 (I) having tapered monovinyl aromatic 
monomer/conjugated diene copolymer block, such as elongation and melt 
index etc. can be advantageously improved; and that by controlling the 
technical conditions for the preparation, the ratio of (I) to (II) can be 
regulated, thus the properties of the resultant copolymer. 
The present inventors have found that such as elongation, tensile strength 
at break and melt index etc. can be regulated within a wide range, which 
renders it suitable for use in various applications. Moreover, the ratio 
of A.sub.1 to A.sub.2 has also larger influence on elongation and melt 
index etc., and therefore the properties can be further controlled by 
appropriately controlling and regulating the ratio of A.sub.1 to A.sub.2. 
The present inventors have further found that by appropriately increasing 
and controlling the polymerization temperature, the random 
copolymerization of monovinyl aromatic monomer and C.sub.4 -C.sub.6 
conjugated diene can be accelerated and controlled, and thus the 
properties, for example elongation etc., of the resultant copolymer can be 
controlled; moreover, by increasing the polymerization temperature, the 
polymerization rate can be increased, thereby the polymerization time can 
be shortened, which leads to an improved production efficiency. 
Therefore, the above-mentioned objects of the present invention can be 
realized by the following thermoplastic elastomeric block copolymer and 
the process for the preparation thereof. 
In an aspect of the present invention, there is provided a thermoplastic 
elastomeric block copolymer having the following two structures. 
A.sub.1 B-A.sub.2 /B-A.sub.2 (I) 
B-A.sub.2 /B-A.sub.2 (II) 
wherein A.sub.1 represents monovinyl aromatic monomer homopolymer block 
formed by the first monovinyl aromatic monomer charge, A.sub.2 represents 
monovinyl aromatic monomer homopolymer block formed by the second 
monovinyl aromatic monomer charge, B represents C.sub.4 -C.sub.6 
conjugated diene homopolymer block, A.sub.2 /B represents random copolymer 
block formed by the second monovinyl aromatic monomer charge and C.sub.4 
-C.sub.6 conjugated diene. 
In another aspect of the present invention, there is provided a process for 
the preparation of the thermoplastic elastomeric block copolymer mentioned 
above, comprising the steps of: 
(1) polymerizing a first monovinyl aromatic monomer charge(A.sub.1) under 
anionic polymerization conditions and in the presence of a first 
organolithium compound initiator charge(Li1), and allowing essentially 
complete polymerization of the monomers to occur; thereafter 
(2) to the reaction mixture obtained in step (1), charging C.sub.4 -C.sub.6 
conjugated diene (B), a second monovinyl aromatic monomer charge(A.sub.2) 
and a second organolithium compound initiator charge(Li2), and allowing 
essentially complete polymerization of the monomers to occur at higher 
temperature. 
The present invention will be described in more detail in the following. 
The C.sub.4 -C.sub.6 conjugated diene suitable for use in the present 
invention is conjugated diene having 4-6 carbon atoms, including, for 
example 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene and 
1,3-pentadiene, which can be used alone or in combination, with 
1,3-butadiene being preferred. The monovinyl aromatic monomer suitable for 
use in the present invention is vinyl aromatic monomer having one vinyl 
group directly attached to aromatic nucleus, including, for example, 
styrene, para-methyl styrene, ortho-methyl styrene, meta-methyl styrene, 
para-ethyl styrene, ortho-ethyl styrene, meta-ethyl styrene, alpha-vinyl 
naphthalene and beta-vinyl naphthalene, which can be used alone or in 
combination, with styrene being preferred. 
In the present invention, the charging ratio of the first monovinyl 
aromatic monomer charge to the second monovinyl aromatic monomer 
charge(A.sub.1 /A.sub.2) is 1/1.0-1/1.5(by weight), preferably 
1/1.1-1/1.3(by weight); the total charging ratio of monovinyl aromatic 
monomers to C.sub.4 -C.sub.6 conjugated diene(A.sub.total /B.sub.total) is 
10/90-60/40(by weight), preferably 25/75-45/55(by weight). 
In the process according to the present invention, the organolithium 
compound used may be hydrocarbyl lithium having one lithium atom, wherein 
the hydrocarbyl may be alkyl, cycloalkyl, aryl, aralkyl or alkaryl, 
preferably saturated alkyl lithium compound, such as methyl lithium, ethyl 
lithium, propyl lithium, butyl lithium, pentyl lithium, hexyl lithium, 
2-ethylhexyl lithium, hexadecyl lithium and cyclohexyl lithium, especially 
preferably butyl lithium, most preferably n-butyl lithium. 
In the process according to the present invention, the organolithium is 
charged in twice, which results that two block copolymers having different 
structures (I) and (II) are obtained, this is an important technical 
feature of the process according to the present invention. The second 
organolithium initiator charge is charged in the second polymerization 
step, which results in a product containing block copolymer (II). The 
charging amount of organolithium is dependent on desired molecular weight 
of the polymer, and the charging ratio of the two organolithium initiator 
charges is dependent on the desired ratio of the two block copolymer 
having different structures(II) and (I). By conveniently controlling the 
charging ratio of the two organolithium initiator charges, the ratio of 
(I) to (I) can be controlled, thereby the properties of the resultant 
copolymer, such as melt index, elongation, hardness and tensile strength 
at break etc. can be controlled. In general, by gradually increasing the 
charging amount of the second organolithium initiator charge, it is 
possible to obtain a series of products having melt index from low to 
high, gradually increased elongation and gradually decreased tensile 
strength at break. In the thermoplastic elastomeric block copolymer 
according to the present invention, the molecular weight of the two block 
copolymers having different structures is about 10,000 to 500,000, and the 
molar ratio of the two block copolymers having different structures II/I 
is generally 0.01 to 0.30, preferably 0.05 to 0.20. 
In the process according to the present invention, the polymerization 
reaction is carried out in an inert hydrocarbon solvent. Suitable solvents 
include saturated aliphatic hydrocarbon, such as pentane, hexane, heptane, 
octane, nonane and decane; naphthene, such as cyclohexane, methyl 
cyclohexane, ethyl cyclohexane and 1,4-dimethyl cyclohexane; aromatic 
hydrocarbon, such as benzene, toluene, ethyl benzene, xylene, diethyl 
benzene and propyl benzene. These solvents can be used alone or in 
combination. In the process according to the present invention, it is 
unnecessary to add any polar compound usually used in conventional 
processes for preparing styrene butadiene block copolymers into the inert 
hydrocarbon solvents, which leads to a simple procedure for recovering 
solvents. Of course, it is possible to add polar compounds to the process 
according to the present invention, which include, for example, 
tetrahydrofuran, ethylene glycol dimethyl ether, diethylene glycol 
dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol 
dimethyl ether, and tetramethyl ethylenediamine etc. 
In the process according to the present invention, there is no particular 
limitation on the temperature of the first polymerization step as long as 
it is sufficient to allow the anionic polymerization of vinyl aromatic 
monomer to occur. Usually, the anionic polymerization reaction is carried 
out at a temperature of 30.degree. C. to 120.degree. C., but preferably 
60.degree. C.-100.degree. C. from the standpoint of reaction rate, most 
preferably 70.degree. C.-90.degree. C. 
The temperature for carrying out the second polymerization step has a 
significant influence on the structure of the resultant thermoplastic 
elastomeric block copolymer and further affects the properties of the 
resultant products. According to the present invention, the second 
polymerization step is usually carried out at a temperature of 30.degree. 
C.-150.degree. C., preferably 60.degree. C.-150.degree. C., and most 
preferably 80.degree. C. -140.degree. C. In the second polymerization 
step, by increasing and controlling the polymerization temperature, the 
random copolymerization of vinyl aromatic monomer and conjugated diene can 
be controlled, and thus random copolymerized block of vinyl aromatic 
monomer/C.sub.4 -C.sub.6 conjugated diene can be obtained. The higher the 
polymerization temperature, the more favorable for forming longer random 
copolymerized block; and the presence of the random copolymerized block 
leads to the increase in elongation and the gradual increase in melt 
index. By controlling the temperature of the second polymerization step, 
it is possible to obtain a copolymer having different length of random 
copolymerized blocks in its molecular chains, which results that a series 
of products having elongation from low to high and gradually increased 
melt index with the increase of the random copolymerized block can be 
obtained, which renders them suitable for use in various applications. 
After the polymerization reaction is completed, it is terminated in a 
conventional manner, for example by adding a termination agent, examples 
being water, methanol, ethanol or isopropanol etc.

PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION 
The invention is illustrated by the following examples which, however, are 
not to be taken as limiting in any respect. 
In the following examples, the various properties are measured according to 
the following method: 
Tensile strength at break: GB/T-528-92 
Elongation at break: GB/T 528-92 
Shore hardness: GB/T 531-92 
Melt index: GB/T 3682-83 
EXAMPLE 1 
A 5-1 stainless steel polymerization reactor is purged by refined nitrogen 
gas for several times. Under an atmosphere of nitrogen, to the reactor are 
added 100 g of styrene, which has been dried and depleted of impurities, 
2750 ml of cyclohexane, which has been dried and depleted of impurities, 
and 15.7 ml of n-butyl lithium, and the first reaction step is carried out 
at a temperature of 80-90.degree. C. After 2.5 minutes, the first reaction 
is completed; then to the reactor are added 127.5 g of styrene, which has 
been dried and depleted of impurities, 372 g of butadiene, which has been 
dried and depleted of impurities, 500 ml of cyclohexane, which has been 
dried and depleted of impurities, and 1.5 ml of n-butyl lithium, and the 
second reaction step is carried out at a temperature of 100-125.degree. C. 
After 4.5 minutes, the second reaction is completed; and immediately 
adding a termination agent to stop the reaction. The resultant polymeric 
product is subjected to property test, and is found to have elongation of 
992%, Shore hardness of 87, melt index of 4.63 g/10 min., and tensile 
strength at break of 15.30 MPa. 
EXAMPLE 2 
A 5-1 stainless steel polymerization reactor is purged by refined nitrogen 
gas for several times. Under an atmosphere of nitrogen, to the reactor are 
added 105 g of styrene, which has been dried and depleted of impurities, 
2750 ml of cyclohexane, which has been dried and depleted of impurities, 
and 15.7 ml of n-butyl lithium, and the first reaction step is carried out 
at a temperature of 80-90.degree. C. After 3.0 minutes, the first reaction 
is completed; then to the reactor are added 135.0 g of styrene, which has 
been dried and depleted of impurities, 360 g of butadiene, which has been 
dried and depleted of impurities, 500 ml of cyclohexane, which has been 
dried and depleted of impurities, and 1.5 ml of n-butyl lithium, and the 
second reaction step is carried out at a temperature of 100-125.degree. C. 
After 4.5 minutes, the second reaction is completed; and immediately 
adding a termination agent to stop the reaction. The resultant polymeric 
product is subjected to property test, and is found to have elongation of 
1015%, Shore hardness of 88, melt index of 8.25 g/10 min., and tensile 
strength at break of 12.26 MPa. 
EXAMPLE 3 
A 5-1 stainless steel polymerization reactor is purged by refined nitrogen 
gas for several times. Under an atmosphere of nitrogen, to the reactor are 
added 110 g of styrene, which has been dried and depleted of impurities, 
2750 ml of cyclohexane, which has been dried and depleted of impurities, 
and 15.7 ml of n-butyl lithium, and the first reaction step is carried out 
at a temperature of 80-90.degree. C. After 3.0 minutes, the first reaction 
is completed; then to the reactor are added 140.0 g of styrene, which has 
been dried and depleted of impurities, 350 g of butadiene, which has been 
dried and depleted of impurities, 500 ml of cyclohexane, which has been 
dried and depleted of impurities, and 1.5 ml of n-butyl lithium, and the 
second reaction step is carried out at a temperature of 100-125.degree. C. 
After 4.5 minutes, the second reaction is completed; and immediately 
adding a termination agent to stop the reaction. The resultant polymeric 
product is subjected to property test, and is found to have elongation of 
1045%, Shore hardness of 89, melt index of 11.63 g/10 min., and tensile 
strength at break of 17.85 MPa. 
EXAMPLE 4 
A 5-1 stainless steel polymerization reactor is purged by refined nitrogen 
gas for several times. Under an atmosphere of nitrogen, to the reactor are 
added 115 g of styrene, which has been dried and depleted of impurities, 
2750 ml of cyclohexane, which has been dried and depleted of impurities, 
and 15.7 ml of n-butyl lithium, and the first reaction step is carried out 
at a temperature of 80-90.degree. C. After 3.0 minutes, the first reaction 
is completed, then to the reactor are added 135.0 g of styrene, which has 
been dried and depleted of impurities, 350 g of butadiene, which has been 
dried and depleted of impurities, 500 ml of cyclohexane, which has been 
dried and depleted of impurities, and 1.5 ml of n-butyl lithium, and the 
second reaction step is carried out at a temperature of 100-125.degree. C. 
After 4.0 minutes, the second reaction is completed; and immediately 
adding a termination agent to stop the reaction. The resultant polymeric 
product is subjected to property test, and is found to have elongation of 
1000%, Shore hardness of 88, melt index of 25.18 g/10 min., and tensile 
strength at break of 12.16 MPa. 
EXAMPLE 5 
A 5-1 stainless steel polymerization reactor is purged by refined nitrogen 
gas for several times. Under an atmosphere of nitrogen, to the reactor are 
added 127.5 g of styrene, which has been dried and depleted of impurities, 
2750 ml of cyclohexane, which has been dried and depleted of impurities, 
and 15.7 ml of n-butyl lithium, and the first reaction step is carried out 
at a temperature of 80-90.degree. C. After 3.0 minutes, the first reaction 
is completed; then to the reactor are added 160.0 g of styrene, which has 
been dried and depleted of impurities, 312.5 g of butadiene, which has 
been dried and depleted of impurities, 500 ml of cyclohexane, which has 
been dried and depleted of impurities, and 1.5 ml of n-butyl lithium, and 
the second reaction step is carried out at a temperature of 
100-125.degree. C. After 4.5 minutes, the second reaction is completed; 
and immediately adding a termination agent to stop the reaction. The 
resultant polymeric product is subjected to property test, and is found to 
have elongation of 944%, Shore hardness of 90, melt index of 19.35 g/10 
min., and tensile strength at break of 12.16 MPa. 
EXAMPLE 6 
A 5-1 stainless steel polymerization reactor is purged by refined nitrogen 
gas for several times. Under an atmosphere of nitrogen, to the reactor are 
added 125 g of styrene, which has been dried and depleted of impurities, 
2750 ml of cyclohexane, which has been dried and depleted of impurities, 
and 15.7 ml of n-butyl lithium, and the first reaction step is carried out 
at a temperature of 80-90.degree. C. After 3.0 minutes, the first reaction 
is completed; then to the reactor are added 127.5 g of styrene, which has 
been dried and depleted of impurities, 347.5 g of butadiene, which has 
been dried and depleted of impurities, 500 ml of cyclohexane, which has 
been dried and depleted of impurities, and 1.5 ml of n-butyl lithium, and 
the second reaction step is carried out at a temperature of 100-125 
.degree. C. After 4.0 minutes, the second reaction is completed; and 
immediately adding a termination agent to stop the reaction. The resultant 
polymeric product is subjected to property test, and is found to have 
elongation of 895%, Shore hardness of 90, melt index of 8.86 g/10 min., 
and tensile strength at break of 22.75 MPa. 
EXAMPLE 7 
The steps of example 6 are repeated except that the amount of styrene used 
in the first reaction step(S1) is changed into 120 g and the amount of 
styrene used in the second reaction step(S2) is changed into 132.5 g. The 
resultant product has elongation of 920%, Shore hardness of 90, melt index 
of 17.16 g/10 min., and tensile strength at break of 20.01 MPa. 
EXAMPLE 8 
The steps of example 6 are repeated except that the amount of styrene used 
in the first reaction step(S1) is changed into 117.5 g and the amount of 
styrene used in the second reaction step(S2) is changed into 135 g. The 
resultant product has elongation of 1000%, Shore hardness of 88, melt 
index of 25.18 g/10 min., and tensile strength at break of 18.92 MPa. 
EXAMPLE 9 
The steps of example 6 are repeated except that the amount of styrene used 
in the first reaction step(S1) is changed into 115 g and the amount of 
styrene used in the second reaction step(S2) is changed into 137.5 g. The 
resultant product has elongation of 1043%, Shore hardness of 90, melt 
index of 15.6 g/10 min., and tensile strength at break of 17.85 MPa. 
EXAMPLE 10 
The steps of example 6 are repeated except that the amount of styrene used 
in the first reaction step(S1) is changed into 110 g and the amount of 
styrene used in the second reaction step(S2) is changed into 142.5 g. The 
resultant product has elongation of 1100%, Shore hardness of 90, melt 
index of 13.4 g/10 min., and tensile strength at break of 16.70 MPa. 
By comparing the data of examples 6-10, it can be seen that as the ratio of 
the amount of styrene used in the second step to that in the first step, 
S2/S1, increases(from 1.0 to 1.3), the elongation of the resultant 
products is gradually increased(from 895% to 1100%), the tensile strength 
at break is gradually decreased(22.75 MPa to 16.70 MPa), and the melt 
index reaches maximum in example 8. Therefore, it is possible to her 
regulate the properties of the resultant products, such as elongation, 
tensile strength at break and melt index by regulating the ratio S2/S1 
according to the present invention. 
EXAMPLE 11 
A 5-1 stainless steel polymerization reactor is purged by refined nitrogen 
gas for several times. Under an atmosphere of nitrogen, to the reactor are 
added 120 g of styrene, which has been dried and depleted of impurities, 
2750 ml of cyclohexane, which has been dried and depleted of impurities, 
and 15.7 ml of n-butyl lithium, and the first reaction step is carried out 
at a temperature of 80-90.degree. C. After 3.0 minutes, the first reaction 
is completed; then to the reactor are added 132.5 g of styrene, which has 
been dried and depleted of impurities, 347.5 g of butadiene, which has 
been dried and depleted of impurities, 500 ml of cyclohexane, which has 
been dried and depleted of impurities, and 0.5 ml of n-butyl lithium, and 
the second reaction step is carried out at a temperature of 
100-125.degree. C. After 5.0 mninutes, the second reaction is completed; 
and immediately adding a termination agent to stop the reaction. The 
resultant polymeric product is subjected to property test, and is found to 
have elongation of 937%, Shore hardness of 90, melt index of 9.67 g/10 
min., and tensile strength at break of 16.57 MPa. 
EXAMPLE 12 
The steps of example 11 are repeated except that the amount of n-butyl 
lithium used in the second reaction step is changed into 1.5 ml. The 
resultant product has elongation of 940%, Shore hardness of 90, melt index 
of 16.3 g/10 min., and tensile strength at break of 15.49 MPa. 
EXAMPLE 13 
The steps of example 11 are repeated except that the amount of n-butyl 
lithium used in the second reaction step is changed into 2.5 ml. The 
resultant product has elongation of 952%, Shore hardness of 88, melt index 
of 22.2 g/10 min., and tensile strength at break of 13.44 MPa. 
EXAMPLE 14 
The steps of example 11 are repeated except that the amount of n-butyl 
lithium used in the second reaction step is changed into 3.0 ml. The 
resultant product has elongation of 985%, Shore hardness of 88, melt index 
of 18.14 g/10 min., and tensile strength at break of 12.94 MPa. 
EXAMPLE 15 
The steps of example 11 are repeated except that the amount of n-butyl 
lithium used in the second reaction step is changed into 3.5 ml. The 
resultant product has elongation of 1000%, Shore hardness of 88, melt 
index of 20.7 g/10 min., and tensile strength at break of 12.16 MPa. 
EXAMPLE 16 
The steps of example 11 are repeated except that the amount of n-butyl 
lithium used in the second reaction step is changed into 4.5 ml. The 
resultant product has elongation of 1087%, Shore hardness of 87, melt 
index of 19.60 g/10 min., and tensile strength at break of 10.88 MPa. 
By comparing the data of examples 11-16, it can be seen that as the amount 
of n-butyl lithium used in the second step increases, that is the ratio 
(II)/(I) in the copolymer increases(from 0.03 to 0.3), the elongation of 
the resultant products is gradually increased(from 937% to 1087%), the 
tensile strength at break is gradually decreased(16.57 MPa to 10.88 MPa), 
and the melt index reaches maximum in example 13. Therefore, it is 
possible to control the properties of the resultant products, such as 
elongation, tensile strength at break and melt index by simply altering 
the amount of the initiator charged in the second step according to the 
present invention. 
EXAMPLE 17 
A 5-1 stainless steel polymerization reactor is purged by refined nitrogen 
gas for several times. Under an atmosphere of nitrogen, to the reactor are 
added 115 g of styrene, which has been dried and depleted of impurities, 
2750 ml of cyclohexane, which has been dried and depleted of impurities, 
and 13.3 ml of n-butyl lithium, and the first reaction step is carried out 
at a temperature of 78-90.degree. C. After 3.3 minutes, the first reaction 
is completed; then to the reactor are added 137.5 g of styrene, which has 
been dried and depleted of impurities, 347.5 g of butadiene, which has 
been dried and depleted of impurities, 500 ml of cyclohexane, which has 
been dried and depleted of impurities, and 2.0 ml of n-butyl lithium, and 
the second reaction step is carried out at a temperature of 75-100.degree. 
C. After 8.1 minutes, the second reaction is completed; and immediately 
adding a termination agent to stop the reaction. The resultant polymeric 
product is subjected to property test, and is found to have elongation of 
897%, Shore hardness of 89, melt index of 9.5 g/10 min., and tensile 
strength at break of 14.70 MPa. 
EXAMPLE 18 
The steps of example 17 are repeated except that the temperature in the 
second reaction step is changed into 87-120.degree. C. The resultant 
product has elongation of 993%, Shore hardness of 88, melt index of 18.1 
g/10 min., and tensile strength at break of 12.35 MPa. 
EXAMPLE 19 
The steps of example 17 are repeated except that the temperature in the 
second reaction step is changed into 95-127.degree. C. The resultant 
product has elongation of 1064%, Shore hardness of 85, and tensile 
strength at break of 11.90 MPa. 
EXAMPLE 20 
The steps of example 17 are repeated except that the temperature in the 
second reaction step is changed into 100-136.degree. C. The resultant 
product has elongation of 1100%, Shore hardness of 79, melt index of 18.57 
g/10 min., and tensile strength at break of 11.31 MPa. 
By comparing the data of examples 17-20, it can be seen that increasing the 
polymerization temperature in the second step results in an increased 
elongation. 
Industrial Applicability 
According to the present invention, it is possible to conveniently obtain 
thermoplastic elastomers having elongation from low to high, tensile 
strength at break from high to low, and melt index and hardness capable of 
being regulated within a wide range, as required by the application 
fields, which can be used in various application fields of conventional 
styrene butadiene block copolymers. For example, the copolymer according 
to the present invention having high elongation and appropriate melt index 
is suitable for use as asphalt modifier and for the preparation of 
adhesives, especially as plastic modifier; the copolymer according to the 
present invention having high strength is suitable for use in shoe 
manufacturing and other non-tyre articles of universal rubber; and the 
copolymer according to the present invention having a compromise between 
strength and elongation can be used for making eraser rubber, 
thermoplastic colour articles or translucent articles. 
The process according to the present invention is simpler, compared with 
the prior art processes. The constituents of the resultant copolymer can 
be controlled by conveniently altering the process conditions, especially 
the polymerization temperature in the second reaction step and the 
charging ratio of the organolithium initiators added in twice, as well as 
the charging ratio of the monovinyl aromatic monomers added in 
twice(A.sub.2 /A.sub.1), which alters the properties of the resultant 
copolymer and renders.multidot.it suitable for various applications. In 
addition, by using higher polymerization temperature, the polymerization 
procedure can be simplified, the polymerization time is shortened as about 
10 minutes and the utilitization of the equipment and the production 
efficiency are greatly improved.