Rubber composition

A rubber composition which gives a rubber vibration insulator having excellent low-temperature properties comprises as a main rubber component a blend composed of (a) 10 to 90 parts by weight of a tapered styrene/butadiene copolymer rubber which has an average bound styrene content of 10 to 40% by weight and a 1,2-bond content in the butadiene portion of at least 50% by weight and in which the bound styrene content increases or decreases in one direction along the copolymer molecular chain and (b) 90 to 10 parts by weight of natural rubber and/or synthetic polyisoprene rubber.

This invention relates to a rubber composition for production of rubber 
vibration insulator, and more specifically to a novel rubber composition 
which gives a rubber vibration insulator rubber having an excellent 
balance between vibration and sound insulating properties at ordinary 
temperature and vibration insulating properties at low temperatures. 
With the recent highly advanced performance of automobiles, it has been 
desired to improve the vibration properties of rubber vibration insulator, 
or decrease the vibration transfer constant, in order to suppress 
vibration or noises and increase riding comfort. Development efforts have 
been directed to rubber vibration insulators which have a large loss 
factor (tan .delta.) in a low-frequency vibration region and a low 
dynamic-to-static modulus ratio (Kd/Ks where Kd is a dynamic spring 
constant and Ks is a static spring constant) in a high-frequency vibration 
region. For this purpose, the present inventors previously found that when 
a rubber vibration insulator is made from a rubbery polymer obtained by 
reacting an alkali metal- and/or alkaline earth metal-added rubbery 
polymer with an organic compound having a 
##STR1## 
bond (in which X represents an oxygen or sulfur atom) or a specific 
(thio)benzophenone, a rubber vibration insulator having an excellent 
balance between the loss factor and the dynamic-to-static modulus ratio 
can be obtained. This technique was applied for a patent (EP No. 198295A). 
These rubber vibration insulators certainly have excellent vibration 
insulating properties at room temperature, but to improve their vibration 
insulating properties and fatigue resistance at low temperatures, it is 
necessary to use them as a blend with natural rubber and/or synthetic 
polyisoprene rubber. In particular, when the rubbery polymer is a 
styrene/butadiene copolymer rubber, there is a problem to be solved. 
Specifically, even when it is blended with natural rubber and/or synthetic 
polyisoprene rubber in order to improve vibration insulating properties at 
low temperatures, sufficient low-temperature properties cannot be 
obtained. Specifically, the ratio of the dynamic spring constant at 
-10.degree. C. (Kd.sub.-10 .degree.C.) of the resulting vulcanized rubber 
composition to that at room temperature (Kd.sub.RT) cannot be sufficiently 
lowered. 
It is an object of this invention therefore to provide a novel rubber 
composition which gives a rubber vibration insulator having an excellent 
balance between vibration and sound insulating properties at ordinary 
temperature and vibration insulating properties at low temperatures. 
The present inventors worked extensively on the relation between the 
composition of a styrene/butadiene copolymer rubber in a blend of the 
styrene/butadiene copolymer rubber and natural rubber and/or synthetic 
polyisoprene rubber and its low-temperature vibration insulating 
properties. This work has led to the discovery that a rubber composition 
obtained by blending natural rubber and/or synthetic polyisoprene rubber 
and a tapered styrene/butadiene copolymer rubber in which the bound 
styrene content increases or decreases in one direction along the 
molecular chain in a specific blending ratio gives a rubber vibration 
insulator having markedly improved vibration insulating properties at low 
temperatures while retaining excellent vibration properties at room 
temperature. 
Thus, according to this invention, there is provided a rubber composition 
capable of giving a rubber vibration insulator having low-temperature 
properties, said composition comprising as a main rubber component a blend 
composed of (a) 10 to 90 parts by weight of a tapered styrene/butadiene 
copolymer rubber which has an average bound styrene content of 10 to 40% 
by weight and a 1,2-bond content in the butadiene portion of at least 50% 
by weight and in which the bound styrene content increases or decreases in 
one direction along the copolymer molecular chain and (b) 90 to 10 parts 
by weight of natural rubber and/or synthetic polyisoprene rubber. 
The tapered styrene/butadiene copolymer rubber used in this invention is 
obtained by a so-called living polymerization between 1,3-butadiene and 
styrene in the presence of a catalyst based on an alkali metal and/or an 
alkaline earth metal. Specifically, it may be, for example, a tapered 
styrene/butadiene copolymer rubber which has an average bound styrene 
content of 10 to 40% by weight, preferably 15 to 30% by weight, and a 
1,2-bond content in the butadiene portion of at least 50% by weight, 
preferably at least 60% by weight, and in which the bound styrene content 
varies from one end of the molecular chain to the other end along the 
molecular chain such that the average bound styrene content of one end 
portion (for example, a portion formed in the early stage of the living 
polymerization) of the molecular chain is not more than 1/5 of the average 
bound styrene content and the bound styrene content of the other end 
portion (for example, a portion formed in the terminal stage of the living 
polymerization) of the molecular chain is at least two times the average 
bound styrene content. 
This tapered styrene/butadiene copolymer rubber can be obtained by, for 
example, anionic polymerization using an alkyllithium catalyst in the 
presence of a polar compound such as an ether or a tertiary amine in a 
hydrocarbon solvent, in which only 1,3-butadiene is placed as a monomer in 
the reaction system before the start of polymerization, and simultaneously 
with the starting of the polymerization, a styrene monomer is added at a 
predetermined rate stepwise or continuously until the end of the 
polymerization. 
The microstructure of the butadiene portion can be caused to have the above 
specified 1,2-bond content (at least 50% by weight) by varying the type 
and amount of the polar compound, and the polymerization temperature. 
If the average bound styrene content of the copolymer exceeds 40% by 
weight, its loss factor at room temperature is large. But its 
dynamic-to-static modulus ratio does not become low even when it is 
blended with natural rubber and/or synthetic polyisoprene rubber, and an 
improvement in low-temperature properties cannot be obtained. If the 
average bound styrene content is less than 10% by weight, the loss factor 
at room temperature is small and no sufficient vibration insulating effect 
can be obtained. If the 1,2-bond content of the butadiene portion of the 
styrene/butadiene copolymer is less than 50% by weight, its compatibility 
with natural rubber and/or synthetic polyisoprene rubber is poor and 
vibration insulating properties at low temperature cannot be improved 
while retaining the excellent vibration insulating effect at room 
temperature. 
By blending the tapered styrene/butadiene copolymer rubber and natural 
rubber and/or synthetic polyisoprene rubber in a weight ratio of from 
10:90 to 90:10, the balance between excellent vibration insulating 
properties at room temperature and vibration properties at low 
temperatures can be markedly improved. If the blending proportion of 
natural rubber and/or synthetic polyisoprene rubber is less than 10% by 
weight, the effect of improving low-temperature vibration insulating 
properties cannot be obtained. If it exceeds 90% by weight, the excellent 
vibration insulating properties at room temperature of the tapered 
styrene/butadiene copolymer cannot be retained. The preferred ratio of the 
tapered styrene/butadiene copolymer rubber and natural rubber and/or 
synthetic polyisoprene rubber is from 80:30 to 20:70, especially from 
70:50 to 30:50, by weight. 
The above-exemplified tapered styrene/butadiene copolymer rubber in which 
the bound styrene content varies from one end of the molecular chain to 
the other end along the molecular chain such that the average bound 
styrene content of one end portion of the molecular chain is not more than 
1/5 of the average bound styrene content and the bound styrene content of 
the other end portion of the molecular chain is at least two times the 
average bound styrene content is especially preferred in this invention. 
If the bound styrene content at one end of the molecular chain is more 
than 1/5 of the average bound styrene content, it is difficult to maintain 
the copolymer and the natural rubber and/or polyisoprene rubber in a 
moderate compatible state, and an effect of improving low-temperature 
properties by blending tends to be difficult to obtain. 
In the rubber composition of this invention, at least a portion of the 
tapered styrene/butadiene copolymer rubber may be replaced with a rubber 
obtained by reacting living tapered styrene/butadiene copolymer with an 
organic compound having the 
##STR2## 
bond (where X represents an oxygen or sulfur atom) in the molecule and/or 
at least one compound selected from the group consisting of ketones, 
aldehydes, thioketones and thioaldehydes which have an amino group and/or 
a substituted amino group. Such a rubber composition of this invention can 
give a rubber vibration insulator having a more improved balance between 
vibration insulating properties at room temperature and vibration 
insulating properties at low temperature. 
The organic compound having the 
##STR3## 
bond in the molecular chain is a compound which reacts with the living 
tapered styrene/butadiene copolymer rubber. Compounds that fall within 
this compound are disclosed, for example, in EP No. 198295A, and can be 
used without any particular limitation. Specific examples of the compound 
include amides such as formamide, N,N-dimethylformamide, 
N,N-diethylformamide, acetamide, N,N-dimethylacetamide, 
N,N-diethylacetamide, aminoacetamide, 
N,N-dimethyl-N',N'-dimethylaminoacetamide, N',N'-dimethylaminoacetamide, 
N'-ethylaminoacetamide, N,N-dimethyl-N'-ethylaminoacetamide, 
N,N-dimethylaminoacetamide, N-phenyldiacetamide, acrylamide, 
N,N-dimethylacrylamide, N,N-dimethylmethacrylamide, propionamide, 
N,N-dimethyl propionamide, 4-pyridylamide, N,N-dimethyl-4-pyridylamide, 
benzamide, N-ethylbenzamide, N-phenylbenzamide, N,N-dimethylbenzamide, 
p-aminobenzamide, N',N'-(p-dimethylamino)benzamide, 
N',N'-(p-diethylamino)benzamide, N'-(p-methylamino)benzamide, 
N'-(p-ethylamino)benzamide, N,N-dimethyl-N'-(p-ethylamino)benzamide, 
N,N-dimethyl-N',N'-(p-diethylamino)benzamide, 
N,N-dimethyl-p-aminobenzamide, N-methylbenzamide, 
N-acetyl-N-2-naphthylbenzamide, succinamide, maleinamide, phthalamide, 
N,N,N',N'-tetramethylmaleinamide, N,N,N',N'-tetramethylphthalamide, 
succinimide, N-methylsuccinimide, maleimide, N-methylmaleimide, 
phthalimide, N-methylphthalimide, oxamide, N,N,N',N'-tetramethyloxamide, 
N,N-dimethyl-p-amino-benzalacetamide, nicotinamide, 
N,N-diethylnicotinamide, 1,2-cyclohexanedicarboximide, 
N-methyl-1,2-cyclohexanedicarboximide, methyl carbamate, methyl 
N-methyl-carbamate, ethyl N,N-diethyl-carbamate, ethyl 
N,N-diethyl-carbamate, ethyl carbanilate, and ethyl 
p-N,N-diethylaminocarbanilate; ureas such as urea, N,N'-dimethylurea, 
N,N,N',N'-tetramethylurea, 1,3-dimethylethyleneurea; anilides such as 
formanilide, N-methylacetanilide, aminoacetanilide, benzanilide, and 
p,p'-di(N,N-diethyl)-aminobenzanilide; lactams such as 
epsilon-caprolactam, N-methyl-epsilon-caprolactam, 
N-acetyl-epsilon-caprolactam, 2-pyrrolidone, N-methyl-2-pyrrolidone, 
N-acetyl-2-pyrrolidone, N-phenyl-2-pyrrolidone, 2-piperidone, 
N-acetyl-2-piperidone, 2-quinolone, N-methyl-2-quinolone, 2-indolinone and 
N-methyl-2-indolinone; imidazolidinones such as 
1,3-diethyl-2-imidazolidinone, 1-methyl-3-ethyl-2-imidazolidinone, 
1-methyl-3-(2-methoxyethyl)-2-imidazolidinone; isocyanuric acids such as 
isocyanuric acid and N,N',N"-trimethylisocyanuric acid; and 
sulfur-containing compounds corresponding to the foregoing compounds. 
Those in which an alkyl or aryl group is bonded to the nitrogen are 
especially preferred. 
Examples of the ketones, aldehydes, thioketones and thioaldehydes having an 
amino group and/or a substituted amino group include ketones such as 
4-aminobenzophenone, 4-dimethylaminobenzophenone, 
4-dimethylamino-4'-methylbenzophenone, 4,4'-diaminobenzophenone, 
4,4'-bis(dimethylamino)benzophenone, 4,4'-bis(diethylamino)benzophenone, 
4,4'-bis(ethylamino)benzophenone, 
3,3'-dimethyl-4,4'-bis(diethylamino)benzophenone, 
3,3'-dimethoxy-4,4'-bis(dimethylamino)benzophenone, 
3,3',5,5'-tetraaminobenzophenone, 2,4,6-triaminobenzophenone, 
3,3',5,5'-tetra(diethylamino)benzophenone, 4-dimethylaminoacetophenone, 
1,3-bis(diphenylamino)-2-propanone and 
1,7-bis(methylethylamino)-4-heptanone; the corresponding thioketones; 
aldehydes such as 3-dimethylaminopropionaldehyde, 
3-diethylaminopropionaldehyde, 4-dimethylaminobenzaldehyde, 
3-dicyclohexylaminopropionaldehyde and 3,5-bis(dihexylamino)benzaldehyde; 
and the corresponding thioaldehydes. 
The substituted amino group preferably has an alkyl group as the 
substituent, and a dialkyl-substituted amino group is especially 
preferred. Substituents other than the amino group and the substituted 
amino group may be present in these compounds if the other substituents do 
not adversely affect the reaction. 
The amount of the above compound to be reacted with the living tapered 
styrene/butadiene copolymer is equimolar to, or slightly larger in moles 
than, the amount of the metal bound to the copolymer. After the reaction, 
the tapered styrene/butadiene copolymer having the above compound bound 
thereto can be obtained by adding a coagulating agent such as methanol to 
the reaction solution or blowing steam into the reaction solution to 
perform hydrolysis. 
It is believed that after the reaction, the above compound is introduced 
into the carbon atom at the end of the molecular chain of the rubbery 
polymer as an atomic grouping of the following formula. 
##STR4## 
The use of a polymer obtained by the reaction of a living tapered 
styrene/butadiene copolymer rubber having butadienyl metal terminals with 
the above compound leads to a further improvement in vibration insulating 
properties. 
After the end of the above reaction, the product may further be reacted 
with an acid and/or a halogen compound. In this case, an unsaturated 
rubbery polymer having a salt or charge transfer complex of the above 
atomic grouping introduced thereinto is obtained and can be used as the 
rubber component (A) in the rubber composition of the invention. The 
Mooney viscosity (ML.sub.1+4, 100 .degree.C.) of the tapered 
styrene/butadiene copolymer rubber having the above compound introduced 
thereinto is usually in the range of 10 to 200, preferably 20 to 150. If 
it is less than 10, the mechanical properties, such as tensile strength, 
of the copolymer is inferior. If it exceeds 200, the miscibility of the 
copolymer with another rubber is poor, and the processing operation 
becomes difficult. Moreover, the mechanical properties of a vulcanizate 
from the resulting rubber compound are undesirably reduced. 
The tapered styrene/butadiene copolymer rubber may, wholly or partly, be 
used as an oil-extended rubber. A rubber vibration insulator may be 
obtained by kneading the rubber composition of the invention with various 
compounding agents such as sulfur, stearic acid, zinc oxide, various 
vulcanization accelerators, reinforcing agents or fillers (e.g., SRF, FEF 
and HAF carbon blacks, silica, and calcium carbonate), process oils, and 
plasticizers by means of a mixer such as a roll or a Banbury mixer, 
molding the mixture and vulcanizing it. 
Since the styrene/butadiene copolymer rubber in the rubber composition of 
this invention has a 1,2-bond content in the butadiene portion of at least 
50% by weight, it essentially has mutual solubility to some extent in 
natural rubber and/or synthetic polyisoprene rubber. It also has partial 
compatibility with the natural rubber and/or synthetic polyisoprene rubber 
because the bound styrene content of the molecular chain increases or 
decreases in one direction along the molecular chain. Consequently, as 
shown in FIG. 2 of the accompanying drawings which shows the temperature 
dependence of the loss factor (tan .delta.) of the vulcanized rubber 
compound, the peak position of the tan .delta. at low temperatures (about 
0.degree. to -10.degree. C.) can be shifted toward a lower temperature 
side without decreasing tan .delta. at room temperature. Accordingly, the 
rubber composition of this invention can give a rubber vibration insulator 
having a much lower ratio of the dynamic spring constant at a low 
temperature (-10.degree. C.) (Kd.sub.-10 .degree.C.) to the dynamic spring 
constant at room temperature (Kd.sub.RT), Kd.sub.-10 .degree.C. 
/Kd.sub.RT, than in the case of using a conventional copolymer rubber 
whose bound styrene content does not vary along the molecular chain. 
The following Examples illustrate the present invention more specifically. 
In these examples, all parts and percentages are by weight unless 
otherwise specified. 
For evaluation of vibration insulating properties in these examples, loss 
factor (tan .delta.) at 25.degree. C., 15 Hz and a compression strain of 
.+-.0.2%, dynamic spring constant Kd.sub.23 .degree.C. at 23.degree. C. 
(room temperature), 100 Hz and a compression strain of .+-.0.2%, and 
dynamic spring constant Kd.sub.-10 .degree.C. at -10.degree. C. (low 
temperature), 100 Hz and a compression strain of .+-.0.2% were measured on 
N2 specimens of JIS K-6394 by means of a hydraulic servo-type dynamic 
tester (Model KC-V made by Saginomiya Seisakusho, Japan). Static spring 
constant (Ks.sub.23 .degree.C.) was determined in accordance with JIS 
K-6385. The dynamic-to-static modulus ratio (Kd.sub.23 .degree.C. 
/Ks.sub.23 .degree.C.) and the dynamic spring constant ratio (Kd.sub.-10 
.degree.C. /Kd.sub.23 .degree.C.) were calculated.

EXAMPLE 1 
Styrene/butadiene copolymer rubbers used in this Example were prepared by 
the following methods. 
(1) Sample 1-a of the invention 
A 15-liter stainless steel polymerization vessel was washed, dried and 
purged with dry nitrogen. Then, 640 g of 1,3-butadiene, 4700 g of 
cyclohexane and 6.5 millimoles of N,N,N',N'-tetramethylethylenediamine 
were fed into the vessel, and further, 6.4 millimoles of n-butyllithium 
(n-hexane solution) was added. Polymerization of 1,3-butadiene was started 
at 45.degree. C. Immediately after the start of the polymerization, 
styrene was continuously added at stepwise varying rates of addition. The 
total amount of styrene added was 160 g. After polymerization for about 2 
hours, 5 ml of methanol was added to the reaction mixture to stop the 
reaction. Then, 8 g of 2,6-di-t-butyl-p-cresol (BHT) was added, and the 
mixture was coagulated with steam. It was then dehydrated on rolls and 
further dried in vacuum at 60.degree. C. for 24 hours. During the 
polymerization, a small amount of the polymer solution was sampled from 
the reactor every 5 minutes. The sequence distribution (see FIG. 1) of 
styrene was determined from the conversions and styrene contents measured 
on the samples. 
(2) Comparative sample 1-b 
A 10-liter stainless steel polymerization vessel was washed, dried and 
purged with dry nitrogen. Then, 4000 g of cyclohexane and 8.0 millimoles 
of N,N,N',N'-tetramethylethylenediamine were fed into the vessel, and 6.0 
millimoles of n-butyllithium (n-hexane solution) was added. While the 
reactor was maintained at 45.degree. C., a mixture of 160 g of styrene and 
640 g of butadiene was continuously added at a rate of 5.0 g per minute. 
During the polymerization, a small amount of the polymer solution was 
sampled from the reactor every 20 minutes, and the sequence distribution 
of styrene was determined from the conversions and styrene contents 
measured on the samples. This led to the determination that the product 
was a random copolymer without styrene tapering (see FIG. 1). After the 
conversion reached 100%, 5 ml of methanol was added to stop the reaction. 
The reaction mixture was coagulated and dried by the same methods as in 
the preparation of sample 1-a. 
(3) Comparative sample 2-a 
Comparative sample 2-a was prepared by the same method as in the 
preparation of sample 1-a except that 200 g of styrene, 600 g of butadiene 
and 1.2 millimoles of N,N,N',N'-tetramethylethylenediamine were used. 
(4) Comparative sample 2-b 
Comparative sample 2-b was prepared by the same method as in the 
preparation of sample 1-b except that 200 g of styrene, 600 g of butadiene 
and 1.2 millimoles of N,N,N',N'-tetramethylethylenediamine were used and 
the rate of addition was changed to 2.0 g/min. 
(5) The vinyl bond content and the bound styrene content of each of the 
polymer rubber samples obtained above were determined by infrared 
spectroscopy [Hampton, Anal. Chem., 21, 923 (1949)]. 
(6) Rubber compositions were prepared from the samples obtained in 
accordance with the compounding recipe shown in Table 1. The rubber 
compositions were each vulcanized to prepare samples. The vibration 
insulating properties of the samples were measured. The temperature 
dependence of the loss factor was measured at 15 Hz and a shear strain 
amplitude of 0.5% by means of a rheometric dynamic analyzer (made by 
Rheometric Company). 
The results are shown in Table 2. 
TABLE 1 
______________________________________ 
compounding recipe 
Ingredient Amount (parts) 
______________________________________ 
Natural rubber (RSS#3) 
60 
Polymer rubber (samples in Table 2 or 3) 
40 
FEF carbon black 40 
Aromatic process oil 15 
Zinc oxide No. 3 5 
Stearic acid 2 
Vulcanization accelerator (N-cyclohexyl- 
1.2 
2-benzothiazyl sulfenamide) 
Sulfur 1.8 
Antioxidant (N-isopropyl-N'-phenyl-p- 
1.5 
phenylenediamine) 
Antioxidant (polymer of 2,2,4-trimethyl- 
1.5 
1,2-dihydroquinoline) 
Total 168.0 
______________________________________ 
TABLE 2 
______________________________________ 
Sample 
1-a 1-b* 2-a* 2-b* 
______________________________________ 
Polymer structural properties 
Average bound styrene content (%) 
20 20 25 25 
Styrene tapering yes no yes no 
Vinyl bond content (%) 
60 60 35 35 
Polymer Mooney viscosity 
46 44 52 50 
(ML.sub.1+ 4, 100.degree. C.) 
Vibration insulating properties 
Static spring constant (Ks.sub.23.degree. C.) 
8.26 8.33 8.48 8.56 
(23.degree. C.) (kg/mm) 
Dynamic spring constant (Kd.sub.23.degree. C.) 
15.7 15.7 15.5 15.5 
(100 Hz, 23.degree. C.) (kg/mm) 
Dynamic-to-static modulus ratio 
1.90 1.88 1.83 1.81 
(Kd.sub.23.degree. C. /Ks.sub.23.degree. C.) 
Loss factor (tan .delta.) (15 Hz, 23.degree. C.) 
0.128 0.127 0.119 
0.121 
Low temperature properties 
Dynamic spring constant (Kd.sub.-10.degree. C.) 
45.3 60.6 50.4 49.9 
(100 Hz, -10.degree. C.) (kg/mm) 
Dynamic spring constant ratio 
2.89 3.87 3.25 3.29 
(Kd.sub.-10.degree. C. /Kd.sub.23.degree. C.) 
______________________________________ 
*Comparisons 
Table 2 shows that sample 1-a (invention) is equivalent in vibration 
insulating properties (dynamic-static modulus ratio and loss factor) at 
room temperature to comparative sample 1-b of the same composition, and 
had a less rise in dynamic spring constant (Kd.sub.-10 .degree.C.) at a 
low temperature (-10.degree. C.) and a lower Kd.sub.-10 .degree.C. 
/Kd.sub.23 .degree.C. value than comparative sample 1-b. It was 
furthermore determined from the results given in Table 2 that when there 
is tapering in the bound styrene content but the vinyl bond content is low 
(sample 2-a), a blend of the styrene/butadiene copolymer with natural 
rubber and/or synthetic polyisoprene rubber does not give a vulcanizate 
having improved low-temperature vibration insulating properties. 
EXAMPLE 2 
Copolymer rubbers used in this example were prepared by the following 
methods. 
(1) Sample 3-a of the invention 
In accordance with the same polymerization recipe and conditions as in the 
preparation of sample 1-a in Example 1, the polymerization was carried out 
for 2 hours. Then, 1.0 g of 4,4'-bis(diethylamino)benzophenone was added, 
and reacted for 30 minutes. Then, 5 ml of methanol was added to stop the 
reaction. The reaction mixture was coagulated and dried by the same 
methods as in Example 1. 
(2) Sample 4-a of the invention 
This sample was prepared in the same way as in the preparation of sample 
3-a except that 140 g of styrene, 660 g of butadiene and 6.0 millimoles of 
N,N,N',N'-tetramethylethylenediamine were used. 
(3) Sample 5-a of the invention 
This sample was prepared by the same method as in the preparation of sample 
3-a except that 200 g of styrene, 600 g of butadiene and 6.2 millimoles of 
N,N,N',N'-tetramethylethylenediamine were used, and 
N-methyl-epsilon-caprolactam was used instead of 
4,4'-bis(diethylamino)benzophenone. 
(4) Comparative sample 3-b 
This sample was prepared in the same way as in the preparation of 
comparative sample 1-b except that 170 g of styrene, 630 g of butadiene 
and 6.8 millimoles of N,N,N',N'-tetramethylethylenediamine were used, and 
before stopping the reaction by methanol, 1.0 g of 
4,4'-bis(diethylamino)benzophenone was added and reacted for 30 minutes. 
(5) Comparative sample 4-b 
This sample was prepared by the same method as in the preparation of 
comparative sample 1-b except that 140 g of styrene, 660 g of butadiene 
and 6.0 millimoles of N,N,N',N'-tetramethylethylenediamine were used. 
(6) Comparative sample 5-b 
This sample was prepared in the same way as in the preparation of 
comparative sample 3-b except that 200 g of styrene, 600 g of butadiene 
and 5.0 millimoles of N,N,N',N'-tetramethylethylenediamine were used, and 
N-methyl-epsilon-caprolactam was used instead of 
4,4'-bis(diethylamino)benzophenone. 
The microstructures of the polymer rubber samples were measured as in 
Example 1. 
Rubber compositions were prepared from the samples in accordance with the 
compounding recipe shown in Table 1. The rubber compositions were 
vulcanized to obtain samples for measuring vibration insulating 
properties. The results of measurement of the vibration insulating 
properties are shown in Table 3. 
TABLE 3 
__________________________________________________________________________ 
Sample 
3-a 4-a 
5-a 3-b* 
4-b* 
5-b* 
__________________________________________________________________________ 
Polymer structural properties 
Average bound styrene content (%) 
21.0 
17.4 
25.0 
21.5 
17.0 
24.5 
Styrene tapering yes yes 
yes no no no 
Vinyl bond content (%) 
69.7 
66.0 
60.5 
70.1 
67.0 
59.0 
Molecular chain end modifier 
EAB.sup.1 
EAB 
NMC.sup.2 
EAB 
EAB 
NMC 
Polymer Mooney viscosity 
50 48 48 51 45 46 
(ML.sub.1+ 4, 100.degree. C.) 
Vibration insulating properties 
Static spring constant (Ks.sub.23.degree. C.) 
7.61 
7.62 
7.74 
7.66 
7.71 
7.81 
(23.degree. C.) (kg/mm) 
Dynamic spring constant (Kd.sub.23.degree. C.) 
13.7 
13.4 
15.7 
13.7 
13.3 
15.6 
(100 Hz, 23.degree. C.) (kg/mm) 
Dynamic-to-static modulus ratio 
1.81 
1.75 
2.03 
1.78 
1.73 
2.00 
(Kd.sub.23.degree. C. /Ks.sub.23.degree. C.) 
Loss factor (tan .delta.) (15 Hz, 23.degree. C.) 
0.143 
0.118 
0.162 
0.143 
0.116 
0.153 
Low temperature properties 
Dynamic spring constant (Kd.sub.-10.degree. C.) 
48.5 
37.1 
62.2 
104.5 
71.6 
142.6 
(100 Hz, -10.degree. C.) (kg/mm) 
Dynamic spring constant ratio 
3.54 
2.77 
3.96 
7.65 
5.37 
9.13 
(Kd.sub.-10.degree. C. /Kd.sub.23.degree. C.) 
__________________________________________________________________________ 
*Comparisons 
.sup.1 4,4bis(diethylamino)benzophenone, 
.sup.2 Nmethyl-epsilon-caprolactam 
The results given in Table 3 show that vibration insulator rubbers prepared 
by using samples 3-a, 4-a and 5-a of the invention had markedly improved 
low-temperature vibration insulating properties over rubber vibration 
insulators prepared by using the corresponding comparative samples 3-b, 
4-b and 5-b.