1. FIELD OF THE INVENTION
This invention relates to copolymers prepared from a monomer mixture of styrene and butadiene-1,3, hereinafter referred to as "butadiene." More specifically this invention relates to copolymers of styrene and butadiene which contain block polystyrene. Still more specifically it relates to a highly branched copolymer of styrene and butadiene having block polystyrene at the terminals of a plurality of branches.
2. RELATED PRIOR ART
Block copolymers of styrene and butadiene are known. Some of these, referred to as "S-B-S" polymers, have a central core of block polybutadiene and two terminals of block polystyrene. Other block polymers having the designation "B-S" have one block of polystyrene and one block of polybutadiene. Such copolymers which contain significant amounts of block polystyrene have not been acceptable for pneumatic tire applications. The presence of as little as 5-10 percent block polystyrene has caused excessive heat build-up in tires made from such copolymers. Hence the S-B-S and B-S types of block copolymer have not been used in tires. Among the polymers which have been used successfully in pneumatic tires are natural rubber, styrene-butadiene emulsion copolymers (SBR), and various stereospecific isoprene and butadiene homopolymers and copolymers with styrene. However, except for natural rubber and isoprene homopolymers, these polymers possess a low uncured or "green" tensile strength. For the manufacture of tires, it is desirable that a styrene-butadiene copolymer should combine the two desirable properties high green strength and low heat build-up or running temperature.
U.S. Pat. No. 3,440,304 discloses the batch polymerization of a mixture of butadiene and styrene and teaches that although the initial proportions in the monomer mixture are 75 parts of butadiene and 25 parts of styrene, the copolymer formed initially contains less than 10 percent, generally about 5-7 percent by weight of styrene, and after all the butadiene monomer is consumed, the residue of styrene monomer results in a terminal block of polystyrene being formed.
U.S. Pat. No. 3,094,542 shows the preparation of random copolymers of butadiene and styrene with all the styrene copolymerized without formation of block polystyrene. This is effected by adding a mixture of butadiene and styrene to the polymerization reactor at a rate slower than the normal polymerization rate. In this way, all or substantially all of the styrene is consumed in random copolymerization and there is less than 2 percent and generally no block polystyrene formed. The patentee also demonstrates that batch polymerization or very rapid addition of the monomer mix produces about 15-20 percent of block polystyrene.
British Pat. No. 1,136,189 discloses a continuous process for copolymerizing butadiene and styrene in a reactor in which vigorous agitation gives a homogeneous reaction mass to produce a random copolymer, with the reaction solution as it is withdrawn from the reactor containing less than 3 percent styrene. In order to avoid separation and recovery of styrene, this solution is further processed to polymerize the remainder of the styrene, thereby giving a single, small polystyrene block at the end of the polymer molecule. When the residual monomer is less than 3 percent, this is considered by the patentee as not objectionable in converting the residual styrene to a terminal block.
None of these prior art polymers have the desired combination of high green strength and low running temperature when used in tire compositions.