Thermoplastic rubbers, radial block

Phillips Petroleum Co., Chemicals and Polymers R D Division, Bartlesville, Okla. 

Butadiene and styrene have been combined in rubbery polymers in ntanerous ways to produce a wide variety of properties. 

One result of solution polymerization technology is the preparation of block polymers - both those that are essentially pure block and those that contain some mixtures of block and random structures (1, 2, j). It was found that copolymers with styrene blocks on the aids of the polymer molecule provide strong, elastomeric materials without vulcanization. This behavior has been broadly documented CS,. 6) polymers 

Block polymers of dienes and styrene are usually prepared in solution polymerization systems using alkylmetal initiators although exact methods of producing commercial polymers are not often disclosed. Block structures can be foimed by sequential addition of different monomsrs combined with coupling of polymer chains if desired. Possible synthesis methods have been described in a number of publications (6, 

ACS Symposium Series American Chemical Society Washington, DC, 1974. 

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Haws, J. R., Radial Block Thermoplastic Rubbers (R. D. Deanin, ed.), American Chemical Society, Washington, DC (1972), pp. 1-14. 

Blends of radial block rubbers with other polymers provide useful variations in properties. Thermoplastics such as polyethylene and polystyrene have already been mentioned. There are several polymers >diich can be added to butadiene-styrene block... 

As discussed previously, thermoplastic elastomers are materials which have the functional properties of conventional vulcanized rubbers but which may be processed as normal thermoplastics (see section 2.9). Effects of this kind are shown by styrene-butadiene block copolymers. Two types of styrene-butadiene block copolymers are produced commercially, namely triblock and radial block copolymers. The triblock copolymers (denoted by SBS) consist of a centre block of butadiene units with two terminal blocks of styrene units. The radial block copolymers (denoted by (SB) X) consist of three or more styrene-butadiene diblock copolymers radiating from a central... 

Block copolymer chemistry and architecture is well described in polymer textbooks and monographs [40]. The block copolymers of PSA interest consist of anionically polymerized styrene-isoprene or styrene-butadiene diblocks usually terminating with a second styrene block to form an SIS or SBS triblock, or terminating at a central nucleus to form a radial or star polymer (SI) . Representative structures are shown in Fig. 5. For most PSA formulations the softer SIS is preferred over SBS. In many respects, SIS may be treated as a thermoplastic, thermoprocessible natural rubber with a somewhat higher modulus due to filler effect of the polystyrene fraction. Two longer reviews [41,42] of styrenic block copolymer PSAs have been published. 

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