Styrene diene polymers, hydrogenated

The three most important commercial VI improver families each represent one of the most important commercial techniques for manufacturing high molecular weight polymers, thus polymethacrylates by free-radical chemistry, olefin copolymers by Ziegler chemistry and hydrogenated styrene-diene or copolymers by anionic polymerization. 

Since oxidation is a chemical process, the established carbon-hydrogen bond strengths can be used to estimate ease of hydrogen abstraction. Table 5.3 [75]. These suggest that PMA backbones should be most stable to oxidative attack OCP polymers would be expected to be less stable whilst the stability of styrene-diene polymers could vary depending on the extent of 1,2 vs. 1,4 structures present. The benzylic hydrogens from styrene are potential sites for attack, and it is obviously critical that hydrogenation of the olefinic unsaturation is as complete as possible. 

Table IV. Effect of Hydrogenation and Block Structure in 25% Styrene-Diene Polymers...

The hydrogenation of unsaturated polymers and copolymers in the presence of a catalyst offers a potentially useful method for improving and optimizing the mechanical and chemical resistance properties of diene type polymers and copolymers. Several studies have been published describing results of physical and chemical testing of saturated diene polymers such as polybutadiene and nitrile-butadiene rubber (1-5). These reports indicate that one of the ways to overcome the weaknesses of diene polymers, especially nitrile-butadiene rubber vulcanizate, is by the hydrogenation of carbon-carbon double bonds without the transformation of other functional unsaturation such as nitrile or styrene. 

This field is still active today numerous recent patents [46, 63-89] and several reports [90-92] have detailed the efficiency of such types of systems for the hydrogenation of unsaturated polymers resulting from the polymerization of dienes (butadiene, isoprene, 1,3-cyclohexadiene) or the co-polymerization of dienes and styrenes (block co-polymers of butadiene and styrene SB, SBS, SBSB polymers). 

In summary the hydrogenation of styrene-diene block polymers is a practical route to tough, clear heat-resistant plastics. Variations in overall physical properties can be obtained by controlling composition, microstructure, and the nature of the block sequence using conventional anionic polymerization techniques. 

They provide an amphiphillic function in blends of dissimilar polymers, where favorable interactions exist between one of the homopolymers and one of the block copolymer segments.Hydrogenated block copolymers which reduce surface tension in polystyrene-polyoletin blends is one example. Another example is the use of a derivatized block copolymer, such as terminally carboxylated styrene-diene di-block copolymers,... 

In a sense, the styrene-butadiene block copolymers, SB or SBS, (first reported in 1956) constituted the next stage of PS modification. The triblock styrene-diene thermoplastic elastomers were patented in 1962, and soon incorporated in blends with PS, PP, LDPE, HDPE, PPE, PET, PBT, or PC, either as impact modifiers or compatibilizers [Bull and Holden, 1977]. In the 1977-78 patents (applications in 1976) it was disclosed that selective hydrogenation of these copolymers leads to new materials, with properties particularly attractive for polymer blends. For example, blending hydrogenated-SBS, or SEES, generated phase co-continuity in blends with PP, PA, PC, PBT, PES, etc. [Gergen et al., 1987]. More recent modification of these copolymers involved incorporation of acidic or acid-anhydride moieties. 

An outstanding property of these polymers is their shear stability. The sonic shear stability testsfci indicate that these polymers are superior to some of the currently used polymers of ethylene-propylene or methacrylate type. The excellent stability of the hydrogenated diene-styrene polymers is attributed to their relatively low molecular weight and narrow distribution consistent with the established theory of shear degradation of polymers. The most recent developments in this field are block polymer VI improvers with dispersancy properties, built into the molecule by chemical modification of the rubber block. 2... 

Orientations in elongated mbbers are sometimes regular to the extent that there is local crystallization of individual chain segments (e.g., in natural rubber). X-ray diffraction patterns of such samples are very similar to those obtained from stretched fibers. The following synthetic polymers are of technical relevance as mbbers poly(acrylic ester)s, polybutadienes, polyisoprenes, polychloroprenes, butadiene/styrene copolymers, styrene/butadiene/styrene tri-block-copolymers (also hydrogenated), butadiene/acrylonitrile copolymers (also hydrogenated), ethylene/propylene co- and terpolymers (with non-conjugated dienes (e.g., ethylidene norbomene)), ethylene/vinyl acetate copolymers, ethyl-ene/methacrylic acid copolymers (ionomers), polyisobutylene (and copolymers with isoprene), chlorinated polyethylenes, chlorosulfonated polyethylenes, polyurethanes, silicones, poly(fluoro alkylene)s, poly(alkylene sulfide)s. 

By-products formed during their preparation (e.g., ethylbenzene and divin-ylbenzenes in styrene acetaldehyde in vinyl acetate) added stabilizers (inhibitors) autoxidation and decomposition products of the monomers (e.g., peroxides in dienes, benzaldehyde in styrene, hydrogen cyanide in acrylonitrile) impurities that derive from the method of storage of the monomer (e.g., traces of metal or alkali from the vessels, tap grease etc.) dimers, trimers, and polymers that are generally soluble in the monomer, but sometimes precipitate, for example, polyac-rylOTiitrile from acrylonitrile. Likewise, in polycondensation reactions it is important to remove reactive impurities because they can cause considerable interference during the polyreaction. 

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