Poly isobutylene Branches

Polymerization of IB from the PS macroinitiator was accomplished with BCl3 as coinitiator in CH2Cl2 at -78 °C. Due to the living nature of the polymerization of IB, high grafting efficiencies ( 85%) were reported. The resulting 15% homoPIB was most probably due to initiation from adventitious moisture or direct initiation (haloboration). 

A similar multifunctional macroinitiator was obtained by Puskas [65] in a radical copolymerization of S and 4-(l-hydroxy-l-methylethyl)styrene. The macroinitiator was then used to initiate the living cationic polymerization of IB. With relatively short backbone and 8-23 branches with Mn=10,000-20,000 g mob1, starlike structures, spherical in shape were obtained. The structure was verified by core destruction followed by SEC analysis of the surviving arms. 

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Polystyrene is particularly susceptible to branching because the 3° benzylic cation produced by a hydride transfer is so stable. In poly(isobutylene), there is no hydrogen on the carbon with the stabilizing substituents any hydride transfer would generate a 2° carbocation at the expense of a 3° carbocation at the end of a growing chain—this is an increase in energy and therefore unfavorable. 

FIGURE 13.16 AFM phase images of poly(styrene-b-isobutylene-b-styrene) (SIBS) triblock copolymers. The images are for (a) polyisobutylene homopolymer (b) branched SIBS with 16% styrene content and (c) linear SIBS with 30% styrene. The darker regions have greater hardness and thus represent the polystyrene domains. (From Puskas et al. (2000) reprinted with permission from John Wiley Sons, Ltd., copyright 2003.)... 

Thermoplastic graft copolymers can be considered as branched block copolymers with many branch points. A graft copolymer of isobutylene on poly (ethylene) and a graft copolymer of styrene/acrylonitrile on acrylic rubber is used in industry. 

The branched chain poly tertiary-butyl acrylate has been considered by several workers [36] at low temperatures. Schaefgen and Sarasohn [37] have studied this degradation at several low temperatures. At 160 °C isobutylene was lost quantitatively while above 180 °C approximately half of the weight of the polymer was lost after 12 hours of heating with the gaseous products being 36% isobutylene, 11% water, and 3% carbon dioxide. Elemental analysis of the pyrolysis residue corresponded approximately to polyacrylic anhydride (C H Oj). 

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