Chemical and Biomolecular Engineering Faculty Research


New Thermoplastic Elastomers of Rubbery Polyisobutylene and Glassy Cyclopolyisoprene Segments

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New thermoplastic elastomers (TPEs) consisting of a central rubbery polyisobutylene (PIB) and glassy outer cyclopolyisoprene (cy-PIP) segments have been prepared by living carbocationic polymerization by sequential monomer addition. First the homopolymerization of isobutylene (IB) and isoprene (IP) have been investigated. The living polymerization of isobutylene initiated by the p-dicumyl methoxy (p-DiCumOMe)/TiCl4 system in CH3Cl/n-C6H14 mixtures at −80°C gave living ⊕PIB⊕ dications of desired molecular weights. The polymerization of IP induced by cumyl acetate (CumOAc)/TiCl4 or cumyl methyl ether (CumOMe)/TiCl4 systems in the same solvent at −40°C readily yielded gel-free off-white powders comprising trans-1,4-PIP units and cy-PIP sequences. The polymerization most likely proceeds by trans-1,4-PIP ⊕ cations that undergo inter/intramolecular alkylation and cyclization leading to branched multiblocks. Block copolymers were synthesized by the addition of IP to charges containing ⊕PIB⊕ dications of different molecular weights. Under select conditions pure (tr-1,4-PIP)-b-PIB-b-(tr-1,4-PIP) triblocks with relatively short tr-1,4-PIP sequences have been obtained. Extended reaction times result in multiblocks; most likely ⊕ (tr-1,4-PIP)-b-PIB-b-(tr-1,4-PIP) ⊕ dications induce alkylation and cyclization of PIP sequences. The extent of alkylation can be regulated by the viscosity of the medium (molecular weight and/or concentration of the PIB midblock). The rate of IP incorporation decelerates after an initial fast polymerization; however, it can be accelerated by adding fresh Lewis acid to the system. Although tr-1,4-PIP endblocks of linear triblocks can be completely cyclized by acid treatment to saturated products, multiblocks cannot, probably because of their branched structure. Transmission electron microscopy of multiblocks shows phase separation with PIB (95–70 mol%) the continuous phase (Tg = −73°C) and cy-PIP blocks (5–30 mol %) the discrete phase. The Tg of the cy-PIP domain is in the range 95–190°C and it is affected by the extent of cyclization and/or the length of the cy-PIP sequence. The new multiblock TPEs are soluble in organic solvents and can be melt-processed above ∼ 140°C. A multiblock TPE film containing 15 mol % glassy cy-PIP block and 85 mol % rubbery PIB block of M̄n = 55,500 cast from CCl4 exhibited to MPa tensile strength [120 MPa engineering strength (sigma true)] and 1200% elongation. The completely cyclized linear triblock shows higher Tg (≡ 180°C) but, due to its low cy-PIP content, exhibits unsatisfactory tensile properties.

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Journal of Applied Polymer Science





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