Sequential synthesis of multiblock copolymers by atom transfer radical and cationic polymerization

ABCBA-type pentablock copolymers of methyl methacrylate (MMA), styrene (S), and isobutylene (IB) were prepared by a three-step synthesis, which included atom transfer radical polymerization (ATRP) and cationic polymerization: (1) poly(methyl methacrylate) (PMMA) with terminal chlorine atoms was prepared by ATRP initiated with an aromatic difunctional initiator bearing two trichloromethyl groups under CuCl/2,2'-bipyridine catalysis; (2) PMMA with the same catalyst was used for ATRP of styrene, which produced a poly(S-b-MMA-b-S) triblock copolymer; and (3) IB was polymerized cationically in the presence of the aforementioned triblock copolymer and BCl3, and this produced a poly(IB-b-S-b-MMA-b-S-b-IB) pentablock copolymer. The reaction temperature, varied from -78 to -25 degreesC, significantly affected the IB content in the product; the highest was obtained at -25 degreesC. The formation of a pentablock copolymer with a narrow molecular weight distribution provided direct evidence of the presence of active chlorine at the ends of the poly(S-b-MMA-b-S) triblock copolymer, capable of the initiation of the cationic polymerization of IB in the presence of BCl3. A differential scanning calorimetry trace of the pentablock copolymer (20.1 mol % IB) showed the glass-transition temperatures of three segregated domains, that is, polyisobutylene (-87.4 degreesC), polystyrene (95.6 degreesC), and PMMA (103.7 degreesC blocks. One glass-transition temperature (104.5 degreesC) was observed for the aforementioned triblock copolymer. (C) 2004 Wiley Periodicals, Inc.