New dual initiators for polyisobutylene-based block and star polymers
Polyisobutylene (PIB), available solely by living carbocationic polymerization (LCP), is a commercially important polymer with excellent thermal stability, good flexibility and extraordinary impermeability to gases. Due to these attractive properties, coupling PIB to other polymer blocks is expected to result in new and useful products. Two types of new dual initiators possessing initiating sites for both LCP and atom transfer radical polymerization (ATRP), have been designed for the preparation of AB linear and A2B miktoarm star copolymers, where A is PIB-based block copolymer that grows cationically and B is polyacrylate or other radically-derived polymer block, without intermediate modification. Mono-cationic mono-radical dual initiators, 3,3,5-trimethyl-5-chlorohexyl 2-bromopropionate (IB2 BP) and 3,3,5-trimethyl-5-chlorohexyl 2-bromo-2-methylpropionate (IB2 BMP) were synthesized and used to prepare AB linear polymers. PIBs obtained from both initiators showed high efficiency in ATRP initiations of methyl acrylate (MA), yielding polymers with targeted block length and narrow polydispersity index (PDI). However, IB 2 BMP and IB 2 BP displayed slow cationic initiation of isobutylene (IB) leading to moderate initiation efficiencies (0.50 < Ieff < 0.80) at low temperature (-70 °C) and low monomer/initiator ratio (82). 3,3,5,5,7-Pentamethyl-7-chlorooctyl 2-bromo-2-methylpropionate (IB 3 BMP), which differs from IB2 BMP by the inclusion of one additional isobutylene (IB) repeating unit, was then synthesized. It showed quantitative initiation efficiency (Ieff ∼1) in TiCl4 -co-initiated LCP of IB under various reaction conditions. Ieff and PDI of the resulting PIBs were identical to those obtained with the standard mono-cationic initiators. The superiority of IB 3 BMP compared to IB2 BMP in carbocationic initiation was attributed to elimination of through-space interactions between the tert -chloride initiating site and the TiCl4 :carbonyl complex at the ATRP initiating site. Di-cationic mono-radical dual initiator 3-[3,5-bis(1-chloro-1-methylethyl)phenyl]-3-methylbutyl 2-bromo-2-methylpropionate (DCCBMP) was syntheized for the preparation of miktoarm star copolymers. Initiation efficiency of DCCBMP was high (0.89 < Ieff < 0.98) for LCP of IB and it was comparable to the standard di-cationic initiator. Using sequential monomer addition under LCP conditions, narrow-polydispersity poly(styrene-b -isobutylene- b -styrene) (PS-PIB-PS) triblock copolymers were prepared, yielding poly(acrylic acid-b -styrene-b -isobutylene) 2 -s -poly(acrylic acid) [(PAA-PS-PIB)2 - s -PAA] amphiphilic miktoarm star polymer after ATRP of tert -butyl acrylate (t BA) and thermolysis of poly( tert -butyl acrylate) (Pt BA) block. Upon thermolyzing PS-PIB-PS macroinitiator, poly(styrene-b -isobutylene) 2 -s -poly(acrylic acid) [(PS-PIB)2 - s -PAA] stars were obtained in the same manner.