Living Coupling Reaction in Living Cationic Polymerization. 2. Synthesis and Characterization of Amphiphilic A2B2Star−Block Copolymer: Poly[bis(isobutylene)-star-bis(methyl vinyl ether)

Abstract

Amphiphilic A2B2 star−block copolymers (A = polyisobutylene (PIB) and B = poly(methyl vinyl ether) (PMeVE)) have been prepared via the living coupling reaction of living PIB, using 2,2-bis[4-(1-tolylethenyl)phenyl]propane (BDTEP) as a living coupling agent, followed by the chain ramification reaction of methyl vinyl ether (MeVE) at the junction of the living coupled PIB. Model reactions for the synthesis of A2B2 star−block copolymers indicated that the fine-tuning of Lewis acidity to the reactivity of MeVE is a crucial step for the structural integrity of the resulting A2B2 star−block copolymers. Side products were negligible using a [Ti(OEt)4]/[TiCl4] ratio of 0.7 and the minimum tuning time (∼5 min). Fractionation of the crude A2B2 star−block copolymer was carried out on a silica gel column, and on the basis of the weights of fractions, the purity of the crude A2B2 star−block copolymer was calculated to be ≥93.5%. Two Tgs (−60 °C for PIB and −20 °C for PMeVE) were observed for the star−block copolymer by DSC indicating the presence of two microphases. An A2B2 star−block copolymer with 80 wt % PMeVE composition ((IB45)2-s-(MeVE170)2) exhibited a critical micelle concentration (cmc) of 4.25 × 10-4 M in water, which is an order of magnitude higher than cmcs obtained with linear diblock copolymers with same total Mn and composition (IB90-b-MeVE340) or with same segmental lengths (IB45-b-MeVE170). This suggests that block copolymers with star architectures exhibit less tendency to micellization than their corresponding linear diblock copolymers. Average particle sizes in aqueous solution above the cmc were measured to be from 41 to 177 nm, depending on the architecture and/or the molecular weight.