Poly(styrene‐b‐isobutylene) multiarm star‐block copolymers

Abstract

Multiarm star-branched polymers based on poly(styrene-b-isobutylene) (PS-PIB) block copolymer arms were synthesized under controlled/living cationic polymerization conditions using the 2-chloro-2-propylbenzene (CCl)/TiCl4/pyridine (Py) initiating system and divinylbenzene (DVB) as gel-core-forming comonomer. To optimize the timing of isobutylene (IB) addition to living PS⊕, the kinetics of styrene (St) polymerization at −80°C were measured in both 60 : 40 (v : v) methyl cyclohexane (MCHx) : MeCl and 60 : 40 hexane : MeCl cosolvents. For either cosolvent system, it was found that the polymerizations followed first-order kinetics with respect to the monomer and the number of actively growing chains remained invariant. The rate of polymerization was slower in MCHx : MeCl (kapp = 2.5 × 10−3 s−1) compared with hexane : MeCl (kapp = 5.6 × 10−3 s−1) ([CCl]o = [TiCl4]/15 = 3.64 × 10−3M; [Py] = 4 × 10−3M; [St]o = 0.35M). Intermolecular alkylation reactions were observed at [St]o = 0.93M but could be suppressed by avoiding very high St conversion and by setting [St]o ≤ 0.35M. For St polymerization, kapp = 1.1 × 10−3 s−1 ([CCl]o = [TiCl4]/15 = 1.82 × 10−3M; [Py] = 4 × 10−3M; [St]o = 0.35M); this was significantly higher than that observed for IB polymerization (kapp = 3.0 × 10−4 s−1; [CCl]o = [Py] = [TiCl4]/15 = 1.86 × 10−3M; [IB]o = 1.0M). Blocking efficiencies were higher in hexane : MeCl compared with MCHx : MeCl cosolvent system. Star formation was faster with PS-PIB arms compared with PIB homopolymer arms under similar conditions. Using [DVB] = 5.6 × 10−2M = 10 times chain end concentration, 92% of PS-PIB arms (Mn,PS = 2600 and Mn,PIB = 13,400 g/mol) were linked within 1 h at −80°C with negligible star–star coupling. It was difficult to achieve complete linking of all the arms prior to the onset of star–star coupling. Apparently, the presence of the St block allows the PS-PIB block copolymer arms to be incorporated into growing star polymers by an additional mechanism, namely, electrophilic aromatic substitution (EAS), which leads to increased rates of star formation and greater tendency toward star–star coupling. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1629–1641, 1999