Living Cationic Sequential Block Copolymerization of Isobutylene with 4‐tert‐Butoxystyrene: Synthesis and Characterization of Poly(p‐hydroxystyrene‐b‐isobutylene‐b‐p‐hydroxystyrene) Triblock Copolymers

The living polymerization of p‐tert‐butoxystyrene (tBuOS) was studied in methylcyclohexane (MeChx)/methylchloride (MeCl) 60/40 v/v solvent mixture at –80°C. The model initiator 1,1,‐ditolylethylene (DTE) capped 2‐chloro‐2,4,4‐trimethylpentane (TMPCl) was formed in situ in conjunction with TiCl₄. Lowering the Lewis acidity by the addition of Ti(OIp)₄ was necessary to induce a rapid and controlled polymerization of tBuOS. Well‐defined polymers with controlled molecular weights, however, were only obtained at a narrow [Ti(OIp)₄]/[TiCl₄]=0.83–0.86 ratio. Above this ratio, the polymerization of tBuOS was slow and became absent at [Ti(OIp)₄]/[TiCl₄]≥1.18. At ratios lower than 0.83, the polymerization was too rapid and the initiator efficiency was lower than 100%. The living polymerization of tBuOS was also studied with SnBr₄ as Lewis acid. After capping TMPCl with DTE, Ti(OIp)₄ was added to reach [Ti(OIp)₄]/[TiCl₄]=1.2, followed by the addition of tBuOS and SnBr₄. SnBr₄ induced a well‐controlled living polymerization approximately first order in [SnBr₄], and the polymers exhibited close to theoretical M _ns and low polydispersity indices (PDI<1.2). The success of the method was also demonstrated by the clean synthesis of poly(isobutylene‐b‐p‐tert‐butoxystyrene) PIB‐b‐PtBuOS diblock copolymers. PtBuOS‐b‐PIB‐b‐PtBuOS triblock copolymer thermoplastic elastomers were prepared by employing 5‐tert‐butyl‐1,3‐bis(1‐methoxy‐1‐methylethyl)benzene (DCE) as a difunctional initiator for the living polymerization of IB followed by capping with DTE and substitution of TiCl₄ with SnBr₄ for the polymerization of tBuOS. Deprotection of the triblock copolymer in the presence of catalytic amount of HCl yielded poly(p‐hydroxystyrene‐b‐isobutylene‐b‐p‐hydroxystyrene) (PHOS‐b‐PIB‐b‐PHOS). PHOS‐b‐PIB‐b‐PHOS with 39.3 wt% p‐hydroxystyrene content exhibited typical characteristic of a thermoplastic elastomers (TPEs) with tensile strength of 18 MPa and ultimate elongation of 300%.

Keywords:

• carbocationic polymerization
• block copolymer
• poly(p‐hydroxystyrene‐b‐isobutylene‐b‐p‐hydroxystyrene)
• thermoplastic elastomer

5 Acknowledgment

Support by Boston Scientific Corporation is gratefully acknowledged.