Living cationic sequential block copolymerization: synthesis and characterization of poly(4-(2-hydroxyethyl)styrene-b-isobutylene-b-4-(2-hydroxyethyl)styrene) triblock copolymers

INTRODUCTION With the development of living cationic polymerization of isobutylene (IB)1 and styrenic monomers,2–5 the synthesis of ABA triblock copolymer thermoplastic elastomers (TPEs) with polyisobutylene (PIB) as rubbery middle segments and styrenic polymers as plastic outer segments became possible. TPEs with PIB rubbery segments inherently exhibit superior oxidative, chemical and thermal stability, as well as outstanding barrier and damping properties, compared with polybutadieneor polyisoprene-based block copolymer TPEs.6–13 Recently, some of these PIB-based TPEs have been under investigation as matrices for controlled drug delivery and scaffolds for tissue engineering. Particularly, the poly(styrene-b-isobutylene-b-styrene) triblock copolymer is currently used as the drug carrier matrix for the TAXUS Express Paclitaxel-Eluting Coronary Stent system (Boston Scientific Corporation, Natick, MA, USA).14–16 PIB-based TPEs are relatively hydrophobic. Hydrophilic drugs, such as paclitaxel, are incompatible with PIB-based TPEs. The polymer and the drug separate into separate domains, and drug diffusion through PIB-based TPEs is extremely slow. Recently, it was shown that the polarity and the hydrophilic nature of the end blocks can improve the drug-polymer miscibility.17 Therefore, the introduction of polar groups is an important strategy to modulate properties of these PIB-based TPEs. However, many polar monomers do not undergo cationic polymerization. Although the combination of cationic polymerization with other living polymerization techniques has been successful for the synthesis of PIB block copolymers with, for example, poly(L-lactide)18 and poly(e-caprolactone)19 end segments, mechanistic transformation reactions are rather complicated. It is more useful to introduce (protected) polar functional groups to monomers that undergo cationic polymerization, as block copolymer synthesis may be accomplished in a relatively simple one-pot procedure using these monomers. Faust and co-workers have recently reported on the synthesis of poly(p-hydroxystyrene-b-isobutylene-b-p-hydroxystyrene) (PHOSb-PIB-b-PHOS) by deprotection from poly(p-(tert-butyldimethylsilyloxy)styrene-b-isobutylene-b-p-(tert-butyldimethylsilyloxy)styrene)6 or poly(p-tert-butoxystyrene-b-isobutylene-b-p-tert-butoxystyrene).20 The preparation of these precursor triblock copolymers was accomplished by living sequential block copolymerization of IB with p-(tertbutyldimethylsiloxy)styrene or p-tert-butoxystyrene in a one-pot capping-tuning process, which involves capping the living PIB chain ends with 1,1-diphenylethylene (DPE) followed by the addition of the second monomers. This work aims at the application of the capping-tuning method for the synthesis of a new triblock copolymer of poly(4-(2-(tert-butyldimethylsiloxy)ethyl)styrene-b-isobutylene-b-4-(2-(tert-butyldimethyl siloxy)ethyl)styrene) (PTBDMES-b-PIB-b-PTBDMES). Hydrolysis of this triblock copolymer gives rise to poly(4-(2-hydroxyethyl)styreneb-isobutylene-b-4-(2-hydroxyethyl)styrene) (PHOES-b-PIB-b-PHOES), which can further be chemically modified to modulate the polarity of the triblock copolymer.