Hybrid anionic block copolymerization: From organocatalysis-streamlined crossover to internally microphase-separated aggregates

Abstract

Polyolefin-b-poly(ethylene oxide) (PEO) represents the most widely investigated amphiphilic block copolymers. So far, one-pot continuous synthesis of such hybrid block copolymers has only been fulfilled by anionic polymerization through sequential addition of vinyl monomers and ethylene oxide (EO). It still remains challenging to achieve altogether high block efficiency, high polymerization efficiency, and high molar mass for PEO. Here, we report a one-pot hybrid block copolymerization approach to polyisoprene/polystyrene(PI/PS)-b-PEO, in which PI/PS are formed by sBuLi-initiated anionic vinyl-addition polymerization, then in situ employed as macroinitiators for the anionic ring-opening polymerization (ROP) of EO aided by an organic Lewis pair. The cooperative (dual-ion-complexing) catalytic effect of organobase and triethylborane is proven, for the first time, effective for lithium alkoxide initiator system, allowing to achieve at room temperature high ROP activity (complete EO conversion and PEO of 3-64 kg/mol reached in 1-6 h), narrow molar mass distribution, controlled block lengths and composition. Density functional theory calculation shows that phosphazene bases are particularly effective, compared with N-heterocyclic bases, for complexing with Li⁺ and enhancing the nucleophilicity of oxyanion. The rate of ROP is also affected by Li⁺-induced aggregation of the chain-end ion pairs, which though can be offset by adequate catalyst loadings. The versatility of this approach is further demonstrated in the one-pot synthesis of tri-/tetrablock ter-/quaterpolymers constituted by PI, PS, PEO, and poly(propylene oxide). Of great interest, PS-b-PI-b-PEO triblock terpolymer with a specific composition is found to form internally microphase-separated micellar aggregates when dispersed in water.

Keywords

aggregation / anionic polymerization / block copolymer / organocatalysis / ring-opening polymerization

Author summay

Lijun Liu and Heng Li contributed equally to this study.