Dendritic-linear A_xB_x block copolymers prepared via controlled ring-opening polymerization of lactones from orthogonally protected multifunctional initiators

Abstract

A new concept in initiator design using orthogonal protecting groups provides a versatile synthetic route to novel dendritic-linear AB, A2B, AB2, A2B2, and A3B3 amphiphilic block copolymers. The A blocks are composed of the first- through third-generation dendrons from 2,2-bis(hydroxymethyl)-propionic acid (bis-MPA), and the B blocks are poly(ε-caprolactone) prepared by living ring-opening polymerization (ROP). The enabling feature in the preparation of these dendritic-linear block copolymers is the synthesis of orthogonally protected multifunctional initiators. These initiators allow for the selective coupling of the dendritic fragments and, upon selective and quantitative deprotection of the initiating species, controlled ROP of ε-caprolactone from this central core molecule. Because of the sensitivity of the poly(ε-caprolactone) and dendrons toward hydrolysis, protection and deprotection schemes for the initiators as well as the dendrons were judiciously designed to be performed under mild conditions. Libraries of new initiators were prepared, composed of the first- through third-generation protected dendrons and hydroxyl groups protected with either benzyl ether or benzylidene acetal groups. Deprotection of these hydroxyl groups by catalytic hydrogenolysis yielded the requisite nucleophilic initiators for the controlled ROP of ε-caprolactone in the presence of a suitable organometallic promoter. Narrowly dispersed products with predictable molecular weights were obtained. NMR and GPC studies of the block copolymers confirmed the versatility of the orthogonally protected multifunctional initiator approach to dendritic-linear copolymers. Upon deprotection of the surface hydroxyl groups on the dendrons, the macromolecules became amphiphilic, with a polar, hydrophilic head and a nonpolar, hydrophobic tail. These materials provide a new class of polymeric surfactants, which might lead to novel supramolecular architectures in the constrained geometry of a monolayer film or in solution as self-assembled micelles.