Dendritic-linear A(x)B(x) block copolymers prepared via controlled ring-opening polymerization of lactones from orthogonally protected multifunctional initiators

A new concept in initiator design using orthogonal protecting groups provid es a versatile synthetic route to novel dendritic-linear AB, A(2)B, AB(2), A(2)B(2), and A(3)B(3) amphiphilic block copolymers. The A blocks are compo sed of the first- through third-generation dendrons from 2,2-bis(hydroxymet hyl)propionic acid (bis-MPA), and the B blocks are poly(epsilon -caprolacto ne) prepared by living ring-opening polymerization (ROP). The enabling feat ure in the preparation of these dendritic-linear block copolymers is the sy nthesis of orthogonally protected multifunctional initiators. These initiat ors allow for the selective coupling of the dendritic fragments and, upon s elective and quantitative deprotection of the initiating species, controlle d ROP of epsilon -caprolactone from this central core molecule. Because of the sensitivity of the poly(epsilon -caprolactone) and dendrons toward hydr olysis, protection and deprotection schemes for the initiators as well as t he 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 eit her benzyl ether or benzylidene acetal groups. Deprotection of these hydrox yl groups by catalytic hydrogenolysis yielded the requisite nucleophilic in itiators for the controlled ROP of epsilon -caprolactone in the presence of a suitable organometallic promoter. Narrowly dispersed products with predi ctable molecular weights were obtained. NMR and GPC studies of the block co polymers confirmed the versatility of the orthogonally protected multifunct ional initiator approach to dendritic-linear copolymers. Upon deprotection of the surface hydroxyl groups on the dendrons, the macromolecules became a mphiphilic, 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.