Revisiting AB2 + A-R copolymerization: Direct access to Janus and peripherally clickable hyperbranched polyesters

Abstract

Hyperbranched polymers (HBPs) are often prepared by the self-condensation of AB2 type monomers, which results in compact globular structures with numerous B-groups on their periphery; the number of terminal B-groups increases with the DPn of the polymer. Since the AB2 monomer has one equivalent of B-group in excess, copolymerization with up to one equivalent of an A-R type molecule provides direct access to HBPs whose periphery is decorated with the desired R-unit. Here, we demonstrate that direct melt condensation of a simple AB2 monomer, dimethyl 5-(6-hydroxyhexyloxy) isophthalate (DMHHI), with MPEG-OH (A-R1) and/or cetyl alcohol (A-R2), generates peripherally decorated HBPs; when two different A-R molecules are used together, the relative amounts installed is controlled by the mole-ratio of the two, taken during the polymerization. We show that the terminal methyl ester groups in the HBP is almost completely replaced (>95 %), making this a very effective and direct strategy for peripheral installation of multiple segments. Importantly, we show that when alkyl and PEG segments are co-installed, self-segregation occurs to generate a Janus structure; evidence for this was obtained from DSC and X-ray scattering studies. Further, we also demonstrate the direct synthesis of peripherally clickable HBPs using this AB2 + A-R copolymerization strategy, wherein 10-undecen-1-ol or 10-undecyn-1-ol were used as the A-R comonomer; the terminal unsaturation was then used to install PEG segments, using radical-initiated thiol-ene or thiol-yne click reactions, to yield core-shell type amphiphilic structures. As expected, aqueous solutions of shell-PEGylated HBPs exhibited a LCST and the transition temperatures increased with the length of the PEG segment and its number density within the shell-region. This general copolymerization approach is scalable and can readily be extended to install different types of functional motifs on the periphery of globular hyperbranched constructs, thereby providing opportunities for a variety of potential applications.