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 DP_n value 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 are controlled by the mole ratio of the two, taken during the polymerization. We show that the terminal methyl ester groups in the HBP are 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, with evidence for this being obtained from DSC and X-ray scattering studies. Furthermore, 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 is used as the A-R comonomer. The terminal unsaturation is 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 an 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.