Polymers with quadruple hydrogen-bonding end groups: controlling molecular weight using a small molecule photoswitch

Abstract

The molecular weight and topology of polymer chains is typically defined during their synthesis, after which these parameters remain fixed. This limitation has motivated efforts to develop reversibly reconfigurable polymers, which offer opportunities for advanced applications and/or efficient reprocessing. Herein, we report the preparation of a library of poly(methyl acrylate) (PMA) homopolymers and a poly(methyl methacrylate) (PMMA) homopolymer bearing pendant hydrogen bonding motifs (HBMs) which have the potential to generate reconfigurable materials. This approach includes the synthesis of alkyne-functional HBMs which were successfully coupled to an azide-functionalised RAFT agent via alkyne/azide “click” chemistry. The resulting ureidopyrimidinone (UPy) and amidonaphthyridine (NAP) RAFT agents could successfully be used to prepare PMA, but the attempt using the diamidonaphthyridine (DAN) RAFT agent was unsuccessful. In this example, a post-polymerisation technique was demonstrated as a viable alternative (in this case with a PMMA homopolymer). To demonstrate proof-of-concept that this can be achieved using a supramolecular approach: a small molecule photoswitch comprising ditopic azobenzene linked ureidopyrimidinone (UPy) was shown to effect reconfiguration of polymer molecular weight of DAN-functionalised PMMA under visible/UV light irradiation. In the cis (Z) photostationary state, intramolecular UPy·UPy homodimerization within the photoswitch was preferred, whereas in the trans (E) configuration, the need for UPy to satisfy its hydrogen-bonding requirements resulted in intermolecular DAN·UPy heterodimerization with the DAN motif of poly(methyl-methacrylate) resulting in a doubling of its molecular weight as observed by ¹H DOSY NMR spectroscopy.