Modulation of side chain crystallinity in alternating copolymers

Abstract

A series of crystalline sequence-controlled polymers based on various fatty acid-based maleimide (MF) and methoxy poly(ethylene glycol) (mPEG) containing styrenic (VBP) monomers are prepared through reversible addition–fragmentation chain transfer (RAFT) polymerization. Predominately alternating arrangement of the two monomers throughout the polymer main chain is systematically investigated by both ¹H and ¹³C NMR spectroscopic analysis. The copolymers form a well-ordered lamellar pattern consisting of an alternate layer of mPEG and fatty acid alkyl segments, where the inter-lamellar spacing (d) increases on a regular basis as a consequence of only increasing the mPEG chain lengths, as determined by small-angle X-ray scattering (SAXS) analysis. The significant crystalline nature of the copolymers is found to be in the form of lattice fringes from transmission electron microscopy (TEM). Remarkably, differential scanning calorimetry (DSC) thermograms reveal a gradual enhancement in crystalline melting temperature (T_m) associated with a given length of fatty acid segments as a result of simply increasing the mPEG chain lengths. Notably, this variation is observed only when the corresponding monomers are placed alternatively in the polymer side chains, rather than their random arrangements. The side chain crystallinity was further evidenced from polarized optical microscopy (POM) with the appearance of good birefringent patterns. The study reported herein presents a unique approach to modulate the side chain crystallinity resulting in the development of attractive sequence-controlled polymers.