Synthesis and Properties of a Coil-g-Rod Polymer Brush by Combination of ATRP and Alternating Copolymerization

We synthesized a coil-g-rod polymer brush, poly{styrene-alt-(maleimide-g-poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene})}(P{St-alt-(MI-g-PMPCS)}), by alternating copolymerization of styrene (St) and maleimide-terminated poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (MI-PMPCS) with the grafting through strategy. MI-PMPCS was synthesized by using the protection strategy in which the initiator protected by DielsAlder reaction with furan was used to initiate atom transfer radical polymerization of 2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene (MPCS), and then furan was deprotected by retro-DielsAlder reaction. 1H NMR, gel permeation chromatography (GPC), and GPC coupled with multiangle laser light scattering were used to determine the chemical structures and molecular weights of the polymer brushes. The highest degree of polymerization (DP) of the main chain is 103, which is significantly large for polymer brushes with rigid side chains. The main-chain length increases with increasing feeding ratio and decreases with increasing side-chain length. The thermal properties and transitions of all samples were studied by thermogravimetric analysis and differential scanning calorimetry. Finally, polarized light microscopy and one-/two-dimensional wide-angle X-ray diffraction were used to examine the phase structures of the polymer brushes. To our surprise, when the DP of the side chain is below the critical value of PMPCS for forming liquid crystalline (LC) phases, the polymer brush can form the LC phase. On the one hand, with the longer main chain, the LC phase of the polymer brush becomes less ordered. On the other hand, with the longer side chain, the LC phase of the polymer brush becomes more ordered. For polymer brushes with LC side chains, the LC phase of the polymer brush may be less ordered than that of the LC PMPCS side chain.