Synthesis of ABA Triblock Copolymers via Ring Opening Metathesis Polymerization Using a Bimetallic Initiator: Influence of a Flexible Spacer in the Side Chain Liquid Crystalline Block

Abstract

A series of monomers with side chain liquid crystals (SCLCs) were synthesized for ring opening metathesis polymerization (ROMP). The liquid crystals (LCs) used were 4-hydroxybenzoic acid 4-methoxyphenyl ester (MPOB-H), which is known to exhibit a nematic liquid crystalline phase, and biphenyl-4-carboxylic acid 4-(1-butoxycarbonylethoxy)phenyl ester (BPP4-H), which is known to exhibit a smectic phase. The side chains differed in spacer length, spacer type, and the nature of the LC. Monomers were polymerized using a bimetallic ROMP initiator [Mo(NAr)(OCMe3)2CH]2C6H4 (where Ar = 2,6-diisopropyl phenyl); both homopolymers and ABA type triblock copolymers, where the B block is the LC functional monomer, and A is methyltetracyclodocene (MTD) were prepared. The polymers gave unimodal peak distributions with polydispersities <1.22. Incorporation of a polyoxyethylene spacer decreased the glass transition temperature of the polymer block to −25 from +20 °C, the Tg when an alkyl spacer was used. Although no distinct LC phase was observed with the polyoxyethylene spacer when MPOB was used, when BPP4 was attached via a polyoxyethylene spacer, a distinct liquid crystalline transition was observed. Polymers with an alkyl spacer exhibited liquid crystalline behavior and good phase segregation on the basis of differential scanning calorimetry and small-angle X-ray scattering studies.