Synthesis and characterization of ABA triblock copolymers containing smectic C* liquid crystal side chains via ring-opening metathesis polymerization using a bimetallic molybdenum initiator

A series of monomers suitable for ring-opening metathesis polymerization ( ROMP) containing a side chain liquid crystal mesogen have been synthesized, where the mesogen is biphenyl-4-carboxylic acid 4-(1-butoxycarbonyl- ethoxy) phenyl ester ( abbreviated as BPP4). Two of the monomers have BPP4 attached to norbornene with a 6- or 10-carbon spacer ((R)- 4'-(5-bicyclo[2.2.1] hept-5-en-2-yl-pentyloxy)-BPP4, NB6wBPP4, and (R)-4'-(10- bicyclo[2.2.1] hept-5-en-2-yl-decyloxy)- BPP4, NB10wBPP4, respectively), and the third monomer has BPP4 attached to norbornadiene via a 10-carbon spacer ((R)-4'-(10-bicyclo[2.2.1] hepta-2,5-dien-2-yl-decyloxy)BPP4, NBD10wBPP4). The monomers were polymerized by ROMP using the bimetallic initiator, {Mo(NAr)( ORF6)(2)[= CHC5H4]}(2)Fe (Ar = 2,6-diisopropylphenyl, ORF6 = OCMe(CF3)(2)). ABA triblock copolymers were also synthesized where the B = NBD10wBPP4 or NBnwBPP4 (n = 6, 10) and A = methyltetracyclododecene (MTD). All polymerizations are living with isolated yields greater than 90% and polydispersities ranging from less than 1.05 to 1.31. Polarized optical microscopy (POM) and differential scanning calorimetry (DSC) studies revealed glass transition temperatures for the inner block of the triblock copolymers at similar to 20 degrees C and smectic-to-isotropic transitions around 150 degrees C. Small-angle X-ray scattering (SAXS) indicated that the triblock copolymers exhibit phase segregation. In addition, it was determined from SAXS data that the triblock copolymer containing NB6wBPP4 forms smectic C* monolayers while the triblock copolymers that contain NB10wBPP4 and NBD10wBPP4 form smectic C* bilayers at room temperature. DMA of the triblock copolymers reveals an elastic plateau above the T-g of the center block, indicating these systems exhibit elastomeric behavior at elevated temperature and could form the basis of interesting materials for LC shape-memory elastomers.