Molecular Engineering of Side Chain Liquid Crystalline Polymers by Living Polymerizations

Abstract “Living” anionic, cationic, metalloporphyrin and ring-opening metathesis polymerizations have been used to prepare well-defined side-chain liquid crystalline homopolymers, block and graft copolymers and statistical copolymers. This paper analyzes their successes and failures by reviewing the mechanistic aspects and experimental conditions of each type of polymerization, and identifies other classes of mesogenic monomers that could be polymerized in a controlled manner in the future. The emerging structure/property relationships are then identified using welldefined SCLCPs in which only one structural feature is varied while all others remain constant. The thermal transitions of liquid crystalline polymethacrylates, polynorbomenes and poly(vinyl ether)s reach their limiting values at less than 50 repeat units, which are generally equal to those of the corresponding infinite molecular weight polymers. Increasing spacer length depresses the glass transition of SCLCPs, and consequently often uncovers mesophases that are not observed without a spacer. The crystalline melting of tactic SCLCPs also tends to decrease (with odd-even alternation) with increasing spacer length. Without additional order within the polymer backbone due to high tacticity, mesogenic side-chains generally do not crystallize until the spacer contains at least nine carbon atoms. As the flexibility of the polymer backbone increases, the glass transition temperature decreases, and the side chains are able to crystallize at shorter spacer lengths and form more ordered mesophases. The isotropization temperature (Ti = ΔHiΔSi) also increases since the change in entropy decreases more rapidly than the change in enthalpy with increasing backbone flexibility. However, the mesogenic side groups of most highly tactic polymers, which are less flexible than the corresponding atactic polymers, are evidently in the proper configuration to crystallize and/or form ordered phases. If the mesogen density is taken into account, the increase in ΔHi and ΔSi per methylenic unit in the spacer are equivalent for a given mesophase, and increase as the order of the mesophase increases. The discontinuity and/or change in the slope of ΔΔHi/-CH2- and ΔΔSi/ -CH2- with a change in the type of mesophase can be used to confirm that a phase change has occurred with the addition or subtraction of one methylenic unit in the spacer of a homologous series. Model compounds corresponding to exactly one repeat unit of the polymer, or which take into account only the (appropriately substituted) mesogen and spacer, mimic the phase behavior of the corresponding SCLCPs well. The monomers themselves, which have chemical structures very different from that of the polymer backbone, are the least appropriate model compounds for most SCLCPs. The effect of polydispersity has not been clarified yet, although it may manifest itself in broad phase transitions if the broad polydispersity is accompanied by polydispersity in molecular architecture, and the molecular architectures are immiscible. Liquid crystalline block and graft copolymers microphase separate into classic morphologies, but with the mesogens within the liquid crystalline block organize anisotropically if the blocks are sufficiently long. Although the same mesophase is generally formed by the copolymers and homopolymer, the phase diagram is asymmetric and less ordered mesophases may result if spheres of the liquid crystalline block are dispersed in a matrix of the other block. The morphology and thermotropic behavior of diblock and ABA and BAB triblock copolymers of 2-(cholesteryloxycarbonyloxy)ethyl methacrylate and styrene are identical when the volume fraction of the blocks are equal. Statistical copolymers also require a minimum concentration of the mesogenic monomer to form a mesophase. The isotropization temperatures of statistical copolymers based on two mesogenic monomers whose homopolymers exhibit identical mesophases follow ideal solution behavior as a function of copolymer composition. Copolymers based on structural units which are not isomorphic do not exhibit their respective mesophases over the entire copolymer composition, and intermediate compositions may exhibit an entirely different phase.