Addition polymerization of mesogenic monomers

Addition polymerization of alkenes can be performed in several ways. Most commonly polymerization is initiated using reactive free radicals provided by an added reagent. Initiation is usually achieved thermally, but photochemical initiation is also relevant here. Initiation procedures using anionic or cationic reagents are attractive alternatives since they offer better control of molecular mass some cationic procedures are mentioned below (Section 7.5.2.1). 

The method can be adapted to the preparation of copolymers or cross-linked network polymers. For copolymerization it is desirable to use monomers of very similar reactivity so that their known molar proportion will be accurately expressed in the copolymer composition this should also be checked, for example using spectroscopy. So long as monomers are of similar structure near the reactive double bond (e.g. both methacrylates with several CH2 groups in the spacer) then their reactivities should be virtually identical. 

For network polymer formation a small amount of an appropriate divinyl monomer can be included in the monomer feed as a cross-linking agent. By analogy with copolymerization, as long as the monomer and cross-linker are of similar reactivity then the degree of cross-linking will be statistical and can be controlled by the molar proportion of monomer to cross-linker. An example of the preparation of acrylic network LCPs is shown in Fig. 

Polymerization can be initiated using radicals generated via the UV irradiation of appropriate initiators in the presence of the monomer. This approach to comb LCPs, first reported by Broer et a/., is attractive since it can be performed at a preselected temperature. Therefore, monomers can be polymerized in an aligned LC phase and the effects of ordered polymerization can be examined. 

Bulk polymerization of acrylate (I) shown in Fig. 7.21 was carried out using either of the initiators (II) or (III). The monomer (I) exhibited monotropic nematic and smectic phases on cooling from the isotropic 

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