No abstract

The synthesis of block copolymers, cooligomers or cotelomers performed from radical polymerization and radical telomerization is discussed. In the first part, the two main methods proposed for obtaining block copolymers by radical polymerization are reviewed. The first method concerns the use of multifunctional micro-or macroinitiators that decompose according to two processes or at two different temperatures, allowing the stepwise insertion of different base units. The second method, more recently proposed, tends to control the reactivity of the growing radicals by the use of either additional stable free radicals or organometallic compounds acting as counter radical, allowing reversible termination steps. These methods still in full development and often referred to as "living" processes, allow well defined block copolymers with narrow molecular weight distributions to be produced. The second part of the review deals with methods of preparation by radical telomerization. Besides the traditional process involving the coupling of co-functional oligomers or telomers and the bistelomerization, the dormant or living telomerization methods are also described. In this last case, the "living" character is achieved by the use of well-chosen transfer agents with specific cleavable bonds leading to reversibility of the termination step.

Liquid crystals were obtained from poly(4-vinylpyridine) quaternized with 1-bromooctane and 1-bromododecane w-substituted with 4'-methoxy-4-biphenyloxy mesogenic groups. Their thermotropic polymorphism was studied by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. Three types of smectic mesophase, namely A, B, and E, were identified and described from a structural point of view. Their structure was found to be very similar to that previously reported for their monomer counterparts. The mesogenic pendant groups are arranged in single layers with the polymer backbones inserted in between. As a function of decreasing degree of quaternization, the smectic period increases slowly, reflecting the expansion of the sublayers containing the polymer backbones along with the free pyridine rings.