Polymeric Initiator-Based Modifications

Functional homopolymers can be synthesized by essentially two different methods. The first and more preferred way is to use a functional initiator which will ensure a high rate of chain end functionality. For instance, the polymerization of St initiated by a unimolecular terpyridine-functionalized nitroxide initiator yields well-defined PS homopolymers. The second technique is based on post-polymerization modifications. In this case, the reaction between mPEG and chloroterpyridine yields terpyridine-functionalized PEG building blocks, as illustrated in Scheme 13. 

Recently, Caster et al. described the surface modification of multifilament fibers such as nylon or Kevlar [70]. Coating techniques using preformed ROMP-based polymers and process contact metathesis polymerization (CMP), initially described by Grubbs et al. [71], were both used. The latter involves a procedure where the initiator is physisorbed onto the surface of a substrate and fed with a ROMP-active monomer that finally encapsulates the substrate. These modified fibers showed improved adhesion to natural rubber elastomers. 

The selection and chemical modification of the current generation of chemically and physically robust stationary phases with narrower particle and pore size distributions has been based on the developmental effort that has occurred over the past 20 years. Initially chemically modified, deformable polymeric gels were used, such as the crosslinked agaroses, dextrans, or acrylate-based copolymers, but more recently various classes of highly refined type I and type II silicas and other ceramic materials, or new classes of controlled porosity polymeric organic materials have found increasing application. 

The process of polymerization consists in general of three steps initiation, propagation, and termination. In radical polymerization, a catalyst is usually employed as a source of free radicals, the primary radicals. A fraction of these initiate a rapid sequence of reactions with monomer molecules, the primary radical thus growing into a polymer radical. Radical activity is destroyed by reaction of two radicals to form one or two molecules. This termination reaction is called mutual recombination, if only one molecule is formed. Termination by disproportionation results in two molecules. For many common monomers, recombination is the normal mode of termination and the kinetic treatment here is based on this termination reaction. Only slight modifications are required for polymerizations in which termination occurs by disproportionation. If both termination processes occur, another variable must be introduced to describe the kinetics of the system fully. 

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