Polymer synthesis chain growth

Scheme 11.18 shows a reasonable mechanism for polymerization that consists of first complexation of alkene, followed by 1,2-insertion. Ideally, the two steps will continue until synthesis of high-molecular-weight polymer occurs. Chain growth will cease if chain transfer occurs. Scheme 11.19 indicates two possible mechanisms for chain transfer, both of which are reasonable and indistinguishable by experiment. In both cases, the steric bulk of the diimine ligand seems to hinder the transfer process.102... 

A fourth strategy involves the polymerization of a monomer out of multifunctional initiators, following a divergent ( core-firsf ) approach (Scheme 27.4) [6, 12-14]. All main CLP methods have been applied to the synthesis of core-first star polymers, including chain growth by LAP [43] and hving cationic polymerizations [44], ROMP [45], nitroxide-mediated polymerization (NMP) [46], ATRP [47], or RAFT polymerization via the so-called R-approach [48]. The conden-sative chain-growth polymerization method, as developed by Yokozawa etal, has also been applied to the synthesis of well-defined, core-first, star-shaped polyamides [49]. 

Synthetic polymers are classified by their method of synthesis as either chain-growth or step-growth. The categories ate somewhat imprecise but nevertheless provide a useful distinction. Chain-growth polymers are produced by chain-reaction polymerization in which an initiator adds to a carbon-carbon double bond of an unsaturated substrate (a vinyl monomer) to yield a reactive inter-... 

Conditions that are important to all chemical reactions such as stoichiometry and reactant purity become critical in polymer synthesis. In step growth polymerization, a 2% measuring and/or impurity error cuts the degree of polymerization or the molecular weight in half. In chain growth polymerization, the presence of a small amount of impurity that can react with the growing chain can kill the polymerization. 

Scheme 5. Structural and chronological overview of the macromonomers used for synthesis of dendronized polymers chain growth (a) and step growth polymerizations (b)...

The first type, termed sequential IPN s, involves the preparation of a crosslinked polymer I, a subsequent swelling of monomer II components and polymerization of the monomer II in situ. The second type of synthesis yields materials known as simultaneous interpenetrating networks (SIN s), involves the mixing of all components in an early stage, followed by the formation of both networks via independent reactions proceeding in the same container (10,11). One network can be formed by a chain growth mechanism and the other by a step growth mechanism. 

Nearly all synthetic polymers are synthesized by the polymerization or copolymerization of different "monomers." The chain growth process may involve the addition chain reactions of unsaturated small molecules, condensation reactions, or ringopening chain-coupling processes. In conventional polymer chemistry, the synthesis of a new polymer requires the use of a new monomer. This approach is often unsatisfactory for Inorganic systems, where relatively few monomers or cyclic oligomers can be Induced to polymerize, at least under conditions that have been studied to date. The main exception to this rule is the condensation-type growth that occurs with inorganic dl-hydroxy acids. 

Because the opportunities for controlled chain growth are more restricted in inorganic than in organic systems, an alternative approach to polymer synthesis becomes appealing. This involves the use of substitution processes carried out on a preformed reactive polymeric intermediate. In this way molecular diversity can be introduced by different substitution reactions rather than by a diversification of the polymerization process. 

This chapter is divided into three main sections fluoropolymer synthesis, nonfluorinated polymer synthesis, and polymer characterization in C02. In the fluoropolymer synthesis section, solution, precipitation, biphasic, and continuous polymerizations will be described. Several examples of nonfluorinated heterogeneous chain-growth polymer syntheses will be followed by step-growth polymerizations. A brief summary of polymer characterization methods in C02 will conclude the chapter. 

SCHEME 4. Synthesis of chain-growth polymers containing metal-metal bonds along their backbones. 

Polymer synthesis is a complicated process that can be carried out applying different methodologies starting from appropriate monomers [193-196], The most important methods of polymer synthesis are step-growth polymerization and chain reaction polymerization [194,196],... 

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