Scheme of Radical Chain Polymerization

It is apparent from the above scheme of radical chain polymerization that the initiator fragment originating in the form of radical R are incorporated as end groups R [see Eq. (6.4)] in the polymer structure, sealing one or both ends of the polymer chains. 

Scheme of Radical Chain Polymerization 6.2.1 Overall Scheme... 

The principle of radical chain polymerization can by schematically illustrated by the following reaction scheme ... 

Table 6.2 shows the general range of values of the various concentrations, rates, and rate constants pertaining to the above kinetic scheme. These values are typical of radical chain polymerizations. 

Problem 6.8 Consider the following scheme of reactions for free-radical chain polymerization initiated by thermal homolysis of initiator with cage effect [6] ... 

In the Soviet study110, the following elementary stages were taken into account in the kinetic scheme of vinyl acetate polymerization chain transfer to the monomer, solvent, and polymer, and chain termination caused by the disproportionation of radicals. It was assumed that long-chain branches could be formed by chain transfer both to the acetate group hydrogen atoms and to the main chain hydrogen. 

Radical chain polymerization, as noted above, is a chain reaction which involves mainly three steps - initiation, propagation, and termination, taking place in sequence. The overall scheme of the polymerization of a vinyl or related monomer M, initiated by the decomposition of a free-radical initiator I, may be schematically represented as follows (Ghosh, 1990) ... 

Plasma initiation of the chain polymerization is due to formation of a primary free radical R( ), starting the traditional scheme (9-70), and by formation of positive or negative ion radicals, which are also capable of initiating the MMA polymerization. The primary free radical R( ) as well as the charged centers of polymer growth are formed from the absorbed monomers by electron/ion bombardment and UV radiation from plasma. Formation of a positive ion radical from an adsorbed MMA molectrle on the strrface under electron/ion bombardment and UV radiation can be schematically shown as the ionization process... 

The synthesis of macromolecules by the free radical chain polymerization of low molar mass compounds, denoted as monomers, commences with the generation of free radicals, which is conveniently performed through photoreactions of initiator molecules. The subsequent processes, i.e. propagation, including chain transfer, and termination, are thermal (dark) reactions, which are not affected by light The simplified overall mechanism is described in Scheme 10.1. 

The usual equations that are used to describe free radical chain polymerizations are not readily applicable to the polymerization of PTFE because the reaction does not meet the first requirement for application of the usual kinetic scheme, the steady state assumption [41, 42]. The CF2 radical has a very long lifetime, and usual termination reactions (combination and disproportionation] do not seem to occur. The need to have an extremely high molecular weight in order to have adequate physical properties requires a reaction system essentially free of any material that will chain transfer, eliminating that mode of termination. The result is that molecular weight of PTFE increases with time of polymerization, unlike most other radical-chain polymerizations. 

Polymers may be synthesized by two major kinetic schemes, chain and stepwise polymerization. The most important of the chain polymerization methods is called free radical polymerization. 

The basic chemistry upon curing is the homopolymerization of a (meth)acrylate functionality as is depicted in Scheme 16.26. This polymerization is a radical chain polymerization. The propagating radical is carbon-centered, and therefore this polymerization is sensitive to oxygen inhibition as oxygen can quench carbon-centered radicals very effectively. As a result of this oxygen inhibition the top layer is generally not as thoroughly cured as the bulk of the material. 

Radical termination is a bimolecular reaction that involves two polymeric radicals. Because of high reactivity (fet lO lmoh s" ) and extremely low concentration (10 -10 moir ) of radicals, chain diffusion becomes critically important and often determines the magnitude of termination rate. " In general, radical termination involves three steps, as shown in Scheme 1. First, two radical chains approach each other by translational diffusion. Once in vicinity, two radical centers come together by segmental diffusion. Chemical activation of two radical centers takes almost no time to occur due to their reactive nature. 

Free-Radical Polymerization The free-radical chain polymerization of appropriate monomers can be initiated with the aid of Vis/UV hght. Here, light serves only as an initiating tool, and does not interfere with the propagation and termination stages of the chain process. The initiation affords the presence of an appropriate light-absorbing initiator, as shown in Scheme 3.1. 

Scheme 5.4 Kinetic scheme of free-radical chain polymerization in homogeneous liquid phase. M monomer P polymer.

Thiol-ene polymerization was first reported in 1938.220 In this process, a polymer chain is built up by a sequence of thiyl radical addition and chain transfer steps (Scheme 7.17). The thiol-ene process is unique amongst radical polymerizations in that, while it is a radical chain process, the rate of molecular weight increase is more typical of a step-growth polymerization. Polymers ideally consist of alternating residues derived from the diene and the dithiol. However, when dienes with high kp and relatively low A-, monomers (e.g. acrylates) are used, short sequences of units derived from the diene are sometimes formed. 

Addition of TEMPO post-polymerization to a methacrylate polymerization provides an unsaturated chain end (Scheme 9.52)i07 sw presumably by disproportionation of the PMMA propagating radical with the nitroxide. For polymers based on monosubstituted monomers (PS,1 0" PBA59,[Pg.534]

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