Through-chain

Chemical initiation generates organic radicals, usually by decomposition of a2o (11) or peroxide compounds (12), to form radicals which then react with chlorine to initiate the radical-chain chlorination reaction (see Initiators). Chlorination of methane yields all four possible chlorinated derivatives methyl chloride, methylene chloride, chloroform, and carbon tetrachloride (13). The reaction proceeds by a radical-chain mechanism, as shown in equations 1 through. Chain initiation... 

Transfer constants for polystyrene chain radicals at 60° and 100°C, obtained from the slopes of these plots and others like them, are given in the second and third columns of Table XIII. Almost any solvent is susceptible to attack by the propagating free radical. Even cyclohexane and benzene enter into chain transfer, although to a comparatively small extent only. The specific reaction rate at 100°C for transfer with either of these solvents is less than two ten-thousandths of the rate for the addition of the chain radical to styrene monomer. A fifteenfold dilution with benzene was required to halve the molecular weight, i.e., to double l/xn from its value (l/ rjo for pure styrene (see Fig. 16). Other hydrocarbons are more effective in lowering the degree of polymerization through chain transfer. 

Branching through chain transfer in vinyl polymerizations has been... 

Nonlinear structures may arise in vinyl polymerizations through chain transfer with monomer or with previously formed polymer molecules, but such processes usually occur to an extent which is scarcely significant. A more common source of nonlinearity in the polymerization of a 1,3-diene is the incorporation in a growing chain of one of the units of a previously formed polymer molecule. The importance of both branching by chain transfer and cross-linking by addition of a polymer unit increases with the degree of conversion of monomer to polymer. 

Fig. 20 Chain sliding diffusion model of primary nucleation. Polymer chains are rearranged from Gaussian shape within the melt into a nucleus through chain sliding diffusion within the nucleus and disentanglement within the interface. Bottom graph indicates change in free energy of the nucleus against N...

Grafting through Chain-transfer Fist the base polymer (for example Polyacrylonitrile) is dissolved in a monomer such as styrene and then an initiator is added. The two possible ways of polymerisation are ... 

There has been nothing like the enthusiasm for the application to these systems of the theoretical equations, which we have noted in the previous sections and will encounter in the next. Nevertheless, a number of features are present which are qualitatively consistent with the discussions in Sec. 5.8.1 and which are in part illustrated in Table 5.11. There is a correlation of rate constant with the driving force of the internal electron transfer. -pjjg p. itro-phenyl derivative is a poorer reducing agent when protonated and k is much less than for the unprotonated derivative. Consequently disproportionation (2A 3) becomes important. Although there are not marked effects of structural variation on the values of A , the associated activation parameters may differ enormously and this is ascribed to the operation of different mechanisms."" The resonance-assisted through-chain operates with the p-... 

FIGURE 2.20 Physical cross-linking through chain entanglement. 

Chain reactions, including ionic chain polymerization reactions, consist of at least three steps initiation, propagation, and termination. Termination generally occurs through chain transfer producing a new ion and the dead polymer. 

The biosynthesis of many hydroxylated natural products proceeds through regio- and enantioselective modification of polyketides, which are assembled through chain elongation via acetate or propionate units [2]. The enzymes responsible for the chain elongation and subsequent reduction, elimination, aromatiza-tion, and further modifications are classified as polyketide synthases [3]. These multifunctional enzymes have been used for whole-cell biotransformation toward unnatural metabolites that are within the scope of combinatorial biosynthesis... 

Chaudhuri, D. K. R., and J. J. Hermans Grafting onto cellulosic macromolecules through chain transfer to mercaptoethyl side chains. I. Experimental procedure and results. J. Polymer Sci. 48, 159 (1960). 

Lamellar breakup occurs through chain pulling and unfolding the chains pulled still connect the fragments of the lamellae. 

Several theoretical studies have been carried out on a phenomenological basis to justify the idea that a dynamic slip can indeed occur [43-45]. New experimental techniques and theoretical treatments must be developed to reveal the molecular origin of dynamic slip and characteristic time scales governing the dynamic processes in the melt/wall interfacial region. Our own experiments on polyethylenes [27-29] have persistently indicated that slip, either through chain desorption on weakly interacting surfaces or interfacial chain disentanglement... 

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