Explain the general stable free radical polymerization mechanism.
Free radical polymerizations are of significant importance in the industrial sector for a variety of reasons. First, many monomers capable of undergoing chain reactions are available in large quantities from the petrochemical sector. In addition, free radical mechanisms are well understood and extension of the concepts to new monomers is generally straightforward. A third advantage of free radical routes is that the polymerization proceeds in a relatively facile manner: rigorous removal of moisture is generally unnecessary while polymerization can be carried out in either the bulk phase or in solution. As chain reactions, free radical polymerizations proceed via four distinct processes:
1. Initiation. In this first step, a reactive site is formed, thereby “initiating” the polymerization.
2. Propagation. Once an initiator activates the polymerization, monomer molecules are added one by one to the active chain end in the propagation step. The reactive site is regenerated after each addition of monomer.
3. Transfer: occurs when an active site is transferred to an independent molecule such as monomer, initiator, polymer, or solvent. This process results in both a terminated molecule (see step four) and a new active site that is capable of undergoing propagation.
4. Termination. In this final step, eradication of active sites leads to “terminated,” or inert, macromolecules. Termination occurs via coupling reactions of two active centers (referred to as combination), or atomic transfer between active chains (termed disproportionation), 6 The free radical chain process is demonstrated schematically below (Scheme 1): R’• represents a free radical capable of initiating propagation; M denotes a molecule of monomer; Rm• and Rn• refer to propagating radical chains with degrees of polymerization of m and n, respectively; AB is a chain transfer agent; and Pn + Pm represent terminated macromolecules. Because chain transfer may occur for every radical at any and all degrees of polymerization, the influence of chain transfer on the average degree of polymerization and on polydispersity carries enormous consequences. Furthermore, propagation is a first order reaction while termination is second order. Thus, the proportion of termination to propagation increases substantially with increasing free radical concentrations. Chain transfer and termination are impossible to control in classical free radical processes, a major downfall when control over polymerization is desired.
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Describe techniques of living cationic polymerization. Give examples.
Initransfers in cationic polymerization 
Method of external foundation
16 Discuss diffusion phenomena in radical polymerization. Gel-effect in radical polymerization
Gel effect
The clipping reaction is the only elementary reaction that is controlled by diffusion at all stages of the process. The determining effect of diffusion on the rate of bimolecular breakage of growth radicals in bulk polymerization at large conversions was established, almost at the time of completion of the theory of radical polymerization in the late 1940s. According to Eqs. (5.9) and (5.16), with an increase in the conversion of the monomer (conversion), i.e. with its exhaustion, the rate and degree of polymerization should decrease. However, by the end of the 40-ies. XX century. This phenomenon was called the gel effect when it was observed at sufficiently high degrees of transformation, when the viscosity of the reaction medium increased significantly It was found that the gel effect is characteristic for polymerization in bulk or concentrated solutions of such monomers as methyl methacrylate, butyl methacrylate, vinyl acetate and some others. In Fig. 5.6 given data characterizing the effect of the solvent on the gel effect in the polymerization of methyl methacrylate. In Table. 5.7 reflects the effect of the conversion of monomer to polymer on kinetic constants in the polymerization of methyl methacrylate.
M The ratio I, according to (5.9) and (5.16), is proportional to the rate and degree of polymerization. Based on this, based on the data of Table. 5.7 it can be concluded that as a result of the gel effect, the rate and degree of polymerization of methyl methacrylate increase by approximately an order of magnitude. The increase in the ratio / is due, first of all, to a significant decrease, by several orders of magnitude, of the rate constant of the reaction of the clipping. The gel-effect depends on many factors and, first of all, on the nature of the monomer. As an example, Fig. 5.7 shows the dependence of the reduced polymerization rates on the conversion during the polymerization of methyl methacrylate and styrene. It can be seen that in the latter case, the gel effect occurs at a later stage. Due to this circumstance, it was believed that the gel effect during the polymerization of this monomer is absent.
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