Homopolymerization mechanism

DICY is considered a catalyst and polymerizes epoxy resin through the homopolymerization mechanism. But DICY has also shown behavior with epoxies that indicates some breakdown at cure temperatures to produce a curing agent that contributes to the polyaddition reaction mechanism. 

Chain Homopolymerization Mechanism and Kinetics Free radical and ionic polymerizations proceed through this type of mechanism, such as styrene polymerization. Here one monomer molecule is added to the chain in each step. The general reaction steps and corresponding rates can be written as follows ... 

Step Growth Homopolymerization Mechanism and Kinetics Here any two growing chains can react with each other. The propagation mechanism is an infinite set of reactions ... 

Table 2.4 Simplified homopolymerization mechanism for a preliminary model using population balances and the method of moments...

Scheme 3.2 Basic free-radical homopolymerization mechanism.

In this chapter we deal exclusively with homopolymers. The important case of copolymers formed by the chain mechanism is taken up in the next chapter. The case of copolymerization offers an excellent framework for the comparison of chemical reactivities between different monomer molecules. Accordingly, we defer this topic until Chap. 7, although it is also pertinent to the differences in the homopolymerization reactions of different monomers. 

It may not be appropriate to compare the thermal stability characteristics of VC/VAc copolymer to that of a VC homopolymer (PVC). The copolymerization would involve different kinetics and mechanism as compared to homopolymerization resulting structurally in quite different polymers. Hence, copolymerization of VC with VAc cannot be regarded as a substitution of chlorines in PVC by acetate groups. To eliminate the possibility of these differences Naqvi [45] substituted chlorines in PVC by acetate groups, using crown ethers (18-crown-6) to solubilize potassium acetate in organic solvents, and studied the thermal stability of the modified PVC. Following is the mechanism of the substitution reaction ... 

Aspects of thermal initiation have been reviewed by Moad et al., w Pryor and Laswell, 10 Kurbatov/" and Hall.312 It is often difficult to establish whether initiation is actually a process involving only the monomer. Trace impurities in the monomers or the reaction vessel may prove to be the actual initiators. Purely thermal homopolymerizations to high molecular weight polymers have only been demonstrated unequivocally for S and its derivatives and MMA. For these and other systems, the identity of the initiating radicals and the mechanisms by which they are formed remain subjects of controversy. 

Various mechanisms have been proposed to explain the initiation processes. The self-initiated copolymerizations of the monomer pairs S-MMA and S-AN proceed at substantially faster rates than pure S polymerization. For S-AN333 and S-MAHJJ the mechanism of initiation was proposed to be analogous to that of S homopolymerization (Scheme 3.62) but with acrylonitrile acting as the dicnophile in the formation of the Diels-Alder adduct (Scheme 3.66). 

This chapter is primarily concerned with the chemical microstructure of the products of radical homopolymerization. Variations on the general structure (CHr CXY) are described and the mechanisms for their formation and the associated Tate parameters are examined. With this background established, aspects of the kinetics and thermodynamics of propagation are also considered (Section 4.5). 

The vinylsilanes (e.g. 40, 41) do not readily homopolymerize. Forsyth et al.Mj explored the mechanism of grafting these monomers using dodecane as a model for PE. Their work suggests that multiple monomer units are attached through a sequence of addition and intramolecular hydrogen atom transfer steps by a mechanism analogous to that shown in Scheme 7.33 on page 394. 

The lower yield for olefin than for S02 was explained by scavenging of the formed free olefin on the cationic sites of the irradiated polymer in a homopolymerization reaction, thus reducing G(olefin). Adding cation scavengers it was found that the overall product yield was reduced with concurrent reduction of the S02/olefm ratio towards unity73. Thus it can be concluded that the homopolymerization of the olefin is occurring by a cationic mechanism. 

Majoros, L, Nagy, A. and Kennedy, /, P, Conventional and Living Carboeationic Polymerizations United. 1. A Comprehensive Model and New Diagnostic Method to Probe the Mechanism of Homopolymerizations. Vol 112, pp. 1-113. 

Although the basic mechanisms are generally agreed on, the difficult part of the model development is to provide the model with the rate constants, physical properties and other model parameters needed for computation. For copolymerizations, there is only meager data available, particularly for cross-termination rate constants and Trommsdorff effects. In the development of our computer model, the considerable data available on relative homopolymerization rates of various monomers, relative propagation rates in copolymerization, and decomposition rates of many initiators were used. They were combined with various assumptions regarding Trommsdorff effects, cross termination constants and initiator efficiencies, to come up with a computer model flexible enough to treat quantitatively the polymerization processes of interest to us. 

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