Kinetic considerations chain polymerization

Mechanisms. Because of its considerable industrial importance as well as its intrinsic interest, emulsion polymerization of vinyl acetate in the presence of surfactants has been extensively studied (75—77). The Smith-Ewart theory, which describes emulsion polymerization of monomers such as styrene, does not apply to vinyl acetate. Reasons for this are the substantial water solubiUty of vinyl acetate monomer, and the different reactivities of the vinyl acetate and styrene radicals the chain transfer to monomer is much higher for vinyl acetate. The kinetics of the polymerization of vinyl acetate has been studied and mechanisms have been proposed (78—82). 

The only example of kinetic considerations dealing with the ring-opening polymerization was published by Ballard and Bamford. The authors examined polymerization of N-carboxy-amino acid anhydrides proceeding onto polysacrosine. The authors assumed the equilibrium between adsorbed molecules of monomer, E, and free monomer, M, described by equilibrium constant, K, independent of the position in the template chain ... 

Copolymerization involves the simultaneous chain polymerization of a mixture of two or more monomers (Hillmyer, 2012 Ham and Alfrey, 1964 Odian, 2004a Tirrell, 1986). Aside from the general kinetic considerations which govern these chain reactions, as described earlier, there is imposed an additional... 

The kinetic picture of cationic chain polymerization varies considerably. Much depends upon the mode of termination in any oarticular system. A general scheme for initiation, propagation, and termination is presented below. ° By representing the coinitiator as A, the initiator as RH, and the monomer as M, we can write ... 

Kinetic considerations are of paramount importance in understanding the mechanism of step-growth polymerization. As stated in Chapter 1, chain-growth polymerizations take place in discrete steps. Each step is a reaction between two functional groups for instance, in a polyesterification reaction it is a reaction between -COOH and -OH. The increase in molecular weight is slow. The first step is a condensation between two monomers to form a dimer ... 

Copolymerization involves the simultaneous chain polymerization of a mixture of two or more monomers [89-92], Aside from the general kinetic considerations which govern these chain reactions, as described earlier, there is imposed an additional feature, i.e., the relative participation of the different monomers during the growth of the chain. This new parameter is most important, since it controls the composition of the copolymer. Systems involving more than two monomers are difficult to resolve in this respect, but it has been found possible to treat the case of a pair of monomers with relative ease [91,93-95]. 

The ceiling temperature constraint in the homopolymerization of alphamethyl styrene (AMS) can be circumvented by copolymerization with acrylonitrile (AN) to prepare multicomponent random microstructures that offer higher heat resistance than SAN. The feasibility of a thermal initiation of free radical chain polymerization is evaluated by an experimental study of the terpolymerization kinetics of AMS-AN-Sty. Process considerations such as polyrates, molecular weight of polymer formed, sensitivity of molecular weight, molecular weight distribution, and kinetics to temperature were measured. 

Micro-emulsion polymerization is a new and effective approach for preparing nanosized polymer particles and has attracted significant attention. Emulsion and micro-emulsion polymerization differ in the kinetics of the polymerization. Emulsion polymerization exhibits three reaction rate intervals, whereas only two are detected in micro-emulsion polymerization. Both particle size and the average number of chains per particle are considerably smaller in micro-emulsion polymerization (Schork et al., 2005). 

Emulsion polymerization of unsaturated monomers where the monomer and polymer are mutually insoluble, are of considerable industrial importance, but there is no generally accepted theoretical framework to discuss them. A major problem in the mechanistic investigation of any multiple - phase polymerization system is the number and complexity of the reactions which must be taken into account. For example, in such particles the formation of reaction loci and the growth of polymer chains occur simultaneously and each of these processes is itself complex. In quantifying the kinetics of heterogeneous polymerizations, where monomer and polymer are mutually immissible (insoluble), it is essential to take proper account of the compartmentalized nature of the system. 

We shall have considerably more to say about this type of kinetic analysis when we discuss chain-growth polymerizations in Chap. 6. 

Hydrolysis and Polycondensation. As shown in Figure 1, at gel time (step C), events related to the growth of polymeric chains and interaction between coUoids slow down considerably and the stmcture of the material is frozen. Post-gelation treatments, ie, steps D—G (aging, drying, stabilization, and densification), alter the stmcture of the original gel but the resultant stmctures aU depend on the initial stmcture. Relative rates, of hydrolysis, (eq. 2), and condensation, (eq. 3), determine the stmcture of the gel. Many factors influence the kinetics of hydrolysis and... 

Generalization of Flory s Theory for Vinyl/Divinyl Copolvmerization Using the Crosslinkinq Density Distribution. Flory s theory of network formation (1,11) consists of the consideration of the most probable combination of the chains, namely, it assumes an equilibrium system. For kinetically controlled systems such as free radical polymerization, modifications to Flory s theory are necessary in order for it to apply to a real system. Using the crosslinking density distribution as a function of the birth conversion of the primary molecule, it is possible to generalize Flory s theory for free radical polymerization. 

Since the depolymerization process is the opposite of the polymerization process, the kinetic treatment of the degradation process is, in general, the opposite of that for polymerization. Additional considerations result from the way in which radicals interact with a polymer chain. In addition to the previously described initiation, propagation, branching and termination steps, and their associated rate constants, the kinetic treatment requires that chain transfer processes be included. To do this, a term is added to the mathematical rate function. This term describes the probability of a transfer event as a function of how likely initiation is. Also, since a polymer s chain length will affect the kinetics of its degradation, a kinetic chain length is also included in the model. 

I have discussed two cases of chain microstructure determination in SO2 copolymers. First, the styrene-S02 system, which exhibits the general kinetic and compositional behavior of such systems, particularly as a function of polymerization temperature. Second, and considerably more complicated, is the chloroprene-S02 system. This one represents the limit of what can be handled and more or less completely solved at the present time. To do so requires about all o ir resources at superconducting frequen-... 

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