Step-growth polymerization kinetics

What amounts to invoking the shortsightedness principle has become standard practice in polymerization kinetics where, for most purposes, the rate coefficients of step or chain growth and termination are taken to be independent of the lengths of the polymer chains (see Sections 11.2.1, 11.3.1, 11.4.1,and 11.5.1). In fact, the principle is the basis of Carother s approach to step-growth polymerization kinetics leading to his equations 11.6 and 11.7. 

Step-growth polymerization. Simple step-growth polymerization kinetics can be represented as follows ... 

The preceding discussions of the kinetics and molecular weight distributions in the step-growth polymerization of AB monomers are clearly exemplified by the esterification reactions of such monomers as glycolic acid or co-hydroxydecanoic acid. Therefore one method for polyester synthesis is the following ... 

Step-growth polymerizations have widely been developed in industrial applications whereas knowledge of their mechanisms and of their kinetics has remained far below that of chain polymerization reactions. 

Why are the kinetics of chain growth polymerization more difficult to study than those of step growth polymerization What simplification do we use to treat the kinetics of the chain growth process How does this simplification reduce the complexity of the problem and what are the limitations of this method ... 

Copolymers are synthesized using exactly the same type of chemistry as homopolymers, of course, but everything depends on the way you do it. Step-growth polymerizations of two different monpmers often give you truly random copolymers, because rearrangement reactions like transesterification can scramble any initial non-random sequence distribution imposed by the kinet-... 

Fig. 17. Extent of reaction at vitrification vs. reaction temperature for nonlinear step-growth polymerization (A + 2B2). All kinetic orders have the same p at vitrification. For mpdel parameters and system, see Fig. 16 caption. [Aronhime,...

Chain-growth polymerizations are so called because their mechanisms comprise chains of kinetic events. For successful polymerization, the sequence of reactions must first be initiated by some agent, and monomers must be added consecutively to a growing macromolecule. This chain of events may then be terminated by a reaction that is inherent in the system or by the action of impurities. In any case, we can usefully distinguish between at least three different reaction types in a kinetic polymerization chain. These are initiation, propagation, and termination reactions. (Recall that theie is only one reaction involved in step-growth polymerizations where the monomers add to the end of a macromolecule without the intervention of an active center.)... 

It is not practical to conduct free-radical polymerizations under conditions where there is an equilibrium between polymerization and depolymerization processes. The polymer synthesis is effectively irreversible in normal radical polymerizations. The course of the reaction is then determined kinetically, and the molecular weight distribution cannot be predicted statistically as was done for equilibrium step-growth polymerizations described in Chapters. 

We first consider the polymerization where each kinetic chain yields one polymer molecule. This is the case for termination of the growth of macroradicals by disproportionation and/or chain transfer (A,c = 0). The situation is completely analogous to that for linear, reversible step-growth polymerization described in Section 5.4.3. If we randomly select an initiator residue at the end of a macromolecule, the probability that the monomer residue which was captured by this primary radical has added another monomer is S and the probability that this end is attached to a macromolecule which contains at least i monomers is S . The probability that this macromolecule contains exactly i monomers equals the product of 5 and the probability of a termination or transfer step. The latter probability must be equal to (I — S) since it is certain that the last monomer under consideration will undergo one of these three reactions. That is, the probability that a randomly selected molecule contains t monomer units is 5 (l — S). Since such probabilities are equal to the corresponding mole fraction of this size molecule, jc,, we have the expression... 

The way in which a plasma polymer is formed has been explained by the rapid step growth polymerization mechanism, which is depicted in Figure 5.3. The essential elementary reactions are stepwise recombination of reactive species (free radicals) and stepwise addition of or intrusion via hydrogen abstraction by impinging free radicals. It is important to recognize that these elementary reactions are essentially oligomerization reactions, which do not form polymers by themselves on each cycle. In order to form a polymeric deposition, a certain number of steps (cycle) must be repeated in gas phase and more importantly at the surface. The number of steps is collectively termed the kinetic pathlength. 

Step growth polymerization of rodlike molecules has some features which qualitatively differentiate it from the step growth polymerization of flexible molecules. Experimental studies of the kinetics show that the polymerization becomes diffusion controlled at moderate polymer lengths and the rate of polymerization increases upon shearing the polymerizing mixture. Furthermore, diffusion control results in narrower molecular weight distributions compared to the Flory distribution for flexible molecules, whereas shear flow produces wider molecular weight distributions. Experiments also indicate that the polymerization may be diffusion controlled in the nematic phase, and transition to the nematic phase does not produce an increase in the polymerization rate. 

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