Molecular weight distribution step-growth polymerization

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 ... 

A factor in addition to the RTD and temperature distribution that affects the molecular weight distribution (MWD) is the nature of the chemical reaciion. If the period during which the molecule is growing is short compared with the residence time in the reactor, the MWD in a batch reactor is broader than in a CSTR. This situation holds for many free radical and ionic polymerization processes where the reaction intermediates are very short hved. In cases where the growth period is the same as the residence time in the reactor, the MWD is narrower in batch than in CSTR. Polymerizations that have no termination step—for instance, polycondensations—are of this type. This topic is treated by Denbigh (J. Applied Chem., 1, 227 [1951]). 

In polyolefins, the chain is propagated by an intermediate free-radical species or by an alkyl species adsorbed onto a solid. Both the free radical and the alkyl have the possibility of termination, and this creates the possibility of growth mistakes by chain transfer and chain-termination steps that create dead polymer before all reactants are consumed. The presence of termination steps produces a broader molecular-weight distribution than does ideal addition polymerization. 

An interesting thing is that the polyether with low polydispersity from chain-growth condensation polymerization possessed higher crystallinity than the one with broad molecular weight distribution from conventional step-growth condensation polymerization. The XRD pattern of the former polymer showed a stronger intensity, and the DSC profile showed the... 

Molecular Weight Distribution in Equilibrium Step-Growth Polymerizations... 

The calculations described in this section yield estimates of the molecular weight distribution of the reaction mixture during a step-growth polymerization which is proceeding according to the assumptions outlined in Section 5.4.1. 

The concepts we use to develop relations between the degree of conversion and molecular weight distribution of the reaction mixture in equilibrium step-growth polymerizations are most clearly illustrated with reference to the selfpolymerization of a monomer which contains two coreactive groups. An example would be a hydroxy acid that can undergo self-polymerization according to... 

When dwj fdi = 0, the curve has a maximum and / = I / In p. A series expansion of — 1 / In p gives p/(p — I) as the first term and this fraction approaches I /(p — I) as p approaches 1. This is the value of in linear step-growth polymerizations (Eq. 5-20). Thus, as a first approximation, the peak in the weight distribution of high conversion linear step-growth polymers is located at of the polymer if the synthesis was carried out under conditions where interchange reactions and molecular weight equilibration could occur. 

Step-growth copolymerization involves the use of three or more monomers which do not ordinarily all react with each other. Examples include mixtures of acids and polyols in the synthesis of alkyds, as illustrated in the recipes in Table 5-1. Such polymers will contain a random distribution of monomer residues if they are synthesized under conditions in which the polymerization is reversible and the molecular weight distribution is random. Polymers like alkyds are intended to be homogeneous products with properties which represent an average of those of all the component monomers. The copolymerization of linolcic acid in the recipe in Table 5-1 would confer air-drying properties on all the macromolccules in which it is incorporated. 

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. 

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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