Cyclization versus Linear Polymerization

Although reversible or equilibrium polymerizations would almost always be carried out in an irreversible manner, it is interesting to consider the kinetics of polymerization for the case in which the reaction was allowed to proceed in a reversible manner. (The kinetics of reversible ring-opening polymerizations are discussed in Sec. 7-2b-5). 

Consider the polyesterification of Eq. 2-58 under stoichiometric conditions, where k and k2 are the rate constants for the forward and back reactions. The initial carboxyl and hydroxyl group concentrations are [M]0. The values at any time are [M], which is given by (1 — p) [M]0. The concentrations of the products, [COO] and [H20], at any time are equal because of the stoichiometry and are given by p[M]0. The polymerization rate is the difference between the rates of the forward and back reactions 

Equation 2-68 can be used to calculate the extent of conversion at any time if k and pe are known. pe is experimentally determined, or calculated from Eq. 2-61 if K is known. From experimental values of p as a function of time, Eq. 2-68 yields a straight line when the left side of the equation is plotted against time. The slope of the line then allows one to calculate k.  

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Another factor in step-growth polymerizations is cyclization versus linear polymerization.1516 Since ADMET is a step-growth polymerization, most reactions are carried out in the bulk using high concentrations of the reactant in order to suppress most cyclic formation. A small percentage of cyclic species is always present but is dependent upon thermodynamic factors, typical of any polycondensation reaction. 

CYCLIZATION VERSUS LINEAR POLYMERIZATION 2-5a Possible Cyclization Reactions... 

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