Formation of high polymers on solid surfaces I. Theoretical study of mechanisms

Abstract

Four models are set up for polymerization on solid surfaces. In two of the models, it is assumed that all adsorption sites have the same adsorption energy. The first of these (Rideal mechanism) assumes that polymerization occurs by reaction of monomer in the gas phase with adsorbed monomer or adsorbed, growing polymer chains. The second model (Langmuir-Hinshelwood mechanism) assumes that polymerization occurs by reaction of an adsorbed monomer molecule with an adjacently adsorbed monomer molecule or growing polymer chain. The other two models are the counterparts of the first two with a variation of adsorption energies among the sites. With all adsorption sites of equal energy, it is shown that in both mechanisms there is a unique weight average molecular weight for each number average molecular weight. Weight average-number average molecular weight ratio, ( W ̄ N ̄ ), a measure of the broadness of molecular weight distribution, is shown to be in the range, 1 ⩽ W ̄ N ̄ ⩽ 2, for all values of velocity constants and pressure. In the Rideal mechanism, number and weight average molecular weights and rates of reaction increase without limit as pressure is increased. In the Langmuir-Hinshelwood mechanism, molecular weights and rates approach a maximum value asympotically as pressure is increased. With a distribution of adsorption energies, weight average-number average molecular weight ratios ( W ̄ N ̄ ) can have values far in excess of 2 for both mechanisms. Molecular weights and rates of reaction again level out with increasing pressure for the Langmuir-Hinshelwood mechanism, and increase without restriction in the Rideal mechanism.