Molecular weight distributions in novolactype phenol-formaldehyde polymerization

Abstract

To model the reversible novolac polymerization, five reactive species A to E have been defined. Molecules having bound CH₂OH (Q_n) are distinguished from those without it (P_n) and it is assumed that molecules of Q_n do not have more than one bound CH₂OH group. A kinetic model has been written and, based upon it, balance equations for molecules of novolac polymer in batch reactors have been derived. Based upon our earlier studies, the phenomenon of molecular shielding has been neglected. As a result, the reactivities of the ortho and para positions of phenol which are available in the literature could be used. The kinetic model for the molecular weight distribution (MWD) of reversible novolac polymer formation thus involves only one parameter. The study of the MWD of novolac polymer reveals two very important design variables: the phenol-formaldehyde ratio, [P]₀/[F]₀ , in the feed and the vacuum applied on the reactor. As the [P]₀/[F]₀ ratio is increased, the breadth of the distribution is found to increase and it undergoes a maximum at [P]₀/[F]₀ ⋍ 1.4 for the set of rate constants chosen. At this ratio, the chain length average molecular weight is also found to be the largest. Industrially, the [P]₀/[F]₀ ratio used in producing novolac polymer is 1.67 and it is usually desired that the polymer be linear with minimal branching. On application of vacuum, for a given time of polymerization, the chain length molecular weight is found to increase when the results are compared with those of batch reactors. The breadth of the distribution is also found to reduce thus giving a lower polydispersity index of the polymer formed.