Oligomer desorption during heterogeneous catalytic synthesis of polymers

Product desorption is the final step in all heterogeneous catalytic processes but is rarely the rate-limiting step. There are, however, exceptions, such as heterogeneous polymerization reactions in which desorption of high molecular weight products can influence the overall reaction rate. In such cases, the apparent activation barrier to desorption, ΔEdes‡, increases with increasing polymer chain length, however, the relationship between the desorption barrier and the desorption energy, ΔEdes, is not straightforward and the scaling of ΔEdes‡ with chain length is not necessarily linear. Desorption of a series of alkanes, polyethyleneglycol dimethylethers and polyethylene glycols from the surface of graphite has been studied both experimentally and theoretically in order to shed light on the scaling of ΔEdes‡ with oligomer chain length and on the mechanism of oligomer desorption. In all three cases, the measured values of ΔEdes‡ are non-linear in the chain length. A model has been developed that accurately describes the scaling of ΔEdes‡ with chain length and shows that the origin of the observed non-linearity is the conformational entropy of the adsorbed oligomers. At elevated temperatures, some fraction of the oligomer segments are detached from the surface and thus do not contribute to the ΔEdes‡ measured at that temperature. One of the consequences of this effect is that the measured value of ΔEdes‡ is dependent on the temperature at which the measurement is made. This effect must introduce a complex temperature dependence into the kinetics and selectivity of heterogeneous catalytic synthesis of long chain polymers.