Evidence for Two Chain Length Distributions in the Polymerization of Formaldehyde on Cu(100)

The polymer species formed from the spontaneous polymerization of formaldehyde (H 2 CO) on clean Cu(100) at 85 K were studied using electron energy loss spectroscopy (EELS) and temperature-programmed desorption (TPD). Formaldehyde forms poly(oxymethylene) (POM) with differing chain lengths; the long (α) and short (β) chain species depolymerize to give two features in TPD at approximately 207 and 219 K, respectively. The complex desorption kinetics observed for the α-POM species were successfully modeled using equations based on the ratio of the average number of monomers unzipped from the chain per initiation event to the length of the polymer chain. Losses were observed in EEL spectra of the short-chain species at ∼290, ∼1020, and ∼1120 cm - 1 that can be assigned to the ν(Cu−O), ν(C−O), and ρ(CH 3 ) modes of oxygen and methoxy endgroups, respectively. Preadsorbed methanol increases the proportion of short-chain POM species by increasing the probability of termination for the β species. Lower thermal stability for adsorbed POM was observed compared to bulk POM and is believed to be related to the stability of the surface-bound oxygen endgroup.