Curbing Chain Transfer in Initiated Chemical Vapor Deposition (iCVD) via Molecular Vapor Complexation

Initiated chemical vapor deposition (iCVD) has revolutionized the preparation of high-quality conformal polymer films with excellent control over composition and properties at the nanoscale. It is compatible with over 70 functional monomers. Despite that chemical versatility, side reactions during iCVD are not well understood. For example, chain transfer could happen during the propagation of an important class of monomers that contain nitrogen (N), arresting the polymerization and limiting the molecular weight. Here, we use 1-vinylimidazole (1VI) to demonstrate the chain transfer reaction to the imidazole group during iCVD, which leads to unpredictable deposition kinetics, low molecular weight, and undesirable products. We further introduce a strategy that utilizes a vapor solvent to engineer monomer reactivity and suppress side reactions. By replacing the traditional patch flow, Ar, with acetic acid (AcOH), which forms hydrogen bonding with 1VI, chain transfer is suppressed, and the deposition rate is increased by as much as 280% while restoring its linear dependence on the monomer partial pressure. That linear dependence has not been achieved previously for 1VI. The tunable deposition kinetics, in turn, leads to a broader range of attainable material properties, including nearly doubling the maximum attainable molecular weight (from 8 kDa to 16 kDa) and increasing the elastic modulus (from 3.5 to 4.7 GPa). The vapor solvent is also effective at suppressing chain transfer in other N-containing monomers, like (2-dimethylamine) ethyl methacrylate (DMAEMA), leading to a considerable increase in the molecular weight (from 16 kDa to 38 kDa). The vapor solvent selectively increases the reactivity of N-containing monomers during copolymerization, demonstrated using 1VI and divinylbenzene (DVB) or 1,3,5,7-tetravinyl tetramethylcyclotetrasiloxane (V4D4), increasing the reactivity ratio of 1VI by an order of magnitude according to the Fineman–Ross equation. This robust strategy engineers monomer reactivity without the need for chemical modifications. It improves the chemical precision of iCVD polymerization, particularly for an important class of N-containing monomers, which have found broad applications as the polymer–electrolyte interphase in batteries, antifouling coatings in food and water production, and bioactive and functionalizable coatings in sensors.