Surface-initiated ring-opening metathesis polymerization in the vapor phase: an efficient method for grafting cyclic olefins with low strain energies

Surface grafting of cyclic olefins with low strain energies, including cyclopentene (CP), 1,4-cyclohexadiene (CHD), cycloheptene (CHP), cis-cyclooctene (CO), cis,cis-1,5-cyclooctadiene (COD), 1,3,5,7-cyclooctatetraene (COT), cyclododecene (CD), and trans,trans,cis-1,5,9-cyclododecatriene (CDT), was explored using ring-opening metathesis polymerization in the vapor phase. These monomers do not polymerize when SiROMP is carried out in solution because of pronounced chain transfer on surfaces where chains are in close proximity to one another. In the vapor phase, however, chain transfer is suppressed at the solid-vapor interfaces, which permits the polymerization of most of these monomers. A minimal required strain energy of 2.2 kcal/mol was determined in this study, which is significantly lower than the estimated 13.3 kcal/mol for SiROMP carried out in solution, indicating that the enhancement in monomer polymerizability is significant using the vapor-phase approach. A series of polyalkenamers with a controlled fraction of unsaturation from 8 to 50% along the polymer backbone were grafted to solid substrates. It was observed that the logarithm of the largest grafted layer thickness obtained before the removal of chain-transfer products, which correlates with the extent of polymerization, scales with the monomer strain energy. This confirms that the release of ring strain is the thermodynamic driving force for SiROMP. It was also found that although chain transfer is suppressed in the vapor phase it is important in monomer/polymer systems where the fraction of unsaturated bonds is high. In these cases, the grafted polymer thickness is dominated by chain transfer rather than the monomer strain energy. A quantitative relationship is established for estimating the graft thickness of a particular monomer using its strain energy and fraction of unsaturated bonds in the monomer.