Polymerization processes are probably the most relevant example of a chemical reaction activated by catalysts or radical initiators. Among polymers, polyethylene is by far the most common and largely produced. Here we present a high-pressure synthesis of high-density crystalline polyethylene by using only physical tools such as pressure and light. Low-density polyethylene is obtained by compressing ethylene at room temperature above 3 GPa in the ordered crystal phase, and a highly crystalline polymer is produced in the fluid phase at pressures lower than 1 GPa by using continuous-wave laser lines (lambda < or = 460 nm) as an optical catalyst. The photo-activation is based on a two-photon absorption process to pi* antibonding states, where the change in molecular geometry favours the polymeric chain formation. The high yield and crystallinity of the polymer recovered by the photoinduced reaction and the simplicity of the synthesis make this process appealing for large-scale applications.

The pressure-induced chemical reaction of liquid butadiene was studied by Fourier transform infrared spectroscopy in a diamond anvil cell. Dimerization was found to occur above 0.7 gigapascal, giving vinylcyclohexene according to a cyclo-addiction reaction and only a trace amount of polybutadiene forms. By irradiating the high-pressure sample with a few milliwatts of the 488-nanometer argon+ laser line, the dimerization was completely inhibited, and the rapid formation of pure trans-polybutadiene was observed. The use of different excitation wavelength allows us to emphasize the selectivity of the process and to identify the active role of the 2(1)Ag state in this pressure- and laser-induced chemical reaction.

The polymerization of solid acetylene under pressure has been studied by Fourier transform infrared (FTIR) spectroscopy. Controlled laser irradiation cycles and the employment of infrared sensors to measure the sample pressure, allowed to separate the photochemical and the pressure effect on the injection and on the evolution of the reaction. The careful assignment of all the spectral features and analysis of their relative intensities and frequencies gave evidence to the specific effect of pressure and laser irradiation on the reaction products. Pressure induces an ordered growth of trans-polyenic species, while irradiation produces the opening of the double bonds and a consequent branching of the chains. Constant pressure measurements allowed to obtain precise information on the kinetics of the reaction. A monodimensional growth geometry, involving the molecules on the bc plane, agrees with the parameters extracted by the kinetic curves. Comparison between experiments at different temperatures suggests an activation of the reaction essentially due to the translational lattice modes.

Ethylene is the simplest organic molecule containing a double bond and is the starting monomeric unit in the synthesis of polyethylene, one of the most largely produced polymers. Here we report a high pressure infrared study of ethylene at room temperature. A polymerization reaction is observed when the crystalline phase I is compressed above 3.0 GPa. The reaction kinetics was investigated at two different pressures, 3.6 and 5.4 GPa. The recovered product was identified in both cases as polyethylene, but while a conformationally disordered and branched low-density polymer is obtained at the highest pressure, a high-density crystalline polymer is obtained at 3.6 GPa. A reaction mechanism was proposed on the basis of the kinetic data and the structural information.

A straightforward, energy- and atom-saving process to the production of tailored N-doped and catalytically active metal-free carbon nanostructures, has been set up. Our ex situ approach to the N-decoration of the carbon nanotube sidewalls contributes to elucidate the complex structure–reactivity relationship of N-doped carbon nanomaterials in oxygen reduction reactions, providing fundamental insights on the nature of the N-active sites as well as on the role of neighboring carbons.