A catalyst is a species that speeds up a chemical reaction but is not itself consumed. Catalysts do not change the thermodynamics of the position of equilibrium or a reaction, but react by providing an alternative pathway of lower activation energy. They allow many reactions to be done at lower temperatures than without a catalyst, and also give selectivity in producing a specific product in reactions where various products are feasible thermodynamically. Enzymes are distinct selective biological catalysts.
A catalyst present in the same phase as the reactants is called homogeneous; one in a distinct phase is heterogeneous. Most heterogeneous catalysts are solids, and react by adsorption of liquid or gaseous reactants on a surface. Homogeneous catalysts are specific molecules, usually organ metallic compounds that can be tailored in a more specific way to produce a required product than is possible with heterogeneous catalysts.
Industrial and domestic plastics are mostly prepared by polymerizing alkenes:
The reaction is exothermic and may be started by free radicals but organ metallic catalysts produce more controllable results. Most commonly used is Ziegler-Natta catalysts build by mixing Al2Et6 with TiCl4. Solid TiCl3 is build and catalysis occurs at surface Ti-Et groups, to which alkene molecules undergo and coordinate insertion into the Ti-R bond. An advantage of these catalysts is that they may form stereoregular polymers where all the R parts in -C(R)H CH2- have the same stereo chemical configuration. This produces stronger materials with greater melting points than the random stereochemistry resulting from radical polymerization.