Polymerization Methods and Kinetics

Polymerization methods and initiators

Polymerization methods are first sorted by mechanism. Step-growth (condensation or polyaddition) polymerization joins any two reacting molecules through their functional groups, whereas chain-growth (addition) polymerization adds monomer only to an active centre that is carried along the growing chain. The type of active centre, whether free radical, cation, anion, or a transition-metal–alkyl site, determines which initiator is needed and which monomers it can polymerize. Electron-rich alkenes such as isobutylene favour cationic initiation, electron-poor alkenes such as acrylonitrile and methyl methacrylate favour anionic or radical initiation, and strained cyclic monomers such as lactides and lactones polymerize by ring opening. Interfacial polymerization is a step-growth technique in which two monomers react at the boundary of two immiscible liquids, and Nylon 66 is the classic example, made from an aqueous diamine and an organic diacid chloride. Nylon 6 is not made this way, because it is produced by ring-opening polymerization of caprolactam, and vinyl polymers such as polyacrylonitrile and poly(butyl acrylate) are chain-growth products of free-radical polymerization. Chain-growth polymerization gives high molar mass almost immediately, so its molar mass is nearly constant after the early stage. Step-growth gives low molar mass until conversion is close to 100%, and living polymerization gives a number-average molar mass that rises linearly with conversion. Initiator choice also depends on temperature. Thermal initiators such as AIBN, benzoyl peroxide and potassium persulfate decompose efficiently only at temperatures usually above about 50 °C. A redox pair such as ferrous sulfate with t-butyl hydroperoxide transfers an electron and generates radicals at about 5 °C, because its activation energy is far lower than that of thermal homolysis. Ziegler–Natta catalysts for coordination polymerization of olefins are formed from a group 4 or 5 transition-metal halide, such as TiCl3 or VCl4, and an organoaluminium co-catalyst, such as diethylaluminium chloride, which alkylates the metal and creates the metal–carbon active site. Zinc chloride and titanium dioxide are not the transition-metal components of these systems. Metallocene with methylaluminoxane is a single-site coordination system that polymerizes olefins by coordination addition. Industrial routes can be recalled as pairs: linear low density polyethylene by coordination catalysis, nylon 6 by ring opening, styrene–butadiene rubber by emulsion polymerization, and aromatic polyamides by step-growth condensation of diamines with diacid chlorides. Atactic polystyrene has no stereochemical control, so it is made by free-radical polymerization or by anionic polymerization in common solvents, while ring-opening and polycondensation do not apply to the open-chain vinyl monomer styrene. Ring-opening polymerization is driven by relief of ring strain. Lactide, the cyclic dimer of lactic acid, gives poly(lactic acid), and \varepsilon-caprolactone, a seven-membered lactone, gives poly(\varepsilon-caprolactone). Polystyrene and polyaniline are not formed by ring opening. Butyl rubber is a cationic copolymer of isobutylene with a small amount, roughly 1–2 mol%, of isoprene, carried out well below 0 °C with a Lewis acid initiator such as AlCl3 and a co-initiator. The isoprene units supply the residual double bonds needed for sulfur vulcanization.