Abstract

Polymers are the materials from which plastics, rubbers, and most fibers, surface coatings, and adhesives are made. Biopolymers are important constituents of living organisms. Polymers are classified as thermosets or thermoplastics according to their response to temperature, as addition or condensation polymers according to their mode of synthesis, or as linear, branched, or cross-linked according to their molecular structure. The microscopic structure of a polymer ultimately determines its macroscopic properties. Important structural features include molecular weight, stereoisomerism, and degree of crystallinity. The glass-transition temperature and crystalline melting point are key parameters in determining a polymer's applications. The structure of polymer molecules is determined by the polymerization mechanism, either step-growth (condensation) or chain-growth (addition). Step-growth polymerizations generally require high conversions and a close approach to stoichiometric equivalence to achieve high molecular weights. Free-radical addition is used to manufacture a wide variety of polymers through bulk, solution, suspension, and emulsion processes. Ionic addition permits the synthesis of block copolymers and polymers of uniform molecular weight. Heterogeneous addition (Ziegler-Natta) polymerization provides control of the steric structure of the polymer.