Polymer Structure and Synthesis

A polymer is prepared by stringing together a low molecular weight species (monomer; e.g., ethylene) into an extremely long chain (polymer; in the case of ethylene, the polymer is polyethylene) much as one would string together a series of bead to make a necklace (see Fig. 1.1). The chemical characteristics of the starting low molecular weight species will determine the properties of the final polymer. When two low different molecular weight species are polymerized, the resulting polymer is termed a copolymer—for example, ethylene vinylacetate. This is depicted in Fig. 1.2. Plastics can also be classified as either thermoplastics or thermosets. A thermoplastic material is a high molecular weight polymer that is not crosslinked. It can exist in either a linear or branched structure. Upon heating, thermoplastics soften and melt, allowing them to be shaped using plastics processing equipment. A thermoset has all of the chains tied together with covalent bonds in a three-dimensional network (crosslinked). Thermoset materials will not flow once crosslinked, but a thermoplastic material can be reprocessed simply by heating it to the appropriate temperature. The different types of structures are shown in Fig. 1.3. The properties of different polymers can vary widely; for example, the modulus can vary from 1 MN/ m2 to 50 GN/m2. For a given polymer, it is also possible to vary the properties simply by varying the microstructure of the material.

 

 

There are two primary polymerization approaches: step-reaction polymerization and chain-reaction polymerization.1 In step-reaction (also referred to as condensation polymerization), reaction occurs between two polyfunctional monomers, often liberating a small molecule such as water. As the reaction proceeds higher molecular weight species are produced as longer and longer groups react together. For example, two monomers can react to form a dimer then react with another monomer to form a trimer. The reaction can be described as n-mer + m-mer . (n + m)mer, where n and m refer to the number of monomer units for each reactant. Molecular weight of the polymer builds up gradually with time, and high conversions are usually required to produce high molecular weight polymers. Polymers synthesized by this method typically have atoms other than carbon in the backbone. Examples include polyesters and polyamides.

Chain-reaction polymerizations (also referred to as addition polymerizations) require an initiator for polymerization to occur. Initiation can occur by a free radical, an anionic, or a cationic species. These initiators open the double bond of a vinyl monomer, and the reaction proceeds as shown above in Fig. 1.1. Chain-reaction polymers typically contain only carbon in their backbone and include such polymers as polystyrene and polyvinyl chloride.

Unlike low molecular weight species, polymeric materials do not possess one unique molecular weight but rather a distribution of weights as depicted in Fig. 1.4. Molecular weights for polymers are usually described by two different average molecular weights, the number average molecular weight, Mn, and the weight average molecular weight, Mw. These averages are calculated using the equations below:

where ni is the number of moles of species i, and Mi is the molecular weight of species i. The processing and properties of polymeric materials are dependent on the molecular weights of the polymer as well as the molecular weight distribution. The molecular weight of a polymer can be determined by a number of techniques including light scattering, solution viscosity, osmotic pressure, and gel permeation chromatography.

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