EPT 1201: Polymer Synthesis, Structure, and Properties

Polymer: A large molecule (macromolecule) built by the repetition of small chemical units; derived from Greek poly ("many") and meres ("parts").
Monomer: Any molecule that can be converted to a polymer by combining with other molecules.
Repeating Unit: The structural unit enclosed in square brackets representing the polymer's composition.
Functionality: The interlinking capacity or number of sites available for bonding.
* Bifunctional: Molecules with two bonding sites, resulting in Linear polymers.
* Polyfunctional: Molecules with more than two sites, resulting in Branched or Cross-linked systems.

Homopolymer: Composed of only one type of repeating unit.
Copolymer: Composed of two different repeating units (e.g., styrene and acrylonitrile).
Copolymer Systems:
    * Random: Units arranged randomly.
    * Alternating: Ordered alternating arrangement.
    * Block: Long sequences (blocks) of each unit bound together.
    * Graft: Sequences of one monomer grafted onto the backbone of another monomer type.

Condensation (Step-reaction): Occurs between chemically reactive functional groups; usually eliminates a small molecule such as $H_2O$ or $NH_3$ .
Addition (Chain-reaction): Monomers are added sequentially to a rapidly growing chain, typically involving unsaturated molecules with carbon-carbon double bonds.
Ring-opening: Derived from the cleavage and polymerization of cyclic compounds (e.g., trioxane to yield polyoxymethylene; caprolactam to yield nylon 6).

Initiation: The acquisition of an active site by a monomer. Initiators (e.g., peroxides, azo compounds, Lewis acids) decompose to form free radicals. Between $60\%$ and $100\%$ of generated radicals typically initiate polymerization.
Propagation: Rapid addition of thousands of monomers to the active center. Head-to-tail linkages predominate, where substituents occur on alternate carbon atoms.
Termination: Growth activity is destroyed by reaction with another radical.
* Combination (Coupling): Two chains react to produce a single polymer (predominates at low temperatures).
* Disproportionation: A labile atom (usually $H$ ) is transferred, resulting in two polymers (pronounced at high temperatures).
Chain Transfer: Growing activity is transferred to a previously inactive species (monomer, polymer, or solvent), reducing average chain length and potentially causing branching.

Conjugated dienes such as butadiene, chloroprene, and isoprene undergo polymerization.
Structural units can form in $1,2$ , $3,4$ , or $1,4$ positions (cis- $1,4$ -unit or trans- $1,4$ -unit).

Examples: Polyethylene, Polypropylene, Poly(vinyl chloride), Polyacrylonitrile, Polystyrene, Poly(vinyl acetate), Poly(methyl acrylate), Poly(methyl methacrylate), Poly(vinylidene chloride), Poly(vinylidene fluoride), and Polytetrafluoroethylene.
Kinetics: The time between initiation and termination for a single chain is typically from a few tenths of a second to a few seconds.