 Polyester

A polyester is a polymer whose polymer backbone contains carboxylic ester linkages. The location of the linkage matters: a polymer carrying ester groups only as side groups is excluded from this structural class by the IUPAC definition.^[ref]

Polyglycolic acid gives a compact backbone example:Following the main chain passes through the ester oxygen and carbonyl connection. The manufacturer's structural diagram identifies this polyester repeat unit.^[ref] In polyvinyl acetate, by contrast, the carbon–carbon backbone carries acetate side groups. Those groups can be chemically transformed without making every backbone connection an ester.

Polycondensation of suitable multifunctional monomers is one route to a polyester. An original ideal step-growth polymerization count explains why high conversion matters. Start with $N_0$ bifunctional molecules, allow only intermolecular bond-forming reactions, and exclude rings, branching and unequal functional-group stoichiometry. Each new connection reduces the number of separate molecules by one. If a fraction $p$ of the initial functional groups has reacted, the number of connections is $p{N}_0$, soConversions 0.99 and 0.999 give number-average degree of polymerization values 100 and 1000 in this stipulated model. Unequal functionality, end-group imbalance and ring formation change the calculation. The equation is a mechanism model, not a guarantee for an actual synthesis.

Ester hydrolysis can cut a polyester backbone, linking chemical transformation to changing chain length and material properties. Hydrolysis of a side-chain ester need not cut the carbon–carbon backbone of the contrasting example. “Contains an ester” and “is a polyester” therefore predict different possible consequences of the same reaction type.

A structural class alone does not specify modulus, degradation rate or processing behaviour. Backbone groups, chain-length distribution, morphology and exposure conditions all matter. Measure the relevant molar mass and physical response rather than transferring a numerical property from one polyester solely because the class name agrees.