what are polycondensation reactions in polymer chemistry?

Answer 1

A condensation is a reaction between two functional groups such that the groups join together and split out a "small molecule" like water as a coproduct. For example, C2H5OH + CH3COOH ===> CH3COOC2H5 + H2O. A polycondensation would involve a diacid like terephthalic acid, p-HOOC-C6H4-COOH, with a dialcohol like ethylene glycol, HO-CH2CH2-OH, to produce an "endless chain" of polyester. In this case, it's polyethylene terephthalate, used for blouses and pop bottles.

Polyamides (nylons and aramids) also come from polycondensation of acid derivatives and amines. Maybe you'll see a demonstration of making nylon 66 from adipoyl chloride and hexamethylenediamine solutions layered one over another. The demonstrator pulls an endless white string of nylon resin from between the layers with a glass rod. In that case, the small molecule split out is HCl.

The other major way to produce polymeric resins is addition, as when ethylene, propylene, styrene, or vinyl chloride string together without splitting out a small molecule.

Answer 2

A polycondensation means in fact that H20 is released from the coupling molecules. For example when an carboxylic acid (-COOH) reacts with an amino group (-NH2), the netto reaction gives -CONH- plus H2O. So if you make polymers of molecules containing either COOH and NH2 or both, you get an polycondensation reaction.