Wang Fang

Step-growth polymerization: Monomers with two functional groups react stepwise. Unlike chain-growth, any two molecular species can react. Conversion must exceed 99% for high MW (Carothers equation).

Nylon-6,6 (CAS 32131-17-2): Condensation polymer of hexamethylenediamine and adipic acid. Strong, tough, abrasion-resistant. Tm ~264°C. Hydrogen bonding between amide groups gives high strength.

Polyethylene (CAS 9002-88-4): (C2H4)n. Most common plastic. LDPE: branched chains, flexible (bags). HDPE: linear chains, rigid (bottles). UHMWPE: ultra-high MW, used in armor and joint replacements.