单体Monomer

加聚单体通常含碳碳双键（乙烯基单体、二烯烃）或可开环的环状结构（环氧乙烷、己内酰胺、丙交酯），双键打开相当于两个官能度。缩聚单体含两个或以上可反应的官能团，如二元酸、二元醇、二元胺。官能度为 2 的单体生成线型高分子；含官能度大于 2 的单体（如甘油、三聚氰胺）会产生支化和交联，形成网状结构。

Addition monomers usually contain a C=C bond (vinyl monomers, dienes) or a ring that can open (ethylene oxide, caprolactam, lactide); opening a double bond provides functionality 2. Condensation monomers carry two or more reactive groups, such as diacids, diols and diamines. Difunctional monomers give linear polymers, while monomers of functionality above 2, such as glycerol or melamine, cause branching and crosslinking into networks.

单体结构决定高分子的性能：乙烯得到柔软的聚乙烯，四氟乙烯得到耐腐蚀低摩擦的聚四氟乙烯，苯乙烯的苯环使聚苯乙烯坚硬透明，甲基丙烯酸甲酯得到透明的有机玻璃。单体的取代基也影响聚合活性和机理：给电子基有利于阳离子聚合，吸电子基有利于阴离子聚合。

Monomer structure decides polymer properties: ethylene gives soft polyethylene, tetrafluoroethylene gives chemically inert, low-friction PTFE, styrene's phenyl ring makes polystyrene hard and clear, and methyl methacrylate gives transparent acrylic glass. Substituents also affect reactivity and mechanism: electron-donating groups favour cationic polymerisation and electron-withdrawing groups anionic.

大宗单体主要来自石油和天然气的裂解与精制，生物基单体（乳酸、生物乙烯、1,3-丙二醇、呋喃二甲酸）正在发展。不少单体有毒：氯乙烯是 1 类致癌物，可引起肝血管肉瘤；苯乙烯、丙烯腈、甲醛也需严格控制残留和暴露。

Bulk monomers come mainly from cracking and refining oil and gas, while bio-based ones (lactic acid, bio-ethylene, 1,3-propanediol, furandicarboxylic acid) are developing. Many monomers are toxic: vinyl chloride is a Group 1 carcinogen causing liver angiosarcoma, and styrene, acrylonitrile and formaldehyde also require strict control of residues and exposure.