缩聚反应Step-growth polymerization

反应初期单体迅速消耗生成二聚体、三聚体等低聚物，分子量随反应程度缓慢增长，只有在反应程度很高时才得到高分子（卡罗瑟斯方程 Xₙ = 1/(1−p)）。多数缩聚是可逆平衡反应，必须不断移除小分子（如真空脱水）推动平衡。两种单体官能团必须严格等摩尔，少量过量或单官能团杂质都会显著限制分子量。分子量分布的分散度趋近于 2。

Early on, monomers are quickly consumed into dimers, trimers and other oligomers; molar mass rises slowly and high polymer forms only at very high conversion (the Carothers equation Xₙ = 1/(1−p)). Most condensations are reversible equilibria, so the by-product must be removed continuously, for example by vacuum. The two kinds of functional group must be exactly equimolar; a slight excess or monofunctional impurity sharply limits molar mass. Dispersity approaches 2.

1907 年酚醛树脂的发明是早期缩聚产物。1929–1937 年杜邦的卡罗瑟斯系统研究缩聚反应，合成了聚酯和聚酰胺，1935 年得到尼龙 66，1938 年作为尼龙袜上市。1941 年温菲尔德和迪克森发明聚对苯二甲酸乙二醇酯（PET）。弗洛里在 1930–40 年代建立了缩聚反应的统计理论和凝胶化理论。

Phenolic resin, invented in 1907, was an early condensation polymer. From 1929 to 1937 Carothers at DuPont studied condensation polymerisation systematically, making polyesters and polyamides; he obtained nylon 66 in 1935, and nylon stockings went on sale in 1938. Whinfield and Dickson invented polyethylene terephthalate (PET) in 1941. In the 1930s and 1940s Flory developed the statistical theory of condensation polymerisation and gelation.

缩聚得到的高分子包括聚酯（PET、PBT）、聚酰胺（尼龙 6,6、芳纶）、聚碳酸酯、聚氨酯（逐步加成，无小分子放出）、环氧树脂、酚醛树脂、聚酰亚胺和有机硅。当使用官能度大于 2 的单体时，在某一临界反应程度会突然发生凝胶化，形成不溶不熔的交联网络。

Step-growth polymers include polyesters (PET, PBT), polyamides (nylon 66, aramids), polycarbonates, polyurethanes (step-growth addition with no by-product), epoxies, phenolics, polyimides and silicones. With monomers of functionality above 2, gelation occurs abruptly at a critical conversion, forming an insoluble, infusible network.