加聚反应Chain-growth polymerization

包括链引发、链增长、链终止和链转移四类基元反应。活性中心一旦生成，在极短时间（约 10⁻¹ 至 10 s）内就加成成千上万个单体形成高分子；体系中始终是单体、高分子和极少量增长链共存，转化率随时间增加，但已生成高分子的分子量基本不随时间变化。这与逐步聚合中分子量随转化率逐步增大形成鲜明对比。

It comprises initiation, propagation, termination and chain transfer. Once an active centre forms, it adds thousands of monomers within about 0.1 to 10 s, so the system always contains monomer, finished polymer and a tiny number of growing chains; conversion rises with time, but the molar mass of the polymer formed hardly changes. This contrasts sharply with step-growth polymerisation, where molar mass rises gradually with conversion.

1929 年卡罗瑟斯把聚合反应分为加聚和缩聚两类；1953 年弗洛里改按机理分为链式和逐步聚合。按活性中心分为自由基聚合（聚氯乙烯、聚苯乙烯、PMMA）、阳离子聚合（聚异丁烯、丁基橡胶）、阴离子聚合（SBS 弹性体）和配位聚合（齐格勒-纳塔催化的高密度聚乙烯、等规聚丙烯）。开环聚合部分也属于链式机理。

In 1929 Carothers classed polymerisations as addition or condensation; in 1953 Flory reclassified them by mechanism as chain-growth or step-growth. By active centre, chain-growth includes radical polymerisation (PVC, polystyrene, PMMA), cationic (polyisobutylene, butyl rubber), anionic (SBS elastomers) and coordination polymerisation (Ziegler–Natta high-density polyethylene and isotactic polypropylene). Some ring-opening polymerisations also follow a chain mechanism.

双键打开成两个单键放热（ΔH 约 −70 至 −100 kJ/mol），但单体聚集成链使熵减小（ΔS 约 −100 J/(mol·K)）。当温度高于上限温度 T_c = ΔH/ΔS 时，聚合不再有利，高分子会解聚：α-甲基苯乙烯 T_c 仅约 61 °C，PMMA 约 220 °C，因此 PMMA 可热解回收单体。

Opening a double bond into two single bonds releases heat (ΔH about −70 to −100 kJ/mol), but joining monomers into a chain lowers entropy (ΔS about −100 J/(mol·K)). Above the ceiling temperature T_c = ΔH/ΔS polymerisation is no longer favourable and the polymer depolymerises: α-methylstyrene's T_c is only about 61 °C and PMMA's about 220 °C, so PMMA can be pyrolysed back to monomer.