开环聚合Ring-opening polymerization

开环聚合的主要驱动力是释放环张力：三元环（环氧乙烷）和四元环张力大、极易聚合；五元环（四氢呋喃、γ-丁内酯）张力小、难以聚合或只能低温聚合；六元环（如环己烷）几乎无张力，不能聚合，但丙交酯和 ε-己内酯因构象原因仍可聚合。机理可以是阴离子、阳离子、配位插入或开环易位聚合（ROMP，用格拉布斯催化剂聚合降冰片烯）。

The main driving force is relief of ring strain: three-membered (ethylene oxide) and four-membered rings polymerise readily, five-membered rings (tetrahydrofuran, γ-butyrolactone) have little strain and polymerise poorly or only when cold, and strain-free six-membered rings such as cyclohexane do not polymerise at all, though lactide and ε-caprolactone still can for conformational reasons. Mechanisms may be anionic, cationic, coordination–insertion or ring-opening metathesis (ROMP, as with norbornene and Grubbs catalysts).

1938 年德国法本公司的施拉克以己内酰胺开环聚合得到尼龙 6，绕开了杜邦尼龙 66 的专利。环氧乙烷开环得到聚乙二醇（PEG），广泛用于医药和化妆品。八甲基环四硅氧烷（D4）开环得到硅橡胶。乳酸环二聚体丙交酯在辛酸亚锡催化下开环得到聚乳酸（PLA）。

In 1938 Paul Schlack at IG Farben made nylon 6 by ring-opening polymerisation of caprolactam, getting round DuPont's nylon 66 patents. Ethylene oxide gives polyethylene glycol (PEG), widely used in medicine and cosmetics. Octamethylcyclotetrasiloxane (D4) gives silicone rubber, and lactide, the cyclic dimer of lactic acid, gives polylactic acid (PLA) with tin octoate catalyst.

开环聚合可以是活性聚合，适合制备窄分布聚合物和嵌段共聚物，如 PEG-PLA 嵌段用于药物递送。由于开环聚合往往可逆，存在聚合-解聚平衡，可用于化学回收：一些聚酯在催化剂作用下可解聚回到环状单体，实现闭环循环。

Ring-opening polymerisation can be living, suiting narrow-distribution polymers and block copolymers such as PEG–PLA for drug delivery. Because it is often reversible, a polymerisation–depolymerisation equilibrium exists that enables chemical recycling: some polyesters can be catalytically depolymerised back to cyclic monomer for closed-loop reuse.