Ring opening polymerization

This is a reaction in which both the ring-opening and polymerization reactions of organic monomers are carried out together to obtain a chain-like polymer (polymer compound) from a cyclic monomer. It can be considered as one form of addition polymerization.

Ring-opening polymerization often proceeds with relatively long-lived anions or cations as the propagating active species. In general, heterocyclic compounds (cyclic compounds containing atoms other than carbon as ring constituent atoms) with heteroatoms (atoms constituting the ring other than carbon) in the ring are more likely to undergo ring-opening polymerization than carbocyclic compounds. Ring-opening polymerization begins when a compound with a heteroatom (X) such as oxygen, nitrogen, or sulfur in the ring breaks its CX bond (X=O,N,S) to form a chain of anions or cations. Therefore, compounds with distortion in the ring, such as three-membered and four-membered rings, which are more likely to undergo ring-opening reactions, are more likely to undergo ring-opening polymerization than stable six-membered ring compounds. For this reason, three-membered oxirane (ethylene oxide), four-membered oxetane, and three-membered aziridine (ethyleneimine), which contain oxygen in the ring, and 3-membered aziridine, which contains a nitrogen atom in the ring, are more likely to undergo ring-opening polymerization. Oxiranes and aziridines are used industrially in epoxy resins, adhesives, hardeners, etc. Tetrahydrofuran, which has a five-membered oxygen ring, also undergoes ring-opening polymerization, but the reaction is low in reactivity and is a reversible reaction, making it difficult to obtain a high molecular weight.

Lactams (cyclic amides) open easily through hydrolysis and other reactions, so ring-opening polymerization can also occur in compounds with large rings (four or more members), and ring-opening polymerization is known for lactams with four-membered rings (the smallest lactam ring) through nine-membered rings, excluding six-membered rings. Ring-opening polymerization of caprolactam, a seven-membered amide, is used in the production of nylon and is therefore industrially important.

[Masahiro Hirota]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

有機単量体の開環と重合の両反応を組み合わせていっしょに行い、環状の単量体から鎖状の重合体（高分子化合物）を得る反応をいう。付加重合の一形式とみなすことができる。

　開環重合は、多くの場合、比較的長寿命の陰イオンまたは陽イオンを生長活性種として進行する。一般に、ヘテロ原子（炭素以外の環を構成する原子）を環内にもつ複素環式化合物（環構成原子として炭素以外の原子を含む環式化合物）のほうが炭素環式化合物に比べて開環重合をおこしやすい。環内に酸素、窒素、硫黄などのヘテロ原子（X）をもつ化合物でそのC-X結合（X=O,N,S）が切断して鎖状の陰イオンまたは陽イオンになるのが開環重合の始まりである。したがって、3員環、4員環のように環に歪(ひずみ)があって開環反応をおこしやすい化合物のほうが、安定な6員環化合物に比べると、開環重合をおこしやすい。このような理由から、環内に酸素を含む3員環のオキシラン（エチレンオキシド）、4員環のオキセタン、環内に窒素原子を含む3員環のアジリジン（エチレンイミン）は開環重合をおこしやすい。オキシランやアジリジンは工業的にエポキシ樹脂、接着剤、硬化剤などに使われている。酸素5員環のテトラヒドロフランも開環重合をするが反応性は低く、可逆反応であり高分子量になりにくい。

　ラクタム（環式アミド）は加水分解その他の反応により容易に開環するので、大きい環（4員環以上）をもつ化合物でも開環重合がおこり、6員環を除く4員環（最小のラクタム環）から9員環のラクタムで開環重合が知られている。7員環アミドであるカプロラクタムの開環重合はナイロンの製造に利用されていて、工業的に重要である。

［廣田　穰］

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