离子聚合Ionic polymerization

适用于带给电子取代基、能稳定碳正离子的单体，如异丁烯、乙烯基醚和 α-甲基苯乙烯。引发剂是质子酸或路易斯酸（BF₃、AlCl₃、TiCl₄）与少量水等共引发剂。碳正离子易发生 β-质子转移，链转移严重，所以通常在 −100 °C 左右进行。工业上用于生产聚异丁烯和丁基橡胶（异丁烯与少量异戊二烯共聚），后者气密性极好，用于轮胎内胎。

It suits monomers with electron-donating groups that stabilise carbocations, such as isobutylene, vinyl ethers and α-methylstyrene. Initiators are protic acids or Lewis acids (BF₃, AlCl₃, TiCl₄) with a co-initiator such as a trace of water. Carbocations readily lose a β-proton, so chain transfer is severe and reactions run near −100 °C. Industry uses it for polyisobutylene and butyl rubber (isobutylene with a little isoprene), whose very low gas permeability suits tyre inner tubes.

适用于带吸电子取代基或共轭体系的单体，如苯乙烯、丁二烯、异戊二烯、丙烯腈和甲基丙烯酸甲酯。常用引发剂为正丁基锂、萘钠。在高纯无杂质体系中，碳负离子不会自发终止，链端能长期保持活性——1956 年施瓦克发现的“活性聚合”。由此可制备分子量分布极窄（Đ < 1.05）的聚合物和苯乙烯-丁二烯-苯乙烯（SBS）嵌段热塑性弹性体。氰基丙烯酸酯（“502 胶”）遇空气中微量水即发生阴离子聚合而固化。

It suits monomers with electron-withdrawing groups or conjugation, such as styrene, butadiene, isoprene, acrylonitrile and methyl methacrylate. Common initiators are n-butyllithium and sodium naphthalenide. In very pure systems carbanions do not terminate spontaneously and chain ends stay active indefinitely, the living polymerisation discovered by Szwarc in 1956. This allows very narrow distributions (Đ below 1.05) and styrene–butadiene–styrene (SBS) block thermoplastic elastomers. Cyanoacrylate superglue cures by anionic polymerisation triggered by traces of moisture.

离子聚合中活性中心与反离子形成紧密离子对、溶剂分离离子对或自由离子，溶剂极性决定其比例并影响速率和立体规整性：在非极性烃溶剂中用锂引发丁二烯得到高顺式-1,4 结构，在四氢呋喃中则主要得到 1,2-结构。水、醇、CO₂ 和氧等杂质会使活性中心失活，因此需要高真空或手套箱技术。

Active centres and counter-ions exist as contact ion pairs, solvent-separated pairs or free ions, and solvent polarity sets the balance, affecting rate and stereoregularity: lithium-initiated butadiene gives mostly cis-1,4 units in hydrocarbons but mostly 1,2 units in tetrahydrofuran. Water, alcohols, CO₂ and oxygen kill the active centres, so high-vacuum or glovebox techniques are needed.