Anionic Ring-Opening Polymerization

Abstract

Monomers that undergo anionic ring-opening polymerization (anionic ROP) are cyclic compounds with high electrophilicity. Figure 1 depicts some typical cyclic monomers for anionic ROP. Heterocycles with three-membered ring such as epoxides and episulfides undergo anionic ROP due to their ring strain. Cyclic compounds with carbonyl group such as lactones, cyclic carbonates, and lactams are susceptible to nucleophilic attack by nucleophiles and thus their anionic ROP proceeds, although the efficiency depends on the ring size. Their thiocarbonyland phosphoruscontaining analogues are also applicable as monomers to anionic ROP. For anionic ROP, a wide variety of nucleophiles can be used as initiators. Alkyl metals such as alkyl lithium (RLi), Grignard reagents (RMgBr), alkyl aluminum (R3Al), and alkyl zinc (R2Zn), which are strong nucleophiles, can initiate anionic ROP of various cyclic monomers. Metal amides and alkoxides are also used frequently. Besides these metal-containing initiators, less nucleophilic compounds such as phosphines, amines, thiols, and alcohols are used efficiently in some cases. Onium salts (RnX Y ), where X can be chosen from N, P, and S, can be used as anionic initiators since the counter anion Y can act as nucleophile. Figure 2 generalizes anionic ROP. X and Y represent atoms with different electron negativity. The atom X is the less electron-negative one and thus more electrophilic than the atom Y. Nucleophile (Nuc ) attacks the atom X to induce the scission of the bond X-Y to afford Y at the chain end. This newly formed anion attacks another molecule of the monomer, leading to the chain growth of the polymer.