Polymerization by Coordination Mechanism

The coordination catalysts for these reactions are diverse. They can be compounds of alkaline earth metals, like calcium amide, or calcium amide-alkoxide. They can also be Ziegler-Natta-type catalysts. These can be alkoxides of aluminum, magnesium, or zinc combined with ferric chloride. Others are reaction products of dialkylzinc with water or alcohol. They can also be bimetallic //-oxoalkoxides, such as [(RO)2A102]Zn. Other catalysts are aluminum or zinc metalloporphyrin derivatives (see Fig. 4.1). 

From propylene oxide these catalysts yield crystalline, isotactic polymers. Living polymerizations with metalloporphyrin derivatives are difficult to terminate and are therefore called by some immortal Catalysts like (C6H5)3-SbBr2-(C2H5)3N in combination with Lewis acids also yield crystalline poly(propylene oxide). Others, like pentavalent organoantimony halides, are useful in polymerizations of ethylene oxide. 

Polymerizations of epoxides by a coordinated anionic mechanism result in high molecular weight products. The details of the reaction mechanism have not been fully resolved yet, but it is conunonly believed to involve coordination of the monomers to electrophilic centers of the catalyst. This is followed by an activation for an attack by the anion. The mechanism can be illustrated by the following reactions  

Ferric chloride polymerizes propylene oxide, a monomer with an asymmetric carbon atom, with retention of asymmetry in the backbone. The products of polymerization contain either optically active polymers or racemic mixtures, depending upon the monomers used. When only a pure optical isomer monomer is used the products are crystalline polymers composed of the same optically active units  

The polymers are fairly high in molecular weight, approximately 100 times greater than the products from KOH initiations. Propylene oxide initially reacts with ferric chloride to form an oligomer, a chloropolyalkoxide. The material contains approximately four or five propylene oxide repeat units. This forms two different halogen sites. It can be illustrated as follows  

---