Steric Control in Cationic Polymerization

Ionic polymerizations yield highly stereoregular polymers when control is exercised over monomer placement. The earliest stereospecific vinyl polymerizations were observed in preparation of poly isobutyl vinyl ether) with a BFa-ether complex catalyst at -70 °C. An isotactic polymer formed. °The same catalyst was employed later to yield other stereospecific poly(vinyl ether)s. The amount of steric placement increases with a decrease in the reaction temperature, and, conversely, decreases with an increase in the temperature.  

Various mechanisms were proposed to explain steric placement in cationic polymerization. Most of them pertain to vinyl ethers. There is no general agreement. Some of the suggested mechanisms are discussed in this section. Most were proposed for homogeneous conditions with soluble initiators like BF3-0(C2H5 2. There are, however, also some explanations of steric control with insoluble catalysts, like A1(S04 2 H2S04. 

Control in Homogeneous Polymerizations. Not all explanations of steric control under homogeneous conditions give equal weight to the influence of the counterion. ABawn and Ledwith mechanism for the polymerization of vinyl ethers is based on data which suggest that only one mesomeric form of the ethers exists, presumably trans  

An alkyl substituent composed of a three-carbon chain caused steric blocking of one side of the bond  

Solvations of the new cations might even occur before they are completely formed, maintaining the steric arrangement throughout, provided that the monomers enter as shown above. One weakness of the above mechanism is that it fails to consider the nature of the counterions. 

---