Linear free energy relations are successfully extended to the principle of polylinearity when propagation is the predominant rate-determining step and others (e.g., initiation, termination) can be neglected. The cationic polymerization of vinyl ethers and p-substituted styrenes provides improved insight into the mechanism. On the other hand, the overall rate of the homopolymerization or the relative reactivity of the cross-propagation step in copolymerization can be controlled by means of multi-parameter equations. Some examples are given where multi-parameter correlation analysis with rather empirical parameters are shown useful for controlling industrial processes as well as for definite product formation.