Driving forces in free radical addition-fragmentation processes

Advances and continuing challenges in achieving radical addition-fragmentation (AF) processes have resulted in an expanded understanding of the factors controlling both the addition and the fragmentation efficiency. Numerous works performed in recent years have offered the means for studying some structural constraints controlling the relative rates of intramolecular fragmentation over intermolecular propagation for the adduct radicals formed through addition on unsaturation. Comparison of sets of similar reactions may provide a reasonable guide to relative reactivities. The aim of this review is to discuss the factors which affect the rate and outcome of radical reactions most frequently encountered in radical AF processes. These reactions are generally believed to be mainly under kinetic rather than thermodynamic control, and a high degree of specificity can be exhibited by these radical processes when specific constraints are operative: steric hindrance (i.e. non-bonded interactions between radical and non-radical species), polar effects (relative electronegativities), stereoelectronic factors (i.e. requirement for overlap of frontier orbitals), and bond-strength (i.e. relative strengths of bonds formed or broken in the reaction). In this review, simple principles are provided for achieving greatly enhanced control of AF processes in most systems and for predicting the outcome of radical reactions. These rules may also help chemists in designing new AF agents and corresponding polymers. (C) 1999 Elsevier Science Ltd. All rights reserved.