Steric, Polar, and Resonance Effects in the Propagation Reaction

Steric, Polar, and Resonance Effects in the Propagation Reaction 

The steric effects depend upon the sizes of the substituents. The resonance stabilization of the substituents has been shown to be of the following order  

The reactivities of the propagating polymer radicals, however, exert greater influence on the rates of propagation than do the reactivities of the monomers. Resonance stabilization of the polymer 

There is no such opportunity, however, for resonance stabilization of the poly(vinyl acetate) radicals, because oxygen can accommodate only eight electrons. 

In vinyl monomers both olefmic carbons are potentially subject to free-radical attack. Each would give rise to a different terminal unit  

The reactivities of the propagating polymer-radicals, however, exert greater influence on the rates of propagation than do the reactivities of the monomers. Resonance stabilization of the polymer-radicals is a predominant factor. This fairly common view comes from observations that a methyl radical reacts at a temperature such as 60°C approximately 25 times faster with styrene than it does with vinyl acetate [72]. In homopolymerizations of the two monomers, however, the rates of propagation fall in an opposite order. Also, poly(vinyl acetate)-radicals react 46 times faster with n-butyl mercaptan in hydrogen abstraction reactions than do the polystyrene-radicals [71]. The conclusion is that the polystyrene radicals are much more resonance stabilized than are the poly (vinyl acetate)-radicals. Several structures of the polystyrene-radicals are possible due to the conjugation of the unpaired electrons on the terminal carbmis with the adjacent unsaturated groups. These are resonance hybrids that can be illustrated as follows  

---

Explain the steric, polar, and resonance effects in the propagation reaction. 

The rate of the propagation reaction depends upon the reactivity of the monomer and the growing radical chain. Steric factors, polar effects, and resonance are important factors in the reaction. 

The propagation reaction also depends on the nature of the gegen ion in that a monomer moleeule adds to the growing chain by squeezing itself between the chain and the gegen ion. As a result of this, resonance, polar, and steric effects would be expeeted to play a significant role in determining k. ... 

The propagation rates in ionic polymerizations are influenced by the polarity of the monomers in free-radical reactions, the relative reactivity of the monomers can be correlated with resonance stabihty, polarity, and steric effects we shall consider only radical copolymerizations. 

---