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 in the following order [70]:

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 60C 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 carbons with the adjacent unsaturated groups. These are resonance hybrids that can be illustrated as follows:

There is not such opportunity, however, for resonance stabilization of the poly(vinyl acetate) radicals because oxygen can accommodate only eight electrons. The effect of steric hindrance on the affinity of a methyl radical is illustrated in Table 3.7 [56, 57]. In vinyl monomers, both olefinic carbons are potentially subject to free-radical attack. Each would give rise to a different terminal unit:

The newly formed radicals can again potentially react with the next monomer in two ways. This means that four propagation reactions can occur:

Contrary to the above shown four propagation modes, a “head to tail” placement shown in (3.1), strongly predominates. This is true of most free radical vinyl polymerizations. It is consistent with the localized energy at the a-carbon of the monomer. Also, calculations of resonance stabilization tend to predict head to tail additions [68]. The free-radical propagation reactions that correspond to conversions of double bonds into single bonds are strongly exothermic. In addition, the rates increase with the temperature. It is often assumed that the viscosity of the medium, or change in viscosity during the polymerization reaction does not affect the propagation rate or the polymer growth reaction. This is because it involves diffusion of small monomer molecules to the reactive sites. Small molecules, however, can also be impeded in their process of diffusion. This can impede the growth rate [50]. During chain growth, the radical has a great deal of freedom with little steric control over the manner of monomer placement. Decrease in the reaction temperature, however, lowers mobility of the species and increases steric control over placement. This is accompanied by an increase in stereoregularity of the product [70, 71]. The preferred placement is trans–trans, because of lower energy required for such placement. As a result, a certain amount of syndiotactic arrangement is observed in polymerizations at lower temperatures [72]. Trans–trans configurations (with respect to the carbon atoms in the chains) yield zigzag backbones. This was predicted from observations of steric effects on small molecules [74, 75]. It was confirmed experimentally for many polymers, such as, for instance, in the formation of poly(1,2-polybutadiene) [74] and poly(vinyl chloride) [72]. Also, in the free-radical polymerizations of methyl methacrylate, syndiotactic placement becomes increasingly dominant at lower temperatures. Conversely, the randomness increases at higher temperatures [74]. The same is true in the free-radical polymerization of halogenated vinyl acetate [75]. One proposed mechanism for the above is as follows. The least amounts of steric compression within macromolecules occur during the growth reactions if the ultimate and the penultimate units are trans to each other. Also, if the lone electrons face the oncoming monomers during the transition states [80, 81], as shown below, syndiotactic placement should be favored:

While the above model explains the formation of syndiotactic poly (methyl methacrylate), possible interactions between the free radicals on the chain ends and the monomers are not considered. Such interactions, however, are a dominant factor in syndiotactic placement, if the terminal carbons are sp2 planar in structures [75].

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مواضيع ذات صلة

Autoacceleration

Ceiling Temperature

Effect of Reaction Medium

Propagation

Capture of Free Radicals by Monomers

Optical Activity in Polymers

Spherulitic Growth

Crystallization from Solution

Crystallization from the Melt

The Crystalline State

Rheology and Viscoelasticity of Polymeric Materials

Thermodynamics of Elasticity