Propagation

The transition state in a propagation reaction can be illustrated as follows:

In the above transition state, the macroradical electron is localized on the terminal carbon. Also, the two p electrons of the double bond are localized at each olefinic carbon. Interaction takes place between p-orbital of the terminal atom in the active polymer chain with associated carbon of the monomer. This results in formation of s-bonds [67]. 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 also important factors in the reaction. Another factor that can affect the rate of propagation is interaction between propagating radicals. Siegmann and Beuermann [60] studied the rate of propagation of 1H,1H,2H,2H-tridecafluorooctyl methacrylate and compared it to the rate of propagation of methyl methacrylate. They observed that kp for 1H,1H,2H,2H-tridecafluorooctyl methacrylate polymerization is 1.9 times that of kp for methyl methacrylate. They concluded that this higher rate is due to less interactions occurring between the propagating macroradicals. Bowman and coworkers studied the impact of intermolecular and intramolecular interactions on the polymerization kinetics of monoacrylates [69]. They carried out polymerization studies in the presence of extensive amounts of solvent. This was an attempt to elucidate the effects of intermolecular interactions, such as bulk medium polarity, p–p stacking, and hydrogen bonding and characterize the contribution of intramolecular conformational effects to monomer reactivity. Solution polymerization kinetics of various monomers were measured in the presence of 95 wt% 1,4-dioxane. The results were compared to bulk polymerization kinetics. The studies revealed that aliphatic acrylates such as hexyl acrylate exhibit approximately two to threefold reduction in reactivity upon dilution. Monomers characterized by only hydrogen-bonding features such as hydroxyethyl acrylate exhibit an 8- to 12-fold reduction upon dilution. Monomers possessing only aromatic ring stacking interactions such as phenyl acrylate exhibit approximately a five to tenfold reduction upon dilution under similar conditions. Even at a concentration of 5 wt% monomer in 1,4-dioxane, there were approximately two to fivefold differences in reactivity observed between various acrylates. Bowman and coworkers attributed these reactivity differences between various acrylates, upon extensive dilution, solely to intramolecular interactions [69].

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مواضيع ذات صلة

Autoacceleration

Ceiling Temperature

Effect of Reaction Medium

Steric, Polar, and Resonance Effects in the Propagation Reaction

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