Effect of Reaction Medium

There were some early reports that reaction media influences the polymerizations of vinyl chloride in aliphatic aldehydes at 50C [80, 81]. This was not confirmed in subsequent studies [82–84]. Subsequently, the rate of polymerization was shown to be influenced by the pH of the reaction medium in polymerizations of monomers such as methacrylic acid (MAA) [85, 88]. Also, the rate of polymerization and solution viscosities increase in polymerizations of acrylamide and acrylic acid with an increase in water concentration [83]. It is not quite clear whether this is due to increases in the speeds of propagations or due to decreases in the termination rates. In the free-radical polymerization of vinyl benzoate, the rate of propagation varies in different solvents in the following order [88, 89]:

Similarly, the rate of photopolymerization of vinyl acetate is affected by solvents [88]. In most cases, however, the rate of polymerization is proportional to the square root of the initiator concen tration and to the concentration of the monomer [5]. De Sterck and coworkers [90] studied solvent effect on tacticity of methyl methacrylate in free radical polymerization. They observed that solvents CH3OH and (CF3)3COH, which are H-bonded with the carbonyl oxygens and are located on the same side of the backbone of the growing polymer radical hinder the formation of isotactic poly (methyl methacrylate) to some extent. Methanol is less effective in reducing the isotactic city because of its small size and also because of the relatively loose hydrogen bonds with the carbonyl oxygens. There is a controversial suggestion that the solvent affects the propagation step in some reactions by forming “hot” radicals [91]. These radicals are supposed to possess higher amounts of energy. At the moment of their formation, they obtain surplus energy from the heat of the reaction and from the activation energy of the propagation reaction. This is claimed to provide the extra energy needed to activate the next chain propagation step. The surplus energy may affect the polymerization kinetics if the average lifetime of the hot radicals is sufficient for them to react with the monomer molecules. This surplus energy is lost by the hot radicals in collisions with monomer and solvent molecules.

There is a difference in the rate constants of propagation for hot and ordinary radicals so two different reaction schemes were written [91]: Propagation by ordinary radicals:

R+M→RM (k2) Propagation by hot radicals: R+M→RM (k2) Energy transfer processes:

R +Solvent → R+ Solvent (k3) where, R* is the symbol for hot radicals. The rate expression for the polymerization is then written as follows [92]:

-d(M) dt=K_XX^0:5(M)X(I +1/(y+y+s/(M)) where, Kx=k2(2k1f/k4)^1/2, Y ¼ k1*/k2* and Y= k3*/k2*, S ¼ solvent, [M] ¼ monomer In comparing free-radical polymerizations of ethyl acrylates in benzene and in dimethyl formamide at 50C[218] the rates were found to be proportional to the square roots of the initiator concentration. They were not proportional, however, to the concentrations of the monomer. This was interpreted in terms of hot radicals [93]. Similar results, however, were interpreted by others differently. For instance, butyl acrylate and butyl propionate polymerizations in benzene also fail to meet ideal kinetic models. The results, however, were explained in terms of termination of primary radicals by chain transferring (see Sect. 3.5 for explanation of chain transferring).

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مواضيع ذات صلة

Autoacceleration

Ceiling Temperature

Steric, Polar, and Resonance Effects in the Propagation Reaction

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