Capture of Free Radicals by Monomers

Once the initiating radical is formed, there is competition between addition to the monomer and all other possible secondary reactions. A secondary reaction, such as a recombination of fragments, as shown above, can be caused by the cage effect of the solvent molecules [41]. Other reactions can take place between a radical and a parent initiator molecule. This can lead to the formation of different initiating species. It can, however, also be a dead end as far as the polymerization reaction is concerned.

After the initiating radical has diffused into the proximity of the monomer, the capture of the free radical by the monomer completes the step of initiation. This is a straightforward addition reaction, subject to steric effects:

The unpaired electron of the radical is believed to be in the pure p-orbital of a planar, sp², carbon atom. Occasionally, however, radicals with sp³ configuration appear to form. Using quantum chemical calculations it was demonstrated that nucleophilic and electrophilic alkyl or aryl radicals attack alkenes following a tetrahedral trajectory:

This means that only substituents Y at the attacked olefinic carbon exert large steric effects [47]. In addition to the steric effects, the rates of addition of strongly nucleophilic or electrophilic radicals are governed mainly by polar effects of the substituents Rx, Y, and Z [48]. In borderline cases, however, the stabilities of the adducts and products tend to dominate [47].

Also,it wasdemonstratedthatacyclicradicalscanreactwithhighstereoselectivity[45].Inorderfor the reactions to be stereoselective, the radicals have to adopt preferred conformations where the two faces of the prochiral radical centers are shielded to different extents by the stereogenic centers. Giese and coworkers [49] demonstrated with the help of Electron Spin Resonance studies that ester substituted radicals with stereo genic centers in b-positions adopt preferred conformations that minimize allylic strain [49](shown below).In these conformations ,large(L)and medium sized substituents (M) shield the two faces. The attacks come preferentially from the less shielded sides of the radicals. Stereoselectivity, because of A-strain conformation, is not limited to ester-substituted radicals [50]. The strains and steric control in reactions of radicals with alkenes can be illustrated as follows [50]:

The above considerations can be illustrated on initiation by benzoyl peroxide, a commonly used initiating compound. The half-life of the initial benzoyloxy radicals from decompositions of benzoyl peroxide is estimated to be 10 ^-4 to 10 ^-5 s. Past that time, they decompose into phenyl radicals and carbon dioxide [49]. This is sufficient time for the benzoyloxy radicals to be trapped by fast-reacting monomers. Slow-reacting monomers, however, are more likely to react with the phenyl radicals that form from the elimination reaction. In effect, there are two competing reactions [50]:

1. Decomposition of the free radical:

1. Two types of radicals can add to the monomer (where x represents any typical substituent of vinyl monomers, such as halogens, or esters, or aromatic groups, or nitriles, etc.):

The ratio of the rates of the two reactions, K00/K0, (or K000/K0) depends upon the reactivity of the monomers. It is shown in Table 3.5 [51, 52]. The benzoyloxy radical is used in this table as an illustration. A similar comparison is possible for a redox initiating system. An initiating sulfate radical ion from a persulfate initiator can react with another reducing ion or add to the monomer:

In Table 3.6 are shown the relative reaction rates of SO4• withsomemonomersat25C[53–57].As explained, the rate of addition of a radical to a double bond is affected by steric hindrance from bulky substituents. Polar effect, such as dipole interactions also influence the rate of addition. Forbes and Yashiro studied the addition of the initiating radials to methyl methacrylate in liquid supercritical carbon dioxide [58]. They demonstrated that the rate of addition of the initiating radicals to the monomers, k_add values, can be measured in liquid CO₂. Phenyl or methyl groups located on the carbon atom that is under a direct attack by a free radical can be expected to interfere sterically with the approach. For instance, due to steric hindrance, trans b-methyl stilbene is more reactive toward a radical attack than is its cis isomer [58]. Yet, the trans isomer is more stable of the two. While 1,1-disubstituted olefins homopolymerize readily, the 1,2 disubstituted olefins are hard to homopolymerize [59]. Some exceptions are vinyl carbonate [61] and maleimide derivatives [62]. Also, per fluoroethylene and chlorotrifluoroethylene polymerize readily. Table 3.6 shows the relative reaction rates of SO₄• with some monomers at 25C[50].

Homo polymerizations of diethyl fumarate by free-radical mechanism were reported [63]. The Mn was found to be 15,000. The same is true of homo polymerizations of several other dialkyl fumarates and also dialkyl maleates [64–66]. The polymerization rates and the sizes of the polymers that form decrease with increases in the lengths of linear alkyl ester groups. There is, however, an opposite correlation if the ester groups are branched. Also, the maleate esters appear to isomerize to fumarates prior to polymerization [66].

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مواضيع ذات صلة

Autoacceleration

Ceiling Temperature

Effect of Reaction Medium

Steric, Polar, and Resonance Effects in the Propagation Reaction

Propagation

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