Radical-monomer reactions steric effects

The rates of radical-monomer reactions are also dependent on considerations of steric hindrance. This is easily observed by considering the reactivities of di, tri-, and tetrasubstituted ethylenes in copolymerization. Table 6-5 shows the kn values for the reactions of various chloroethylenes with vinyl acetate, styrene, and acrylonitrile radicals. The effect of a second substituent on monomer reactivity is approximately additive when both substituents are in the 1- or a-position. However, a second substituent when in the 2- or (3-position of the monomer results in a decrease in reactivity due to steric hindrance between it and the radical to which it is adding. Thus 2-10-fold increases and 2-20-fold decreases in the reactivities of vinylidene chloride and 1,2-dichloroethylene, respectively, are observed compared to vinyl chloride. 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

Steric influences may retard some radical polymerizations and copolymerizations. Double bonds between substituted carbon atoms are relatively inert (unless the substituents are F atoms) and 1,2-substituted ethylenes do not homopolymerize in normal radical reactions. Where there is some tendency of such monomers to enter into polymers, the trans isomer is more reactive. When consideration is restricted to monomers that are doubly substituted on one carbon atom, it is usually assumed that steric effects can be neglected and that the influence of the two substituents is additive. Thus vinylidene chloride is generally more reactive in copolymerizations than is vinyl chloride. 

The unpaired electron is thought to reside in a pure p-orbital so that RiR2R3C- is a planar molecule. R4R5C=CRgR7 is also planar with a filled jr-orbital so that reaction can occur only on parallel approach of the reactants from above or below the plane of the RjRgRgC radical. Steric effects due to the bulkiness of the substituents R to Ry markedly reduce the rate of reaction. For example 1,2-disubstituted olefin monomers are known to be unreactive in homopolymerization (5). A polymer crystal lattice must have a similar effect if the reactive free radical end of the molecule is fixed in the crystal lattice. 

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. 

Attack of monomer at the methylene carbon atom is less sterically hindered and yields a free radical that is more stable because the substituent group X stabilizes the free-radical site by steric hindrance and, in many cases, also by mesomeric stabilization. (Inductive effects are not important because the free-radical site bears no charge.) Thus the reaction is regioselective with mode (I) addition predominating. 

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 ... 

Figures shows the potential range where some heterocycles polymerizeThe cathodic cutoff for the polymerization (around 1.2 V) occurs when the stability of the radical cation is enhanced (intrinsically or via a substituent). When becomes greater than kp - - k ([S] + PC ]) diffusion of R+ from the electrode results in the production of soluble products. The anodic cutoff (around 2.1 V) occurs when k,([S] -t- [X ]) > (kp -t- k ). Then R" becomes unstable and reacts with the solvent or anions. Between around 1.2 and 2.1 V good conditions for the electropolymerization of such monomers exists where kp > k q- k ([S] -f- [X ]). The influence of substituents in pyrroles, thiophenes, indoles, azulenes, fluorenes, and pyrenes on whether electropolymerization of the monomers or other reactions can occur has been discussed in detail including consideration of electronic or steric effects...

The other class of compounds useful for degenerative transfer reactions are those with either C = C or C = S double bonds. Methacrylate derivatives have transfer rates similar to that of the propagation of methacrylates, and are successful only for the polymerization of methacrylates [35,36]. Due to steric effects the intermediate radical shown in Scheme 12 cannot react directly with monomer but only fragment. Unfortunately, mono substituted alkenes such as styrenes and acrylates react with the intermediate radicals and give branched structures, i.e., there is inefficient fragmentation. 

When such comparisons are made it becomes clear that the reactivities of radicals, monomers, or transfer agents depend on the particular reaction being considered. It is not possible to conclude, for example, that polyfvinyl acetate) radical will always react x times more rapidly than polystyrene radical in addition reactions or y times as rapidly in the atom abstraction reactions involved in chain transfer. Similarly the relative order of efficiency of chain transfer agents will not be the same for all radical polymerizations. This is because resonance, steric, and polar influences all come into play and their effects can depend on the particular species involved in a reaction. 

Normally reactivity ratios lie between 0 and 1 (Table 2.9) and so there is usually a tendency toward alternation in most copolymerization reactions. It is found that for the same pair of monomer molecules the reactivity ratios can differ greatly depending upon the nature of the chain carrier used (i.e. free radical, cationic or anionic). Obviously the rate constants fcii, k 2, ki2 and k2 will be affected in different ways by the nature of the active centre and it is found that the relative reactivity of different monomers can be correlated with resonance stability, polarity and steric effects. Such correlations are beyond the scope of this book and the reader is directed towards more advanced texts. 

The rates of radical-forming thermal decomposition of four families of free radical initiators can be predicted from a sum of transition state and reactant state effects. The four families of initiators are trarw-symmetric bisalkyl diazenes,trans-phenyl, alkyl diazenes, peresters and hydrocarbons (carbon-carbon bond homolysis). Transition state effects are calculated by the HMD pi- delocalization energies of the alkyl radicals formed in the reactions. Reactant state effects are estimated from standard steric parameters. For each family of initiators, linear energy relationships have been created for calculating the rates at which members of the family decompose at given temperatures. These numerical relationships should be useful for predicting rates of decomposition for potential new initiators for the free radical polymerization of vinyl monomers under extraordinary conditions. 

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