Monomers, mechanism, side reactions

The outcome of the polymerization depends strongly on the particular monomer. Polymerizations of S, MMA, MA, VAc and some derivatives have been reported. Studies on model compounds indicate that the primary or secondary dithiocarbamatc end groups arc much less susceptible to photodissociation than benzyl or tertiary derivatives. 

Dithiocarbamatc 16 has been used to prepare low dispersity PM A A ( / M, 1.2), Photopolynierization of S in the presence of dithiocarbamatc 16 

Chain ends formed with monosubstituted monomers, other than S, appear resistant to photolysis and polymerizations of MA and VAc do not show living characteristics. Most polymerizations involve methacrylate esters or S. 

Various side reactions that are likely to lead to a slow loss of living ends have been described. With disulfide initiators, one (initiation by the dithiocarbamyl radical) is unavoidable since tlie experiment relies on the same radical species to both initiate polymerization and tenninate chains. 

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There will be a gradual loss of stable radical with these systems as the di- or triarylmethyl radicals produced from the macroinitiator can add monomer, albeit slowly.99 100 This side reaction provides a mechanism for mopping up the excess stable radical formed as a consequence of termination between propagating radicals and may be essential to maintaining polymerization rates. 

Anionic polymerization represents a powerful technique for synthesizing polymers with low PDI values, thus providing good control over the chain length. This method leads to less side reactions than radical polymerizations. For instance, unlike in radical polymerization, there is no termination by the combination of two active chains. However, the mechanism is more sensitive to impurities and functional groups, and therefore applicable for only a limited class of monomers. 

Up to now it has not been possible to prove a mechanism which is able to explain all these effects. Neither intramolecular termination alone, nor a combination of intramolecular termination and monomer termination fit the data. However, they can be fitted by a hypothetical kinetic scheme which assumes that a slow reaction takes place between the living end and a deactivating species which is produced in the initial stage of the reaction. The dependence of the rate of termination on the initial monomer concentration can only be explained on the basis of a side reaction occuring in the initiation process. To verify this model, however, the deactivating species has to be identified by analyzing the oligomers produced in the initiation step. 

The application of polymeric plasticizers can be effected by mixing a preformed polymer mechanically with a polymerizable monomer containing the catalyst and subsequent polymerization. But side reactions, like grafting, must be expected. Generally, this method has not found wide application because the resulting products are likely to show poor mechanical properties. Besides, mixing the monomer on the rolls or in the extruder leads to losses by evaporation and causes unpleasant odors. Furthermore, some of the more important monomers, like the acrylates, show poor compatibility with many polymers. For instance, PVC is not sufficiently plasticizable with acrylates. 

The fundamental task, in our opinion, is to correlate the principles and methods of the proposed synthesis with those of mechanochemical synthesis. Thus, besides the destruction processes and mechanochemical synthesis discussed in the literature, other lands of transformations sometimes occur as side reactions, or even as major processes. These include chemical fixation of small molecules (methyl chloride or butyl alcohol) on mechanically activated macromolecular backbones grafting of inorganic surfaces (quartz, metals, metallic oxides, inorganic salts, etc.) dispersed by vibratory milling on polymerized fragments synthesized from monomers present in the reaction medium, and activated by centers on the inorganic surface (14) and the possibility of some reactions (such as nitration), achieved so far on macromolecular supports and only as side reactions. 

A number of authors propose a mechanism for oligomer formation. The mechanisms can be divided into two substantially different groups (a) oligomer formation is a side reaction of the polymerization, which means that oligomer is formed directly from monomer (b) oligomers are formed by degradation of polymer. 

Some monomers are also polymerized by a cationic mechanism in a series of steps not too unlike those of anionic chain-growth. Initiators are often Lewis acids such as AICI3. The polymerization is not quite as straightforward as anionic, because for one thing cationic intermediates are subject to more side reactions. Common monomers that undergo cationic polymerization include styrene, isobutylene, and vinyl acetate. Some commercial products... 

For further elucidation of the initiation mechanism in polymerizations of polar monomers with organometals, a method for determining the instantan-neous concentrations of active centres has to be found, as well as an initiating system acting without side reactions. 

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