Polar molecules, reactions with ions Polymerization

The process of setting in resin-modified glass-ionomers is complicated because of the variety of reactions involved [6]. In principle it consists of two distinct parts, namely an acid-base reaction as in conventional glass-ionomers, and a free radial polymerization reaction. However, the two reactions influence each other in various ways. The acid-base reaction is slowed down in these materials for two reasons. One is that, as we have seen, the presence of HEMA causes the molecules to adopt a more coiled configuration and this reduces the ease with which they donate protons and behave as acids. The other reason is that the reduced polarity of the HEMA/water mixture compared with pure water inhibits the activation step of the acid-base reaction. This step involves electronically charged species, namely hydrogen ions from the acid and... 

The polymerization reactions can sometimes be complicated by two different types of propagation paths. Some chains may grow without a terminal counterion as free propagating species. Other polymeric chains, however, may be paired off with counterions. It should be noted that when references are made to free propagating ions, the ions are free from electrostatic influences of the anions. They are, however, still associated with, and interact with, polar or polarizable solvent molecules or monomers. 

Lithium methoxide does not initiate polymerizations of methyl methacrylate. The ketone molecules, however, react with carbanions on the growing chain. The resultant anions are less reactive than methyl methacrylate anions and can only add new methyl methacrylate monomers slowly. Once added, however, the reaction proceeds at a normal rate. Polymerizations of methyl methacrylate in polar solvents, on the other hand, proceed in what might be described as an ideal manner with formations of only one kind of ion pair. ... 

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