Polymerization chain initiation

What species is produced by the reaction of an anionic chain polymerization initiator and the monomer ... 

With most systems of this nature it is important that the two parts be mixed in the correct proportions (that is stoichiometrically) so that the reactions are completed and an excess of neither component is left in the hardened product. The most usual ratio of mixing is equal proportions of both parts (1 1) but differing ratios are by no means uncommon—especially for systems in which chain polymerization initiated by free radicals takes place (when relatively small amounts of free radical initiators will start the setting). 

Bifunctional initiators such as bis-(2-glycylaminophenyl) disulfide (GAPDS) and 2 -azo-2,2 -dimethyldipropiono hydrazide (AMPH) have been used [152] to obtain hydrocaibon chain polymeric initiators with amino end groups able to promote the polymerization of amincmcid A-carboxy anhydrides (NCA) (Scheme 47). 

Problem 6.8 Consider the following scheme of reactions for free-radical chain polymerization initiated by thermal homolysis of initiator with cage effect [6] ... 

Okawa, Y. and Aono, M., Linear chain polymerization initiated by a scanning tunneling microscope tip at designated positions, J. Chem. Phys. 115, 2317-2322, 2001. 

In chain polymerization initiated by free radicals, as in the previous example, the reactive center, located at the growing end of the molecule, is a free radical. As mentioned previously, chain polymerizations may also be initiated by ionic systems. In such cases, the reactive center is ionic, i.e., a carbonium ion (in cationic initiation) or a carbanion (in anionic initiation). Regardless of the chain initiation mechanism—free radical, cationic, or anionic—once a reactive center is produced it adds many more molecules in a chain reaction and grows quite large extremely rapidly, usually within a few seconds or less. (However, the relative slowness of the initiation stage causes the overall rate of reaction to be slow and the conver-... 

The mechanism of cure is quite different for cyanoacrylates on one hand and silicones and isocyanates on the other. In the former case, cure is by Chain polymerization initiated by water on the surface of the substrate, while in the latter, water from the atmosphere diffuses into the adhesive or sealant and then participates in a Step polymerization. One consequence of this is that only catalytic qnantities of water are reqnired to cure cyanoacrylates, while much larger, stoichiometric quantities are needed for silicones and isocyanates. 

Prucker, O., and Ruhe, J. Mechanism of radical chain polymerizations initiated by azo compounds covalently boimd to the surface of spherical particles. Macromolecules 31 602-13. 

The low values of activation energy for mechanical breakdown have been explained by the existence of weak links in the polymer chains [21-23], The type of chain polymerization initiator or other polymerization conditions, such as inclusion of oxygen in the chain [24], can lead to the incorporation of weak bonds in the chains [22, 25], Weak bonds have also been postulated from thermal degradation studies [26],... 

---