Pseudoanionic mechanism

Here we discuss dispersion polymerizations that are not related to vinyl monomers and radical polymerization. The first one is the ring-opening polymerization of e-caprolactone in dioxane-heptane (30). A graft copolymer, poly(dodecyl acrylate)-g-poly(e-caprolactone), is used as a stabilizer. The polymerization proceeds via anionic or pseudoanionic mechanism initiated by diethylaluminum ethoxide or other catalysts. The size of poly(caprolactone) particles depends on the composition of stabilizer, ranging from 0.5 to 5 i,m. Lactide was also polymerized in a similar way. Poly(caprolactone) and poly(lactide) particles with a narrow size distribution are expected to be applied as degradable carriers of drugs and bioactive compounds. 

Methyl methacrylate polymerizations, initiated by oiganomagnesium compounds, also yield abnormal products. Here, the active centers are unusually persistent and stabile. In addition, the a-carbon atoms of the monomers were found to assume tetrahedral configurations.This suggests that the active centers contain covalent magnesium carbon bonds. Also, gel permeation chromatography curves of the products show that more than one active center operates independently. A pseudoanionic mechanism was therefore postulated for polymerizations of acrylic and methacrylic esters by Grignard reagents. 

The conclusion that the mechanism is pseudoanionic is restricted to organomagnesium initiated systems. The gist of our argument is that these polymerizations behave differently from the true anionic mechanisms initiated by (typically)... 

Any mechanism proposed for the formation of microspheres in the anionic or the pseudoanionic ring-opening polymerizations of lactides and e-caprolartone should conform to the following observations ... 

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