Pseudoanionic Polymerizations

Duda A, Penczek S (1991) Anionic and pseudoanionic polymerization of e-caprolactone. Makromol Chem, Macromol Symp 42/43 135-143... 

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. 

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)... 

If pseudoanionic polymerization occurs anywhere else in the periodic table, beryllium and lithium are the most likely candidates. 

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. 

In spite of the living character of the anionic polymerization of PL, it was revealed that in the case of p-substituted p-lactones or medium-size cyclic esters (LA and CL) this process suffers from various side reactions such as chain transfer to monomer, racemization, segment exchange, or formation of macrocydic esters. " Finally, it has been concluded that in the so-called coordination-insertion (pseudoanionic) polymerization, initiated with covalent metal alkoxides (R MtOR c. where Mt is Zn, Al, Sn, Ti, etc.), these side reactions can be kinetically suppressed or even eliminated. Correlation of the selectivity parameters, defined as the ratio of rate constants of propagation and transfer (fep/fetr). with the reactivity of active species (fep) showed that these polymerizing systems conform to the reactivity-selectivity principle. ... 

Systematic kinetic measurements, carried out during the past decade " revealed the importance of aggregation phenomena in the pseudoanionic polymerizations. The solution of the pertinent kinetic scheme involving aggregation " allowed determination of the absolute rate constants of propagation on the covalent species. Then, it has been shown that... 

Anionic and Pseudoanionic Ring-Opening Dispersion Polymerization 647... 

Continuous Media for the Anionic and Pseudoanionic Dispersion Polymerizations of e-Caprolactone... 

The anionic and pseudoanionic ring-opening polymerizations were studied only for 8-caprolactone and for the cyclic dimers of lactic acid (L,L-lactide, D,D-lactide, and o,L-lactide). 

Continuous media for the anionic or pseudoanionic dispersion polymerization of cyclic esters should fulfill the following... 

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 ... 

Figure 10 shows the dependence of the apparent propagation rate constants of the anionic and pseudoanionic dispersion polymerizations of e-caprolactone (fepU) on the concentration of propagating species. In Figure 10, there are also given plots of the ratios of the apparent propagation rate constants for dispersion polymerization and for the polymerization in solution (J ff/fep] ). 

The calculated number-average molecular weight (Ain (calcd)) was obtained assuming that the functionality of initiators of the anionic and pseudoanionic polymerizations was equal to 1 and that all the monomer was converted into polymer ... 

Figure 11 Relation between the experimentally determined molecular weight, dispersity (MJMn), and calculated number-average molecular weight (Mn(calcd)) for the pseudoanionic and anionic polymerization of s-caprolactone. Reproduced with permission from Slomkowski, S. Sosnowski, S. Gadzinowski, M. etal. Macromol. Symp. 2000, 150, 259F...

Anionic and Coordination Polymerization Aggregates of ion pairs, or of covalent species formed reversibly in the anioitic or pseudoanionic polymerization of EO [152, 153], cyclosiloxanes [154] and cychc esters [79, 155-157], were found to be essentially unreactive thus, in polymer chain growth only unimeric, deaggregated species will participate. The pertinent kinetic scheme reads ... 

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