Biomedical Engineering Reference
In-Depth Information
3
Polymerization in Homogeneous Media
Because of the presence of two powerful electro-withdrawing groups in the a-carbon
of the double bond (i.e., ester and nitrile), alkyl cyanoacrylate monomers exhibit a
very high reactivity toward nucleophiles such as anions (hydroxide, iodide, alco-
holate, etc.) or weak bases (alcohol, amine, etc.), thus resulting in a very high
polymerization rate. Traces of one of the above-mentioned compounds in the reac-
tion medium can initiate the polymerization, which explains why batches of alkyl
cyanoacrylates are usually stored with a small amount of acidic stabilizers (e.g., SO
2
,
sulfonic acid, etc.).
Depending on the experimental conditions and reactants, three different mecha-
nisms are usually well-admitted for the polymerization of alkyl cyanoacrylates
(Fig.
2
): (i) anionic; (ii) zwitterionic and (iii) radical (Nicolas and Couvreur
2009
).
Those polymerizations are respectively initiated by charged nucleophiles (Nu
-
),
protic nucleophiles (Nu) and radicals (P
•
). However, due to the very high reactivity
of alkyl cyanoacrylate monomers, anionic and zwitterionic polymerizations are
highly predominant under conventional experimental conditions with respect to a
radical process.
Fig. 2
Initiation and propagation steps involved during anionic (
a
), zwitterionic (
b
), and radical
(
c
) polymerizations of alkyl cyanoacrylates initiated by a charged nucleophiles (Nu-), a protic
nucleophile (Nu) and a radical (P•), respectively
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