Michael Addition Polymerization
(Conjugated Additon Reactions)
The Michael addition reaction (1887)1 is a versatile method for the addition of various nucleophiles to (conjugated) unsaturated compounds with electron withdrawing substituents. It allows for the synthesis of a wide range of highly complex macromolecules under relative mild conditions and in a very efficient manner with often quantitative yield. Basically, any monomer with an activated double bond such as, α,β-unsaturated aldehydes/ketones, vinyl esters, vinyl sulfones, imidazoles, and maleimides etc. will undergo a Michael addition with a nucleophile such as thiol, amine or any stabilized carbanion:
The reaction rate is highly affected by steric hindrance of the nucleophile and the activated diene. For example, aromatic amines and secondary amines with bulky groups do only slowly react with α,β-unsaturated compounds such as simple vinyl ester and require a strong Lewis acid2, whereas primary aliphatic amines react readily with vinyl ester in the presence of a Lewis acid catalyst. The order of reactivity for amine addition to unsaturated esters typically decreases in the order acrylates > maleates > fumarates > methacrylates. The ease of a Michael addition also depends upon the type of nucleophile. Sulfur nucleophiles R-SH usually react faster than primary amines R-NH2 followed by secondary amines R2NH and alcohols R-OH. The last two nucleophiles react very slowly with electron-poor olefins such as conjugated aldehydes, ketones, esters, and nitriles and usually require a strong Lewis acid catalyst.
Michael addition reactions can also be employed to prepare polymers of various architectures. The monomers of this type of
step-growth polymerization are molecules that contain conjugated bisdienes and bisdienophiles (A-A and B-B co-monomers).
A well known example is the addition of a bisthiol to a
Michael addition reactions have also been applied to prepare thermosetting resins4. An example is the aza-Michael addition of a polyamine such as ethylenediamine, diethylenetriamine (DETA), or 1,3-diaminopropane to a diacrylate.5 Theses adducts undergo free radical photopolymerization when exposed to UV light. During the first stage, an off-stoichiometric amount of amino and acrylate groups are partially crosslinked through aza-Michael addition catalyzed by the tertiary amines formed during this reactions. The addition often proceeds at room temperature. In the second stage, the oligomers are fully crosslinked via free-radical photompolymerization of the excess amount of acrylic groups (dual cure).
References & Notes
A. Michael, Ueber die Addition von Natriumacetessig- und Natriummalonsäureäthern zu den Aethern ungesättigter Säuren, Journal für Praktische Chemie, Volume 35, Issue 1, 349-356, 21 March 1887
Feric chloride FeCl3, and aluminum chloride AlCl3 are excellent catalysts for the addition of primary and secondary amines to acrylates with near-quantitative yield.3
J. Cabral, et al., Tetrahedron Letters, Vol. 30, Issue 30, pp. 3969-3972 (1989)
G. Gonzales, et al., Polym. Chem., 6, 6987-6997 (2015)
Aza-Michael addition is the reaction of an amine with an electron poor C=C double bond.
Michael addition reactions are usually thermally reversible, which is known as a retro-Michael reaction. Thus, the adducts will revert to the starting compounds at elevated temperatures. In the case of a bismaleimide, the reversion takes place in the vicinity of 300°C. (US 20110152466)