Coordination−Addition Polymerization and Kinetic Resolution of Methacrylamides by Chiral Metallocene Catalysts

Abstract

This contribution reports the first successful coordination-addition polymerization of N,Ndialkylmethacrylamides and the first example of kinetic resolution of a racemic methacrylamide by chiral metallocene catalysts. The polymerization of methacryloyl-2-methylaziridine (MMAz) by rac-(EBI)Zr + -(THF)[OC(O i Pr)dCMe 2 ][MeB(C 6 F 5 ) 3 ] -(1) is stereospecific and also exhibits a high degree of control over polymerization. This polymerization follows first-order kinetics in both concentrations of monomer and catalyst, consistent with a monometallic propagation mechanism involving the fast step of intramolecular conjugate addition within the catalyst-monomer coordination complex leading to the eight-membered-ring resting intermediate. Substituents on the highly strained aziridine ring stabilize the aziridine moiety against thermally induced crosslinking through its ring-opening reaction; thus, the polymer derived from methacryloyl-tetramethyleneaziridine (MTMAz) exhibits greatly enhanced resistance toward thermal cross-linking over poly(MMAz), marking 57 and 42 °C higher onset cross-linking and maximum cross-linking temperatures, respectively. Enantiomeric catalyst (S,S)-1 demonstrates experimentally and theoretically its ability to kinetically resolve the racemic MMAz monomer with a low stereoselectivity factor s of 1.8. Polymerizability of several methacrylamide monomers has been investigated via a combined experimental and theoretical (DFT) study that examines the degree of conjugation between the vinyl and carbonyl double bonds, relative polymerization reactivity, and relative energy for the formation of amide-enolate intermediates.

Propagation Kinetics of the Radical Polymerization of Methylated Acrylamides in Aqueous Solution

“…According to Okamoto and Yuki, this difference may explain why N , N ‐disubstituted methacrylamides do not undergo radical homopolymerization. The explanation based on the s‐ cis conformation is supported by Miyake et al who reported poor overlap between the π molecular orbitals of the carbon–carbon and the carbon–oxygen double bonds in the case of N , N ‐dimethylmethacrylamide. Differences in conformation of AAm and DM‐AAm as well as of MAAm and M‐MAAm may also explain the difference in activation energies since the energy of the transition state structure should be less dependent on monomer conformation than the energy of the educts.…”

Metal‐Catalyzed Group‐Transfer Polymerization: A Versatile Tool for Tailor‐Made Functional (Co)Polymers

“…They introduced highly active chiral ansa ‐metallocenium enolate cations ( rac ‐(C 2 H 4 (Ind) 2 )Zr + (THF)[OC(O i Pr)=CMe 2 ][MeB(C 6 F 5 ) 3 ] − ) for the isoselective polymerization of methyacrylates (e.g., methyl‐ and n‐ butylmethacrylate) and various acrylamides. The polymerization proceeded in a living, monometallic and intramolecular coordinative‐conjugate‐addition mechanism via a cyclic ester enolate intermediate (Scheme , right) . As sustainability is an emerging topic in the field of macromolecular chemistry, Chen et al.…”

Rare Earth Metal‐Mediated Group Transfer Polymerization of Vinylphosphonates

“…Similar observations were made for different methacrylamides, that is, those for which the steric repulsion of the α‐methyl with the N –alkyl groups leads to a torsion of the C–C single bond thereby disturbing the molecules planarity (such as N , N ‐dimethyl methacrylamide (DMAA), Figure 5). 6, 29…”