Dual role for alkali metal cations in enhancing the low-temperature radical polymerization of N,N-dimethylacrylamide

Abstract

The radical polymerization of N,N-dimethylacrylamide (DMAAm) has been investigated in the presence of several alkali metal salts, including lithium bis(trifluoromethanesulfonyl)imide (LiNTf 2 ). The addition of an alkali metal salt led to a significant increase in the yield and molecular weight of the resulting polymer.NMR analysis of mixtures of DMAAm and LiNTf 2 suggested that DMAAm was being activated by the coordination of Li + to its CvO group. Electron spin resonance analysis of the DMAAm polymerization in the presence of LiNTf 2 suggested that the propagating radical was being stabilized by Li + through a single-electron lithium bond, because a signal for the propagating radical of the acrylamide derivatives was observed for the first time in solution when LiNTf 2 was added. Based on these results, we have proposed a mechanism for this polymerization, where the propagation steps occur between a lithium ionstabilized propagating radical and a lithium ion-activated incoming monomer. Furthermore, polymers with a wide range of stereoregularities, such as isotactic, syndiotactic and heterotactic systems, were successfully prepared using this method by carefully selecting the appropriate combination of solvent and alkali metal salt. † Electronic supplementary information (ESI) available: Dependence of polymer yield on the added amount of LiNTf 2 in toluene, additional 1 H NMR spectra of the main-chain methylene groups of the poly(DMAAm)s prepared in this study and changes in the chemical shifts of DMAAm in the presence of MNTf 2 . See View Article Online a [Monomer] 0 = 1.0 mol L −1 , [MAIB] 0 = 1.0 × 10 −2 mol L −1 , [alkali metal salts] 0 = 1.0 mol L −1 . b Determined from 1 H NMR signals. c Determined by SEC (PMMA standards). d Alkali metal salt was not completely dissolved. e Polymer precipitated during polymerization reaction. Polymer Chemistry PaperThis journal is

Photopolymerization of coordinated ionic liquid monomers: Realizing the benefits of structured media using only common reagents

“…53,54 Indeed, in spectroscopic investigations of vinyl monomers and Li 1 salts, cation complexation has been detected using IR as well as 1 H and 13 C NMR spectroscopy. 55,56,[58][59][60] Based on similar evidence, it can be confirmed that these species are also formed in our study. As displayed in Figure 9, the C@O stretching peaks of MMA undergo shifts to lower wavenumbers following the addition of LiTf 2 N at monomer:salt ratios of 1:1 and 2:1.…”

“…56,58,59 The mechanism of the reduction in the rate of termination in these reactions has been assigned to a variety of factors including radical stabilization and single electron-alkali metal interactions as well as electrostatic repulsion between complexed macroradicals. 56,58,59,62 The improvements in the reaction kinetics of the polar vinyl monomers in this study may be due, in part, to these effects. We attribute improvements in overall monomer conversions to structuring of the monomers about the Li 1 cation (Fig.…”

“…57 The use of alkali Tf 2 N salts in the solution polymerization of acrylamide (ACR) and methacrylamide (MACR) monomers has been shown to increase monomer conversion and product molecular weight while also altering the tacticity of the resulting polymers. 58,59 A 2011 study completed by Pedr on and et al, in which several ethylene oxide methacrylate monomers with 1, 2, or 3 ether groups in the side chain were combined with LiOTf, with reported improvements in the reaction kinetics for these solutions compared to bulk reaction mixtures. 60 However, LiOTf would be expected to have different kinetic effects than a more weakly coordinating Li 1 salt such as LiTf 2 N. Based on our experiences, LiOTf does not sufficiently dissolve in non-ether-containing monomers to form coordinated ILs, while LiTf 2 N is capable of dissolving at up to 1:1 (mol:mol) in monomers without ethers such as MMA without cosolvent.…”

Syndiotactic- and heterotactic-specific radical polymerization of N-n-propylmethacrylamide complexed with alkali metal ions

“…[25][26][27][29][30][31][32][33][34][35][36][37][38][39][40][41] Recently, we have also found that inclusion of alkali metal bis(trifluoromethanesulfonyl)imides (MNTf 2 ), such as LiNTf 2 , in the radical polymerization of N,Ndimethylacrylamide (DMAAm) leads to a significant increase in the yield and molecular weight of the resulting polymer. 42 NMR analysis of a mixture of DMAAm and LiNTf 2 suggests that DMAAm is activated by the coordination of Li + to its CvO group. Furthermore, electron spin resonance (ESR) spectroscopic analysis of the polymerization in the presence of LiNTf 2 suggests that the propagating radical is stabilized by Li + , probably through a single-electron lithium bonding, which is a recently proposed theory.…”

Mechanistic Perspectives on Stereocontrol in Lewis Acid-Mediated Radical Polymerization

“…152 More recently, Hirano and coworkers examined the use of high concentrations of alkali metal triflates and triflimides (MNTf 2 ), finding that the relatively weak LA, NaNTf 2 , in a 1:1 ratio with monomer increased isotactic content to m ¼ 85% in DMA polymerizations. 153 While this example of isotactic control is intriguing, it is probably more readily attributed to electrostatic repulsion between the side-chains rather than a chelation mechanism.…”