Synthesis of Highly Isotactic Poly(N-isopropylacrylamide) by Anionic Polymerization of a Protected Monomer

Abstract

KEY WORDSN-Isopropylacrylamide / Acrylamide / Trialkylaluminum / Isotactic Polymer / Stereospecific Polymerization / Protected Monomer / Isomerization / Stereoregularity of vinyl polymers often affects significantly their properties and functions, and thus stereospecific polymerization has been important from the view points of both polymer chemistry and industry. In vinyl polymerization ionic and coordination processes have been considered favorable for the stereocontrol, in which proper selection of initiators, catalysts, additives and solvents may provide us with a handle for controlling stereoregularity in a wide variety. For example, we have shown that highly isotactic, syndiotactic, and heterotactic polymethacrylates could be prepared in toluene by using several anionic initiators; tert-butylmagnesium bromide (t-BuMgBr) for isotactic polymerization 1, 2 tert-butyllithium (t-BuLi) / trialkylaluminum (R 3 Al) for syndiotactic one 3, 4 and t-BuLi / bis(2,6-di-tert-butylphenoxy)methylaluminum [MeAl(ODBP) 2 ] for heterotactic one. [5][6][7][8][9][10][11][12][13] Polymers of acrylamide and its derivatives including N-isopropylacrylamide (NIPAm) are widely used in industry, agriculture and medicine due to their remarkable properties including good biocompatibility and low toxicity. 14, 15 Electron-withdrawing nature of amide groups might provide the monomers with potential reactivity favorable for anionic polymerization. However, the presence of acidic hydrogen in acrylamide itself and monosubstituted acrylamide inhibits the anionic vinyl polymerization.Thus radical polymerization has been so far the only means to obtain the corresponding polymers with few exceptions such as hydrogentransfer polymerization of α-(aminomethyl)acrylates 16 and N-acryloylurea derivatives with potassium tertbutoxide or 1,8-diazabicyclo[5.4.0]undec-7-ene. [17][18][19][20][21] As a consequence, stereospecific anioic polymerization of acrylamide derivatives have only been known for N, N-disubstituted ones. 22-28 Nakahama and his coworkers have reported successful stereospecific anionic polymerization of N, N-diethylacrylamide (DEA) in tetrahydrofuran (THF) by careful selection of Lewis-acid additives and counter cations. A syndiotactic poly(DEA) was prepared with alkyllithium initiator [1,1-bis(4 -trimethylsilylphenyl)-3-methylpentyllithium]/diethylzinc (Et 2 Zn), an isotactic one with t-BuMgBr/Et 2 Zn, and a heterotactic one with diphenylmethylpotassium (Ph 2 CHK)/Et 2 Zn. [29][30][31][32][33][34] In 1996, Okamoto and his coworkers 35 reported the anionic polymerization of N, N-diphenylacrylamide with t-BuMgBr in toluene at −78 • C, which gave an isotactic polymer with m diad of 80%. More recently, they also found that, in radical polymerizations of acrylamide and its derivatives, rare-earth metal trifluoromethanesulfonates (triflates) as Lewis-acid additives promoted stereospecific polymerization and, in particular, isotactic poly(NIPAm)s with m diad content of 92% were obtained with lutetium and yttrium triflates in methanol at −20 • ...

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

“…The 13 C NMR signals of the CvO groups of NNPMAAm showed downfield shifts following the addition of the alkali metal salts (ESI Fig. S1 †).…”

“…As a result, control of a wide range of stereoregularities, such as isotacticity, syndiotacticity, and heterotacticity, has been achieved. 13,14 However, this protection strategy is unsuitable for N-alkylmethacrylamides, because N,N-disubstituted methacrylamides, which correspond to protected N-alkylmethacrylamides, show non-polymerizability via any reaction mechanism, [15][16][17][18][19][20] unless the N-substituents are small and highly strained ring structures, such as aziridine and azetidine. 1), which are amide analogs of methacrylates, are scarce.…”

Prolong and sustain

“…The phase transition behavior of aqueous solutions of poly(NIPAAm) has been extensively studied 14–16. It was reported recently that isotactic poly(NIPAAm) exhibited a lower‐phase transition temperature than the atactic polymer 17, 18. We have also reported that increase in syndiotacticity of poly(NIPAAm) increased both the phase transition temperature and the cooperativity of phase transition, and reduced the hysteresis between heating and cooling cycles 5(a).…”

Application of multivariate analysis of NMR spectra of poly(N-isopropylacrylamide) to assignment of stereostructures and prediction of tacticity distribution

“…[8][9][10][11] The diad tacticities were easily determined from the 1 H NMR signals of the methylene groups in the main chain; the signals showed typical splitting as a result of diad stereostructures, as measured in deuterated dimethyl sulfoxide (DMSO-d 6 ) at 150 1C. [12][13][14][15] However, assignments, in terms of triad stereostructures, had not previously been made. We therefore assigned the 13 C NMR signals of the methine carbons in the main chain, at triad levels, by comparing the NMR spectra of poly(NIPAAm)s with a wide range of stereoregularities, such as isotactic, syndiotactic and heterotactic polymers, reported previously.…”