Lewis‐Acid‐Mediated Stereospecific Radical Polymerization of Acrylimides Bearing Chiral Oxazolidinones

Lewis acid (MgBr₂)‐catalyzed radical polymerization of acrylimides bearing chiral oxazolidinones gave highly isotactic polyacrylimides with up to >99 % meso tetrad (mmm) selectivity. Polymerization in the absence of Lewis acid gave atactic polymers with 80 % racemo diad (r) selectivity; the selectivity was deliberately tuned from 80 % r to >99 % mmm by varying the polymerization conditions. The polyacrylimide was quantitatively converted to corresponding polyacrylates while preserving the stereoregularity, thus providing a general method for the synthesis of atactic to isotactic polyacrylates.     

Stereospecific polymerization of benzyl α-(alkoxymethyl) acrylates

α-(Alkoxymethyl) acrylates, such as methyl α-(phenoxymethyl) acrylate, benzyl α-(methoxymethyl)acrylate (BMMA), benzyl α-(benzyloxymethyl)acrylate, and benzyl α-(tert-butoxymethyl)acrylate, were synthesized, and their polymerizability and the stereoregularity of the polymers obtained by radical and anionic methods were investigated. The radically obtained polymers were found to be atactic by ¹³C- and ¹H-NMR analyses, but the polymers obtained with lithium reagents in toluene at −78°C were highly isotactic. Further, it is noteworthy that isotactic polymers were also produced with lithium reagents even in tetrahydrofuran. Effects of polymerization temperature and counter cation on stereoregularity were clearly observed in the polymerization of BMMA, and a potassium reagent afforded an almost atactic polymer. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 721–726, 1997     

Preparation of isotactic‐rich poly(dimethyl vinylphosphonate) and poly(vinylphosphonic acid) via the anionic polymerization of dimethyl vinylphosphonate

A vinylphosphonate monomer, dimethyl vinylphosphonate (DMVP), has been polymerized by anionic initiators. Anionic polymerization of DMVP with tert‐butyllithium (t‐BuLi) in combination with a Lewis acid, tributylaluminum (n‐Bu₃Al), in toluene proceeded smoothly to give an isotactic‐rich poly(dimethyl vinylphosphonate) (PDMVP) with relatively narrow molecular weight distribution. Although all the PDMVPs were soluble in water, the isotactic‐rich PDMVP was insoluble in acetone and in chloroform which are good solvents for an atactic PDMVP prepared by radical polymerization. The isotactic‐rich PDMVP showed higher thermal property than that of the atactic PDMVP. Moreover, we successfully prepared poly(vinylphosphonic acid) (PVPA) through the hydrolysis of the isotactic‐rich PDMVP, which formed a highly transparent, self‐standing film. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1677–1682, 2010     

Conformations et stereoregularite de poly 2 vinyl pyridines—I Etude experimentale

Study of the average molecular optical anisotropy 〈γ²〉 of atactic and isotactic poly 2 vinyl pyridines of low polydispersity has been carried out in various solvents by means of depolarized Rayleigh scattering (D.R.S.) Experimental evidence (D.R.S. and viscosity) for free charges both on atactic and isotactic chains has been found in methanol and dimethyl formamide; their effect on 〈γ²〉 has been analyzed. The influence on molecular optical anisotropy of thermodynamic quality of the solvent has also been studied. Finally, comparison of the experimental optical anisotropies of atactic and isotactic structures reveals the strong effect of stereoregularity on the physical property under study.     

Living anionic polymerization of N‐methoxymethyl‐N‐isopropylacrylamide: Synthesis of well‐defined poly(N‐isopropylacrylamide) having various stereoregularity

Anionic polymerization of N‐methoxymethyl‐N‐isopropylacrylamide ( 1 ) was carried out with 1,1‐diphenyl‐3‐methylpentyllithium and diphenylmethyllithium, ‐potassium, and ‐cesium in THF at ?78 °C for 2 h in the presence of Et₂Zn. The poly( 1 )s were quantitatively obtained and possessed the predicted molecular weights based on the feed molar ratios between monomer to initiators and narrow molecular weight distributions (M_w/M_n = 1.1). The living character of propagating carbanion of poly( 1 ) either at 0 or ?78 °C was confirmed by the quantitative efficiency of the sequential block copolymerization using N,N‐diethylacrylamide as a second monomer. The methoxymethyl group of the resulting poly( 1 ) was completely removed to give a well‐defined poly(N‐isopropylacrylamide), poly(NIPAM), via the acidic hydrolysis. The racemo diad contents in the poly(NIPAM)s could be widely changed from 15 to 83% by choosing the initiator systems for 1 . The poly(NIPAM)s obtained with Li⁺/Et₂Zn initiator system possessed syndiotactic‐rich configurations (r = 75–83%), while either atactic (r = 50%) or isotactic poly(NIPAM) (r = 15–22%) was generated with K⁺/Et₂Zn or Li⁺/LiCl initiator system, respectively. Atactic and syndiotactic poly(NIPAM)s (42 < r < 83%) were water‐soluble, whereas isotactic‐rich one (r < 31%) was insoluble in water. The cloud points of the aqueous solution of poly(NIPAM)s increased from 32 to 37 °C with the r‐contents. These indicated the significant effect of stereoregularity of the poly(NIPAM) on the water‐solubility and the cloud point in water © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4832–4845, 2006     

Synthesis,complex formation,and dilute‐solution associative behavior of linear poly(methacrylic acid)‐graft‐poly(2‐ethyl‐2‐oxazoline)*

Poly(methacrylic acid) (PMA) and poly(2‐ethyl‐2‐oxazoline) (PEOZO) are a polyacid/polybase pair capable of forming reversible, pH‐responsive, hydrogen‐bonding complexes stabilized by hydrophobic effects in aqueous media. Linear PMA was modified with long‐chain (number‐average molecular weight: 10,000) PEOZO via statistical coupling reactions in organic media to prepare a series of PMA‐graft‐PEOZO copolymers. Potentiometric titrations revealed that the presence of tethered PEOZO markedly increases the pK_a values for PMA‐g‐PEOZO copolymers as compared with simple PMA/PEOZO mixtures at degrees of ionization, α, between 0.0 and 0.1. The dilute‐solution PMA–PEOZO intramolecular association has been probed by monitoring the PEOZO NMR spin–spin (T₂) relaxation as a function of pH. Covalently attached PEOZO side chains participate in complexation at higher values of α than untethered PEOZO. Surprisingly, most PEOZO side chains did not take part in hydrogen bonding at low α, and the highest level of PEOZO incorporation induced a decrease in the number of PMA/PEOZO hydrogen bonds. The polymer self‐diffusion as a function of α was measured with dynamic light scattering. At low pH, the copolymers had no charge and they were in a collapsed form. At high pH, the expected conformational expansion of the PMA units was enhanced at moderate levels of PEOZO incorporation. However, the highest PEOZO incorporation induced the onset of intramolecular associations between PEOZO units along the copolymer chains. Low shear rheometry and light scattering measurements were used in conjunction with the T₂ NMR measurements to propose a model consistent with the aforementioned behavior. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2520–2533, 2004     

A–B–A stereoblock copolymers of N‐isopropylacrylamide

A–B–A stereoblock polymers with atactic poly(N‐isopropylacrylamide) (PNIPAM) as a hydrophilic block (either A or B) and a non‐water‐soluble block consisting of isotactic PNIPAM were synthesized using reversible addition fragmentation chain transfer (RAFT) polymerizations. Yttrium trifluoromethanesulfonate was used in the tacticity control, and bifunctional S,S′‐bis(α,α′‐dimethyl‐α″‐acetic acid)‐trithiocarbonate (BDAT) was utilized as a RAFT agent. Chain structures of the A–B–A stereoblock copolymers were determined using ¹H NMR, SEC, and MALDI‐TOF mass spectrometry. BDAT proved to be an efficient RAFT agent in the controlled synthesis of stereoregular PNIPAM, and both atactic and isotactic PNIPAM were successfully used as macro RAFT agents. The glass transition temperatures (T_g) of the resulting polymers were measured by differential scanning calorimetry. We found that the T_g of isotactic PNIPAM is molecular weight dependent and varies in the present case between 115 and 158 °C. Stereoblock copolymers show only one T_g, indicating the miscibility of the blocks. Correspondingly, the T_g may be varied by varying the mutual lengths of the A and B blocks. The phase separation of aqueous solutions upon increasing temperature is strongly affected by the isotactic blocks. At a fixed concentration (5 mg/mL), an increase of the isotacticity of the stereoblock copolymers decreases the demixing temperature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 38–46, 2008     

The stereoregularity of polyacrylonitrile and its dependence on polymerization temperature

The stereoregularity of polyacrylonitrile was studied by NMR spectroscopy. The methylene protons in isotactic configuration are equivalent in dimethylformamide-d₇ solution and are nonequivalent in dimethylsulfoxide-d₆ solution. In the case of the latter solution the difference in chemical shift between the isotactic methylene protons is 6.6 cps. The stereoregularity of polyacrylonitrile-α-d instead of polyacrylonitrile was determined on the dimethylformamide-d₇ solution. In the radical polymerization all the polyacrylonitrile-α-d's that polymerized at temperatures between 80 and ?78°C. have a configuration consisting of about 50% of isotactic diads and, accordingly, the stereoregularity of polyacrylonitrile does not depend upon the polymerization temperature. Analysis of the NMR spectra of isotactic polyacrylonitrile prepared from acrylonitrile–urea canal complex was also carried out. The NMR spectra of meso- and dl-2,4-dicyanopentanes, dimer models of isotactic and syndiotactic polyacrylonitriles, respectively, were analyzed by a computer program proposed by Bothner-By.     

Molecular modeling of polymers. 10. Characterization of conformational transitions of poly(acrylic acid) and poly(methacrylic acid) using dyad structures

The conformational profiles of nearest side-chain neighbors, methylene-dyad structures, of poly(acrylic acid), PAA, and poly(methacrylic acid), PMA, were determined as a function of tacticity, extent of ionization, and presence of counterion. The dominant backbone conformer states are quite similar for both isotactic and syndiotactic diads in a common charge state. Thus, the overall dimensional properties of isotactic syndiotactic and atactic chains of PAA or PMA, based upon dyad interactions, are predicted to be alike for a given charge state. Significant deviations from precise t, g⁺, and g^? states are found for the dyad minimum energy conformations. The rod-to-coil and coil-to-rod transitions observed in PAA and PMA, respectively, as a function of increasing counterion concentration can be explained, to a large extent, by the conformational profiles of the corresponding dyad model structures. © 1994 John Wiley & Sons, Inc.     

Synthesis of ultra‐high‐molecular‐weight poly(α‐olefin)s by thiobis(phenoxy)titanium/modified methylaluminoxane system

Polymerization of 1‐butene with thiobis(phenoxy)titanium dichloride (TBPTiCl₂) with modified methylaluminoxane (MMAO) yielded atactic and regioirregular poly(1‐butene) with low molecular weight. However, productivity was increased and the yielded polymer structure was changed when water‐modified MMAO (WM‐MMAO) was used as a cocatalyst. The molecular weight of obtained polymer with TBPTiCl₂/WM‐MMAO was dramatically increased and reached over 9 million g/mol. Poly(1‐butene) produced by this catalytic system was almost regioregular and slightly isotactic. 1‐Hexene and 1‐octene were also polymerized by this catalytic system. In all cases, the ultra‐high‐molecular‐weight polymers were obtained. These results showed that the modified structure of the MMAO cluster by water affects the productivity, molecular weight, regioregularity, and stereoregularity of yielded polyolefins. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1107–1111, 2004     

Stereoregularity of poly(alkyl α-chloroacrylates) and mechanism of isotactic polymerization

The catalytic activity of the complexes prepared by the reaction of Grignard reagents with ketones, esters, and an epoxide as polymerization catalysts of methyl and ethyl α-chloroacrylates was investigated. The modifiers which gave isotactic polymers were α,β-unsaturated ketones such as benzalacetophenone, benzalacetone, dibenzalacetone, mesityl oxide, and methyl vinyl ketone, and α,β-unsaturated esters such as ethyl cinnamate, ethyl crotonate, and methyl acrylate. Catalysts with butyl ethyl ketone, propiophenone, and propylene oxide as modifiers produced atactic polymers but no isotactic polymers. It was revealed that the complex catalysts having a structure ? C?C? O? MgX (X is halogen) gave isotactic polymers. The mechanism of isotactic polymerization was discussed. In addition, for radical polymerization of ethyl α-chloroacrylate, enthalpy and entropy differences between isotactic and syndiotactic additions were calculated to give ΔH_i^* ? ΔH_s^* = 910 cal/mole and ΔS_i^* ? ΔS_s^* = 0.82 eu.     

Etude par spectrométrie de masse de l'ionisation de benzonitriles,de phénylacétonitriles et de N,N-diméthylanilines substitués

Molecular ionization potentials for series of compounds of the type X? C₆H₄? CN, X? C₆H₄CH₂? CN and X? C₆H₄? N(CH₃)₂ have been measured using the retarding potential difference technique (RPD. technique). The effect of the various substituents X is better correlated through the electrophilic Brown σ_p⁺ constants than through Hammett's σ_p values. No meta-para orientation effect is observed. For all the disubstituted phenyl compounds studied, the effect of the second substituent is affected by the electron-releasing power of the original substituent. Ionization potentials calculated by using the semi-empirical method of equivalent orbitals are in good agreement with the experimental values.     

Polimeri vinilici otticamente attivi XXI. Determinazione del tipo e del grado di stereoregolarità in alcuni polivinilchetoni

Using NMR. spectroscopy it has been shown that the stereoregularity in the polyvinylketones prepared with Zn(i-C₄H₉)₂ or LiAlH₄ as initiator is of the isotactic type. A surprisingly high stereoregularity (% isotactic diads > 95) has been observed not only in polymethylvinylketone but also in poly[(S)-1-methyl-propyl]vinylketone.     

Synthesis of highly isotactic poly(N,N‐diethylacrylamide) by anionic polymerization with grignard reagents and diethylzinc

N,N‐Dimethylacrylamide (DMA) and N,N‐diethylacrylamide (DEA) were polymerized with various Grignard reagents in tetrahydrofuran at −78 °C in the presence of diethylzinc (Et₂Zn). Highly isotactic poly(DEA) was produced in quantitative yield with tert‐butylmagnesium bromide and Et₂Zn, whereas atactic poly(DEA) was generated in the absence of Et₂Zn. No stereospecific polymerization of DMA proceeded with Grignard reagent in the presence of Et₂Zn. The highly isotactic poly(DEA) obtained was soluble in water and showed the characteristic coil–globule transition phenomenon. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4677–4685, 2000     

Thermal behavior of carbon fiber precursor polymers with different stereoregularities

The effect of stereoregularity, in terms of isotactic triad content on the thermal behavior of carbon fiber precursor polymers synthesized through different polymerization routes such as solid state and radical solution polymerization techniques, was investigated by the thermogravimetric analysis and differential scanning calorimetric measurements. The isotactic contents of I-PAN and A-PAN were estimated with ¹³C NMR. The thermal cyclization reactions of atactic polyacrylonitrile (A-PAN) with low isotactic content (26.4–29.7 %) occurred at a lower temperature than that of isotactic polyacrylonitrile (I-PAN) with higher content (48.7–51.6 %). The percentage of mass loss observed in I-PAN was less as compared to A-PAN. The molecular mass characteristics of PAN obtained through solid state and radical solution polymerization were [M _n (10.2–14.3 × 10⁴), M _v (2.44–3.26 × 10⁵)] and [M _n (10.2–14.3 × 10⁴), M _v (2.29–2.74 × 10⁵)] Daltons (Da).     

Interactions Intramolécularies—XVII Calcul des constantes de couplage en RMN du cyclohexane et de cyclohexanes substitués. Comparaison des méthodes de Pople et Santry et des perturbations finies

Proton–proton ³J, ⁴J and ⁵J NMR coupling constants have been calculated for cyclohexane and monosubstituted cyclohexane conformers (substitiuents: Li, CH₃, OH, F) by the two methods mentioned. Comparing the two methods on the basis of group theory, we show the necessity to use the second. The results from this method are compared with those of the literature.     

Étude des spectres infrarouges de divers dialcoyl-diméthoxy-stannanes

The infrared absorption spectra of some dialkyldimethoxystannanes have been investigated in the 400–1500 cm^?1 region. The bands associated with v_s(SnC₂) and v_s(SnO₂) vibrations have been found at 510–521 cm^?1 and 466–475 cm^?1. The group of bands between 560 and 620 cm^?1 is assigned jointly to v_a(SnC₂) and v_a(SnO₂) vibrations. v(C? O) of the methoxy groups linked to tin appears at 1064–1068 cm^?1.     

Triad tacticity of polyacrylonitrile

The high-resolution NMR spectra of polyacrylonitrile-β,β-d₂ prepared by radical polymerization were determined, and the stereoregularity of the polymer was studied. The NMR spectra of methine protons of polyacrylonitrile-β,β-d₂ in dimethyl sulfoxide-d₆ and a mixture of nitromethane-d₃ and ethylene carbonate showed three partially resolved multiplets. The deuterium-decoupled spectra of the polymer were measured, and three well resolved peaks were observed in the two solvents and dimethylformamide-d₇. These three peaks were analyzed by comparison with the NMR spectra of model compounds and polyacrylonitrile-α-d, and they were assigned to isotactic, heterotactic, and syndiotactic triads with decreasing magnetic field. This order seems to be unchanged in other solvents. Triad stereoregularity of the polymer was determined according to the assignment. Polymerizations of acrylonitrile-β,β-d₂ by radical initiators between ?78°C and 60°C were explained by the Bernoulli trial propagation step. The polymers had an atactic structure, independent of polymerization temperature. This shows that in free-radical polymerization of acrylonitrile, the chain end is not represented as having any particular stereochemistry. Other stereochemical control is necessary to produce tactic polymers. The triad tacticity of isotactic polyacrylonitrile was also determined.     

Synthesis and chiral recognition ability of optically active poly{N‐[(R)‐α‐methoxycarbonylbenzyl]methacrylamide} with various tacticities by radical polymerization using Lewis acids

The radical polymerization of an optically active methacrylamide, N‐[(R)‐α‐methoxycarbonylbenzyl]methacrylamide, was carried out in the absence and presence of Lewis acids such as yittribium trifluoromethanesulfonate [Yb(OTf)₃] and scandium trifluoromethanesulfonate [Sc(OTf)₃]. Catalytic amounts of the Lewis acids significantly affected the stereoregularity of the obtained polymers. The polymerization with Yb(OTf)₃ in tetrahydrofuran afforded isotactic polymers (up to mm = 87%), whereas the conventional radical method without the Lewis acid produced polymers rich in syndiotacticity (up to rr = 88%). The radical polymerization in the presence of MgBr₂ proceeded in a heterotactic‐selective manner (mr = 63%). Thus, the isotactic, syndiotactic, and heterotactic poly(methacrylamide)s were synthesized by the radical processes. The chiral recognition abilities of the obtained optically active poly(methacrylamide)s were affected by the stereoregularity. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3354–3360, 2003     

Organoaluminum catalyst for polymerization reaction. Synthesis of isotactic polyacetaldehyde by AIR3–alkali metal hydroxide catalysts in situ

The catalytic behavior of binary systems derived from AIR₃ and alkali metal hydroxide in a molar ratio of 1 to 0.5 in situ for stereospecific polymerization of acetaldehyde was studied for the purpose of preparation of isotactic polyacetal. The polymer obtained can be readily stretched to a film. The polymerization proceeds slowly (in ～20 hr). The polymer yield and stereospecificity of the polymerization by AlEt₃–LiOH (1:0.5) catalyst were not significantly changed by the nature of solvent or dilution as far as studied. AlEt₃–NaOH, AlEt₃–KOH, AlEt₃–CsOH, AliBu₃–LiOH and AlMe₃–LiOH in molar ratios of 1 to 0.5 behaved similarly. AlMe₃–NaOH, AlMe₃–KOH and AliBu₃–NaOH also gave isotactic polymer of high stereoregularity but in lower yields.