Molecular motions of tactic poly(2-hydroxyethyl methacrylate) in solutions studied by 13C-NMR relaxation measurement

The spin-lattice relaxation time and the nuclear Overhauser enhancement were measured using Bruker AM 300 spectrometer operating at 75.5 MHz for ¹³C to investigate the molecular motional characteristics and its tacticity effect for tactic poly(2-hydroxyethyl methacrylate) (PHEMA) as a function of temperature in dimethyl sulfoxide and methanol solvents. The observed relaxation data have been analyzed for both backbone motion and methyl internal rotation according to the log-χ² distribution model and the diamond-lattice model. The correlation times thus obtained for the molecular motions show that isotactic PHEMA is more flexible than syndiotactic counterpart. The syndiotactic PHEMA seems to have intramolecular hydrogen bonding which restricts the motion of C-2 carbon at temperatures below 35°C, whereas the isotactic one indicated no hydrogen bonding at all temperatures examined in this study. The methyl group of isotactic PHEMA shows a greater degree of freedom for the internal rotation than that of syndiotactic one. From the temperature dependence of correlation times, the activation energies for both backbone segmental motion and methyl internal rotation are obtained. The activation energies, 20 kJ/mol for backbone motion and 19 kJ/mol for methyl internal rotation, for isotactic PHEMA are substantially lower than the corresponding activation energies of 30 and 32 kJ/mol obtained for syndiotactic one. An examination of these energies indicates that methyl side group and backbone motions in tactic PHEMA are linked together.     

Temperature dependence of unperturbed dimensions for isotactic poly(2-hydroxyethyl methacrylate)

The unperturbed dimensions of isotactic poly(2-hydroxyethyl methacrylate) (PHEMA) were evaluated from intrinsic viscosity measurements in water, ethanol, 1-propanol, 2-propanol, and 2-butanol under θ conditions over the temperature range of 3.7–32.1°C. The smallest value of unperturbed dimensions (K_θ) and the largest negative temperature dependence of unperturbed dimensions and the polymer–solvent interaction parameter (B) were obtained in aqueous θ solvent relative to the corresponding organic θ solvents. These results were interpreted by the hydrophobic interaction between the hydrophobic groups of isotactic PHEMA and water solvent. The temperature coefficient of the unperturbed dimensions, d ln〈r〉/dT, obtained in this study has a negative value of ?1.44 × 10^?3 deg^?1 under chemically similar θ solvents such as ethanol, 1-propanol, 2-propanol, and 2-butanol where specific solvent effects are eliminated or minimized. In order to obtain the thermodynamic parameters for mixing between isotactic PHEMA and solvents, the plots of the polymer–solvent interaction parameter versus reciprocal absolute temperature (1/T) were carried out. Both the entropy of dilution and enthalpy of dilution show the negative values for water, methanol, and t-butanol, whereas the positive ones for ethanol, 1-propanol, 2-propanol, and 2-butanol. This result indicates that the solution of isotactic PHEMA behave as exothermal systems in the former class of solvents and endothermal ones in the latter class of solvents.     

Conformational analysis of poly(2-hydroxyethyl methacrylate)

The poly(2-hydroxyethyl methacrylate) (PHEMA) is a disubstituted vinyl chain in which the substituents CO₂CH₂CH₂OH and CH₃ differ in size and shape. In order to verify the various characteristics of the PHEMA chain, the conformational energy calculations for meso and racemic diads, which are the segments consisting of the stereoregular isotactic and syndiotactic chains, respectively, were carried out using ECEPP/2 potential. From these calculations, the averaged geometry and the statistical weights were obtained in a local minima. The characteristic ratio, C∞ = (〈r²〉_o/nl²)∞, was determined from the statistical weights and geometries. The calculated C∞ for the isotactic and syndiotactic chain are 10.2 and 2.3, respectively. The characteristic ratio for isotactic chain is larger than that for syndiotactic chain. This shows that the syndiotactic chain is more folded than the isotactic chain is, and that the calculated tendency is in reasonably agreement with the experimental tendency of acrylate polymers.     

Effect of tacticity on conformation of p-tert-butylphenol acetaldehyde resins as studied by molecular simulation

p-tert-Butylphenol acetaldehyde resins can have isotactic, syndiotactic, and atactic sequences. Structural characteristics of the p-tert-butylphenol acetaldehyde resin with different tacticities were studied using molecular mechanics and molecular dynamics. Trimer–decamer isotactic and syndiotactic resins and 12 stereoisomers of a hexamer were calculated. In the p-tert-butylphenol acetaldehyde resin, the hydroxyl groups cluster in the center of the molecule through intramolecular hydrogen bonding and the tert-butyl groups are extended out. It has been found that the energy-minimized structures of the isotactic resin are more stable than those of the syndiotactic resin by 7–17 kcal/mol. From the results of molecular dynamics at 303, 373, 474, and 573 K for 300 ps, the isotactic resin was also found to be more stable than the syndiotactic resin. For atactic resins, the closer to isotactic their structures are, the more stable they are. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1355–1361, 1998     

Radical polymerizations and copolymerizations of dimethylstannyl dimethacrylate and trimethylstannyl methacrylate

The radical polymerizations and copolymerizations of dimethylstannyl dimethacrylate (DSM) and trimethylstannyl methacrylate (TSM) in dimethylformamide (DMF) were studied. These monomers did not polymerize thermally, but easily underwent polymerization in the presence of α,α′-azobisisobutyronitrile and on irradiation with ultraviolet light. The polymer obtained from TSM was soluble in DMF and methanol, but that from DSM was insoluble in any organic solvents; this polymer probably consists of a network structure. These polymers were converted to poly(methyl methacrylate) (PMMA) by means of acid hydrolysis and then methylation with diazomethane. The content of syndiotactic triad was determined from infrared spectra of PMMA derived from the polymers of DSM and TSM. It was noted that the content of syndiotactic triad was greater in the radical polymerization of TSM than those of DSM at every temperature investigated. The differences in the activation enthalpy (ΔΔH?) and in the activation entropy (ΔΔS?) between isotactic and syndiotactic additions were determined as follows: for DSM, ΔΔH? = ～0 cal/mole, ΔΔS? = ?0.856 eu; for TSM, ΔΔH? = 229 cal/mole, ΔΔ = ?1.09 eu. From the radical copolymerizations of DSM and TSM with styrene at 60°C, the copolymerization parameters, Q and e, were evaluated as follows: for DSM, Q = 1.36, e = 0.41; for TSM, Q = 0.45, e = ?0.37. These results were compared with the reported effects of stannic chloride and zinc chloride on the radical polymerization of methyl methacrylate.     

Stereoregulation in the polymerization of methyl α-ethylacrylate by n-BuLi in toluene

The polymerization of methyl α-ethylacrylate was carried out in toluene by n-BuLi at various temperatures. The yield of the polymer decreased with increase in the polymerization temperature and at 30°C and above no polymer was obtained, indicating that the ceiling temperature of this monomer lay between 0 and 30°C. The isotacticity increased with an increase in the polymerization temperature and at 0°C a highly isotactic polymer was obtained. The fractionation of the polymer obtained at ?78°C showed that the polymer was a mixture of isotactic and syndiotactic ones. Upon the addition of a small amount of methanol or water in the polymerization mixture the isotacticity of the polymer increased while the yield decreased. Syndiotactic polymer was obtained in the polymerization by n-BuLi in tetrahydrofuran as well as by diisobutyl aluminum diphenylamide in toluene.     

Participation of water in conformational change of isotactic poly(2-hydroxyethyl methacrylate) as studied by viscometry in various electrolytic solutions

Conformational properties of isotactic poly(2-hydroxyethyl methacrylate) (PHEMA) have been studied by viscometry in various electrolytic solutions. The intrinsic viscosity of isotactic PHEMA at 0.01M salt solution increases with decreasing the B coefficient in Jones—Dole's equation. In respective to water structures, a polymer chain is more expanded in the salt solution including water structure breaker ions. As the concentration of ions increases, the interactions between polymer segments and ions make a major contribution to conformational changes of isotactic PHEMA. Depending on the kind of ions, a salting-in or out effect is observed at higher concentrations than 0.1M salt solution. We observed that the denaturing effects of various anions in isotactic PHEMA salt solutions are as follows; SO₄^2- < F^? < I^? NO₃^? < SCN^-. This order is similar to the Hofmeister series. To investigate the influences of denaturing agents on solvent structures, we also compared the guanidine hydrochloride effect with the tetrabutylammonium chloride effect in isotactic PHEMA solution.     

Conformational properties of isotactic poly(2-hyroxyethyl methacrylate) in the mixed water-alcohol solvents

Conformational properties of isotactic poly(2-hydroxyethyl methacrylate) (PHEMA) have been studied in mixtures of water and several aliphatic alcohols by viscometry and fluorometry. The highest fluorescence intensity of auramine has been detected in aqueous isotactic PHEMA solution compared with several aliphatic alcohol systems. Upon aliphatic alcohol addition, there was the decrease of fluorescence intensity of auramine. As the number of aliphatic group of alcohol increases, the decrease of fluorescence intensity of auramine has been pronounced. And the sharp increases of the reduced viscosity of isotactic PHEMA in water-alcohol solvents were observed in the lower region of alcohol volume percentage with the increasing number of aliphatic group of alcohol. Nevertheless, the sharp increase of the reduced viscosity of isotactic PHEMA in ethyleneglycol-water solvent was obtained even at a low-volume percentage region of ethyleneglycol at which compact structures exist. Therefore, we consider that the increase of viscosity at this region is due to the hydrophilic side group of polymer-solvent interaction, forming hydrogen bonds. The experimental results suggest that compact structures of isotactic PHEMA in aqueous solution are caused by hydrophobic interactions by methyl group of polymer backbone, and the hydrophobic interaction by adding alcohols. Finally, our study brings the fact that the solvating sites of alcohols on the isotactic PHEMA molecule are strongly influenced by the number of hydroxyl and aliphatic group in alcohols.     

Identification of a stereoblock poly(methyl methacrylate) sample with a stereocomplex

Acid hydrolysis of a stereoblock poly(methyl methacrylate) sample leads to a mixture of isotactic and syndiotactic poly(methacrylic acid) which can be separated by electrophoresis. The experiment confirms the stereochemical identity between the so-called “stereoblock” poly(methyl methacrylate) and the stereocomplex which syndiotactic and isotactic poly(methyl methacrylate) form in the ratio 2:1. A possible mechanism of replica polymerization is suggested to account for this effect.     

Solid matrix polymerization and stereoisomeric complexes of poly(methyl methacrylate)

The polymerization of methyl methacrylate within solid matrices of stereoregular poly(methyl methacrylate) has been studied by proton NMR and wide angle X-ray diffraction. The semi-crystalline isotactic (i-) PMMA matrix was synthesized in the laboratory by anionic polymerization initiated by phenylmagnesium bromide, and the syndiotactic (s-) PMMA matrix was synthesized through a Ziegler–Natta reaction. Matrix polymerization of the monomer was initiated through the redox activation of benzoyl peroxide with N,N-dimethyl-p-toluidine. NMR measurements of triad distributions in matrix-polymerized chains suggest that the well-known stereospecific replica polymerization in PMMA (syndiotactic sequences promote isotactic sequences and vice versa) plays only a limited role in the systems studied. Experimental results indicate that chains grown within the i-PMMA or s-PMMA solid matrices have greater degrees of configurational disorder. The greater concentration of atactic triads in these chains could be the result of limited free volume or steric effects during polymerization in a highly condensed environment. X-ray diffraction studies of solution cast blends of isotactic PMMA and PMMA with conventional tacticity reveal some crystallinity with a structure characteristic of the stereocomplex formed by isotactic and syndiotactic PMMA from suitable solvents. Evidence was obtained for the presence of this complex in solidified mixtures of the i-PMMA solid matrix and liquid monomer. This observation is an example of special intermolecular structures that can form under conditions of in situ growth of chains within a pre-polymerized matrix.     

Optoeletronic properties of poly(N‐alkenyl‐carbazole)s driven by polymer stereoregularity

Stereoregular polymers like isotactic poly(N‐butenyl‐carbazole) (i‐PBK), isotactic and syndiotactic poly(N‐pentenyl‐carbazole) (i‐PPK and s‐PPK), and poly(N‐hexenyl‐carbazole) (i‐PHK and s‐PHK) are synthesized using the stereospecific homogeneous “single site” Ziegler‐Natta (Z‐N) catalysts: rac‐dimethylsilylbis(1‐indenyl)zirconium dichloride ( 1 )/methylaluminoxane (MAO) and diphenylmethylidene(cyclopentadienyl)‐(9‐fluorenyl)zirconium dichloride ( 2 )/MAO. Catalytic activity is rationalized by density functional theory (DFT) calculations. All synthesized polymers are fully characterized by NMR, thermal, wide‐angle X‐ray diffraction, and fourier transform infrared spectroscopy analysis. Fluorescence measurements on isotactic and syndiotactic polymer films indicate that all polymers give rise to excimers, both “sandwich‐like” and “partially overlapping.” Excimer formation is essentially driven by the polymer tacticity. Isotactic polymers generate both sandwich‐like and partially overlapping excimers, while syndiotactic polymers give rise especially to partially overlapping ones. A theoretical combined molecular dynamics–time dependent DFT approach is also used to support the experimental results. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 242–251     

Separation of Poly(propylene) Samples According to Tacticity Using a Hypercarb Column

Summary: Samples of polypropylene having different stereoregularities, i.e., differing in the isotactic or syndiotactic stereosequence distribution, were separated by means of high-temperature gradient adsorption liquid chromatography. The porous graphite was used as stationary phase in the column packing (Hypercarb®). Predominantly isotactic samples eluted in 1-decanol, while predominantly syndiotactic samples eluted in a binary gradient composed of 1-decanol and 1,2,4-trichlorobenzene. Their elution volumes increased with the average content of the syndiotactic units (racemo dyads mole fraction as determined with the NMR spectroscopy) in the samples. Thus these chromatographic separations represent a new method for the analysis and characterization of stereoregular polyolefins. It requires substantially less time and solvents than the commonly used methods.     

Stereospecific polymerization of 9-fluorenyl methacrylate: Tacticity effects on the thermal and photophysical properties of the polymers

Poly(9-fluoreneyl methacrylate) was obtained through anionic polymerization with t-BuLi and t-BuMgBr and through radical polymerization with α,α′-azobisisobutyronitrile. Anionic polymerization with t-BuLi in tetrahydrofuran and radical polymerization afforded syndiotactic polymers (rr ∼ 90%), whereas anionic polymerization with Li and Mg initiators in toluene and CH₂Cl₂ led to isotactic polymers. The thermal and photophysical properties of the polymers were examined. A syndiotactic polymer tended to show higher glass transition and decomposition temperatures than an isotactic polymer. However, polymers with different tacticities were not likely to assume specific, distinctive conformations such as a helix or a π-stacked conformation in solution. An isotactic polymer showed stronger interactions in a CH₂Cl₂ solution with 2,4,7-trinitro-9-fluorenylidenemalononitrile, an electron-acceptor molecule, than a syndiotactic polymer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4656–4665, 2004     

Radical polymerization of methyl methacrylate in the presence of isotactic poly(methyl methacrylate)

Methyl methacrylate (MMA) was polymerized by radical initiation at 25°C in DMF in the presence of preformed isotactic PMMA (iMA) with about 90% isotactic triads and different M?_v's, viz., iMA-1: 7.2 × 10⁵; iMA-2, 5.0 × 10⁵; iMA-3, 3.5 × 10⁵; iMA-4, 1.25 × 10⁵; and iMA-5, 1.15 × 10⁵. The MMA:iMA ratio was 6:1. The collected polymers were separated into two fractions by extraction with boiling acetone and characterized by 60 MHz NMR. It is found that the M?_v of the polymer formed ran parallel to the M?_v of iMA. In all cases syndiotactic PMMA (s-PMMA) was produced which associated with the isotactic substrate to form acetone-insoluble stereocomplexes. The syndiotactic polymers probably consist of long syndiotactic and heterotactic sequences. The syndiotacticity decreased with conversion and was generally highest in the presence of iMA-1. With iMA-1 even the formation of some additional i-PMMA (in the acetone-insolubles) was indicated, especially in the later stages of the polymerization. Characterization of the acetone-soluble fractions indicated that i,s-stereoblock polymers were also produced, of which the persistence ratios ρ increased with the M?_v of iMA. From these results it is concluded that this reaction differs from the conventional radical polymerization and can be considered a stereospecific replica polymerization, the driving force being the strong tendency of i- and s-PMMA to associate. The formation of i,s-stereoblock polymers and additional i-PMMA indicates that s-PMMA in its turn can also act as a polymer matrix.     

Reactive poly(glycidyl methacrylate) microspheres prepared by dispersion polymerization

Poly(glycidyl methacrylate) [poly(GMA)] microspheres of narrow size distribution were prepared in a simple one‐step procedure by dispersion radical polymerization. Depending on the solvent used, poly(GMA) particle size could be controlled in the range of 0.5–4 μm by changing the solubility parameter of the reaction mixture. In N,N′‐dimethylformamide (DMF)/methanol mixture, the particle size increased and the size distribution broadened with decreasing initial solubility parameter. While in the DMF/methanol solvent system, hydroxypropyl cellulose (HPC) or cellulose acetate butyrate (CAB) were taken as steric stabilizers of the dispersion polymerization, poly(vinylpyrrolidone) (PVP) was used in alcoholic media. Contrary to the DMF/methanol system, narrow particle size distributions were obtained with PVP‐stabilized polymerizations in ethanolic, methanolic, propan‐1‐olic or butan‐1‐olic medium. Both the particle size and polydispersity were reduced with increasing stabilizer concentration. If lower molecular‐weight PVP was used, larger microspheres were obtained. Poly(GMA) samples prepared in a neat alcoholic medium virtually quantitatively retained oxirane group content after the polymerization. Reactivity of the poly(GMA) microspheres was confirmed by their hydrolysis and aminolysis. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3855–3863, 2000     

Syndiotactic‐specific radical polymerization of N,N‐dimethylacrylamide in the presence of tartrates: A proposed mechanism for the polymerization

Radical polymerization of N,N‐dimethylacrylamide (DMAAm) was investigated in the presence of tartrates, such as diethyl L ‐tartrate, diisopropyl L ‐tartrate, and di‐n‐butyl L ‐tartrate, in toluene at low temperatures. Syndiotactic polymers were obtained in the presence of tartrates, whereas isotactic polymers were obtained in the absence of tartrates. The syndiotactic‐specificity increased with increasing amount of tartrates and with decreasing polymerization temperature. NMR analysis suggested that DMAAm and tartrates formed a 1:1 complex through double hydrogen bonding. A mechanism for the syndiotactic‐specific radical polymerization of DMAAm is proposed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1192–1203, 2009     

Effect of ureas on the viscometric behavior of water-soluble isotactic poly(2-hydroxyethyl methacrylate)

Isotactic poly(2-hydroxyethyl methacrylate) (PHEMA) is soluble and has the compact random coil structure in water solution. The effect of six ureas (thiourea, urea, methylurea, ethylurea, 1,3-dimethylurea, and tetramethylurea) on the viscometric behavior of aqueous solutions of isotactic PHEMA as a function of urea concentrations at 25°C has been investigated. The compact random coil structure is found to disappear as ureas are added. The result is discussed in terms of water structure breaking and making effect of solutes, and hydrophobic interactions. In addition, experiments were performed with aqueous isotactic PHEMA solutions at four different temperatures. Isotactic PHEMA in pure water has the most compact conformation at 20°C.     

The dependence of the glass transition temperature of poly(N-vinylcarbazole) upon steric microstructure and molecular weight

Poly(N-vinylcarbazole) (PVK) samples were found to exhibit up to three glass transition temperatures T_g, corresponding to the whole chain and the syndiotactic and isotactic stereoblocks. An increasing tendency to multiple T_gs, and hence to phase separation, was observed with increasing isotacticity. Limiting values at infinite molecular weight for syndiotactic and isotactic PVK were obtained from correlations of the T_gs corresponding to the syndiotactic and isotactic stereoblocks with their respective average stereoblock lengths derived from ¹³C NMR measurements. They were found to be 549 and 399 K, respectively. The conventional T_g for PVK was found to exhibit the following dependence upon the syndiotactic dyad mole fraction X_s: The molecular weight dependences of the conventional T_gs for several fractionated PVK samples obeyed a Fox–Flory-type relation with values of ?dT_g/d(1/M) varying between 7.6 × 10³ for isotactic PVK and 2.7 × 10⁵ for Luvican M 170.     

Thermal and dynamic mechanical relaxation behavior of stereoregular poly(2-hydroxyethyl methacrylate)

Thermal, dynamic mechanical, and dielectric relaxation techniques were used to determine the relaxation behavior of isotactic and syndiotactic poly(2-hydroxyethyl methacrylate) (pHEMA). Activation energies E_a were determined for the dielectric γ relaxation and compared with those of poly(2-methoxyethyl methacrylate) (pMEMA) to determine the influence of hydrogen bonding on side-chain relaxation processes. No difference in E_a was observed between syndiotactic pHEMA and atactic (predominantly syndiotactic) pMEMA. Isotactic pHEMA, however, had E_a + 1 kcal/mole higher than that of syndiotactic pHEMA. This was attributed to improved side-chain packing in the isotactic polymer.     

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