Tacticity Of Poly Research Articles

Effect of Tacticity Sequence of the Poly(N-isopropylacrylamide) Oligomer on Phase Transition Behavior in Aqueous Solution.

The tacticity of poly(N-isopropylacrylamide) (PNIPAM) has a strong impact on the lower critical solution temperature (LCST) in aqueous solution. The sequence of meso diads (m) and racemo diads (r) further contributes to such an effect. In this work, the phase transition behaviors of poly(N-isopropylacrylamide) pentamers with four kinds of sequences, i.e., rrmm, rmmr, mrrm, and rmrm, in water were studied applying replica exchange molecular dynamics with a modified OPLS/AA force field. The difference in local component concentration in the system was used as an order parameter to quantitatively describe the phase separation extent. It was found that the phase separation degree of rrmm and rmmr is higher than that of mrrm and rmrm at the same temperature. The LCSTs of rrmm and rmmr are lower than those of mrrm and rmrm. The radial distribution function and hydrogen bond analysis revealed that the average values of hydrogen bonds between pentamers for rrmm and rmmr are greater than those of mrrm and rmrm, whereas the average values of hydrogen bonds between pentamers and water for rrmm and rmmr are less than those of mrrm and rmrm. It was demonstrated that the isotactic triad (mm) plays an important role in the thermosensitive behaviors of the PNIPAM pentamer. The increase of isotactic triad (mm) content in the PNIPAM chain promotes the formation of intermolecular hydrogen bonds between amide and amide and leads to a higher aggregation of the pentamer with the sequence of rrmm or rmmr. Finally, the effect of the isotactic triad was qualitatively explained with the mean-field theory.

Ab Initio Quantitative Prediction of Tacticity in Radical Polymerization of Poly(methyl methacrylate) by a Molecular Simulation Technique with the Conformation Indexing for Multiple Transition States

Free-radical polymerization is a critical route for generating vinyl polymers owing to its versatility and productivity, while the realization of stereospecific propagation has been considered very difficult and demanding. Therefore, a concrete investigation on the mechanism of tacticity obtained in radical polymerization is urgent. Experimentally, tacticity of poly(methyl methacrylate) produced by radical polymerization in bulk was reported to be syndiotactic and temperature-dependent. In this study, we intended to rationalize those behaviors on the microscopic scale via density functional theory calculations and a hybrid Monte Carlo/molecular dynamics simulation [Red Moon methodology]. We devised a unique conformation indexing based on the multidimensional conformational analysis and obtained as many as 576 transition state conformations for the propagation reaction. Boltzmann factors of the 576 conformations implied the overall higher stability of pro-racemo conformations than pro-meso ones and, in addition, the more syndiotactic tendency at low temperature. By the Red Moon simulations utilizing the energy barriers of those conformations, we successfully simulated the propagation reaction process and achieved quantitative agreement of the product tacticity. These results exhibit the validity of our ab initio molecular simulation procedure with the conformation indexing to realize quantitative analysis of the selectivity of reactions.

Regio- and Stereoselective Ring-Opening Metathesis Polymerization of Enantiomerically Pure Vince Lactam

The ring-opening metathesis polymerization (ROMP) of (+)-Vince lactam [(S)-azabicyclo[2.2.1]hept-5-en-3-one] (1) and its N-benzyl, N-trimethylsilyl (TMS), and N-tert-butoxycarbonyl (Boc) derivatives (2a–c) is reported. Highly cis-syndiotactic (st) poly(Vince lactam) was readily accessible by using the cyclometalated ruthenium complex Ru[CH(2-OiPr-Ph)](Piv)(1-mesityl-3-C4H8-imidazol-2-ylidene) (Piv =2,2-dimethylpropanoate) (4); however, small amounts of trans double bonds (ca. 5%) formed. Highly cis-st (>98%) polymers were accessible by the action of the monoaryloxide pyrrolide (MAP) type complexes W(N-2,6-iPr2C6H3)(CHCMe2Ph)(Pyr)(HMTO) (Pyr = pyrrolide, HMTO = 2,6-(2,4,6-Me3C6H2)2C6H3O) (7) and W(O)(CHCMe2Ph)(PMe2Ph)(Me2Pyr)(TPPO) (TPPO = 2,3,5,6-tetraphenylphenolate) (8). Complementary, cis-isotactic (>98% cis-it) polymers were prepared by the action of Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OBiphen) (OBiphen = 3,3′-di-tert-butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diolate) (5) and its tungsten analogue W(N-2,6-Me2C6H3)(CHCMe2Ph)(OBiphen) (6). Notably, none of these Mo- and W-based initiators polymerize unprotected Vince lactam. Deprotection of poly(N-TMS Vince lactam) and poly(N-Boc Vince lactam) with neat trifluoroacetic acid allowed for the isolation of all-cis highly tactic poly(Vince lactam).

Synthesis of meso-lactide by thermal configurational inversion and depolymerization of poly(l-lactide) and thermal configurational inversion of lactides

Meso-lactide (MLA) was synthesized by thermal degradation of poly(l-lactide) [i.e., poly(l-lactic acid) (PLLA)] at relatively high temperatures of 250 and 300 °C for thermal degradation periods up to 750 and 360 min, respectively. Maximum MLA yields as high as 30 and 40 mol% were successfully attained at 250 and 300 °C, respectively, when thermal degradation was continued for 750 and 180 min, respectively. This indicates that the intentional thermal configurational inversion and depolymerization of PLLA and configurational inversion of lactides (LAs) at the higher temperature are favorable for the synthesis of MLA in a short period. The molecular weight and optical purity and tacticity of poly(lactide) [i.e., poly(lactic acid) (PLA) decreased and the d-lactyl unit content of PLA increased with an increase in reaction time. The higher MLA fraction for 250 °C compared to that for 300 °C at the reaction times when PLA samples had similar number-average molecular weights can be ascribed to the lower tacticity or higher racemization at chiral carbons in PLA chains.

Hydrogen bond directed surface dynamics at tactic poly(methyl methacrylate)/water interface.

The complexity of induced ordering for tactic poly(methyl methacrylate) (PMMA) thin films in contact with water is examined through all-atom molecular dynamics with validated potentials. We observe that for the water molecules that are hydrogen bonded to the PMMA surface, the isotactic and atactic PMMA show a 33% longer relaxation time compared to syndiotactic PMMA. Almost 94% of hydrogen bonds are with the carbonyl groups of PMMA, irrespective of temperature and tacticity. The stability in re-orientation and nature of hydrogen bond participation for the carbonyl groups as well as about 20% higher interaction energies of carbonyl group hydrogen bonded with water for atactic form indicates existence of cooperative effects. Quantifying the dynamics of hydrogen bond at the tactic interface is important in understanding the role tacticity plays in controlling adhesion and biocompatibility, a design choice that has been gaining ground in the soft material science community.

Control of Diffusion and Conformation Behavior of Methyl Methacrylate Monomer by Phenylene Fin in Porous Coordination Polymers

Radical polymerization in nanoscale channels of porous coordination polymers has been explored as a promising technique to control the properties of product polymers. In particular, tacticity of poly(methyl methacrylate) was successfully controlled by utilizing one-dimensional channels of [M2(L)2TED]n type of porous coordination polymers by changing the dicarboxylate ligand L. Toward understanding the atomistic mechanism of this tacticity control, we computationally studied the behavior of methyl methacrylate monomers in the one-dimensional channels by molecular dynamics simulations. Smooth monomer diffusion in the direction along the channel was shown as expected from the channel shape. In addition, we confirmed that the monomers can pass through the narrow apertures between the channels and can diffuse slowly in the direction perpendicular to the channel. In the channels, the ratio of the s-trans and s-cis conformations of the monomers is different from that in monomer liquid. We found two factors affec...

New Insights into the Ring‐Opening Polymerization of β‐Butyrolactone Catalyzed by Chromium(III) Salphen Complexes

AbstractThe heterogeneous nature of β‐butyrolactone (BL) polymerization towards tactic poly(3‐hydroxybutyrate) (PHB) in the presence of chromium(III) salphen (salphen=N,N′‐disalicylidene‐o‐phenylenediamine) complexes is supported by a number of experimental observations. Depending on the substitution pattern, initially soluble chromium(III) salphen chloride complexes can generate microcrystalline agglomerates under the polymerization conditions, driven by formation of μ‐OH bridges between metal centers. Coordinated water molecules are suggested to be the source of such bridging ligands. The formation of these/this heterogeneous species is a prerequisite for the stereocontrolled ring‐opening polymerization (ROP) of BL, whereas both iso‐ and syndioselective enchainment occurs simultaneously. According to the analysis of the 13C NMR spectra of the polymers, the ratio of the corresponding triads depends on a number of parameters in a not yet understood manner. Besides dual stereoselectivity, the heterogeneous chromium(III) salphen species feature catalytic sites with different activities, which is reflected in the very broad molecular mass distribution of the produced PHB. Highly active catalytic sites cause the formation of polymer chains with a high molecular mass at the beginning of polymerization. The described behavior is not inherent to truly homogeneous chromium salphen complexes and is more in line with a bimetallic ROP mechanism proposed earlier, which requires a particular mutual spatial orientation of two salphen complexes for the efficient catalysis of BL polymerization.

Effect of the composition of methanol−water mixtures on tacticity of poly(N‐ethylacrylamide) gel

ABSTRACTTwo series of macroporous poly(N‐ethylacrylamide) (PNEAM) gels are synthesized in different composition of methanol–water mixtures (xm = 0, 0.06, 0.13, 0.21, 0.31, and 0.43; where xm = mole fraction of methanol) in presence as well as in the absence of 0.1M Y(OTf)3 Lewis acid as additive. The gels synthesized in the absence of Lewis acid are atactic and in the presence of the same are isotactic. Synthesis of the corresponding linear PNEAM homopolymers shows that, the isotacticity (meso dyad, %) of the resulted polymers increases for the gels prepared in the presence of Lewis acid (LA) and remains constant for the gel prepared in the absence of LA, respectively, with the increase in the concentration of the synthesis solvent methanol. SEM micrographs reveal that, the hydrogels synthesized in the presence of LA are more porous than the gels prepared in the absence of LA. Swelling ratio of all the hydrogels decreases with the increase in the temperature and LA gels show higher swelling ratio values than non LA gels (NLA). Deswelling rate of the hydrogels prepared in methanol–water mixture in presence of LA is faster than the hydrogels prepared in absence of LA. Moreover, reswelling rate increases with increase in the isotacticity of the PNEAM segment in the gel. All these results have been explained on the basis of the formation of highly porous hydrogels with higher isotactic PNEAM chain segment in the presence of LA in methanol–water mixtures. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41668.

Configurational effects on the crystalline morphology and amorphous phase behavior in poly(3-hydroxybutyrate) blends with tactic poly(methyl methacrylate)

Poly(3-hydroxybutyrate) (PHB) blends with two tactic poly(methyl methacrylate)s [PMMAs; isotactic poly(methyl methacrylate) (iPMMA) and syndiotactic poly(methyl methacrylate) (sPMMA)], being chiral/tactic polymer pairs, were investigated with regard to their crystalline spherulite patterns, optical birefringence, and amorphous phase behavior with polarized optical microscopy and differential scanning calorimetry. The PHB/sPMMA and PHB/iPMMA blends exhibited upper critical solution temperatures of about 225 and 240°C, respectively, on the basis of the results of thermal analysis and phase morphology. The interactions of two constituents in the blends (PHB/iPMMA or PHB/sPMMA) were measured to be insignificantly different for the PHB/sPMMA and PHB/iPMMA blends. However, syndiotacticity in PMMA exerted a prominent effect on the alteration of the PHB spherulite morphology, whereas, by contrast, isotacticity in PMMA had almost no effect at all. At high sPMMA contents (e.g., 30 wt %) in the PHB/sPMMA blend, the spherulites were all negatively birefringent and ringless when they were crystallized at any crystallization temperature between 50 and 90°C. That is, not only was the original ring-banded pattern in the neat PHB spherulites completely disrupted, but the optical sign was also reverted completely from positively to negatively birefringent in the sPMMA/PHB blend; this was not observed in the iPMMA/PHB one. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Simultaneous control over the molecular weight and tacticity of poly(vinyl acetate) using a low-temperature photoinitiated RAFT process in fluoroalcohols

A photoinitiated RAFT process in fluoroalcohols (FAs) was successfully applied to the preparation of poly(vinyl acetate) and its hydrolyzed polymer, poly(vinyl alcohol), permitting the simultaneous control over the molecular weight (MW) and tacticity. The polymerization reaction displayed living polymerization characteristics, evidenced with linear relationships between ln([M]0/[M]) and time, and between the MW and the conversion. Polymers of higher syndiotacticity could be prepared by using the FA solvent, achieving a 62.4% racemic diad at −20 °C. The syndiotacticity of the polymer increased with increasing concentrations of FAs or with decreasing polymerization temperatures, and the dependence was stronger than the one observed in conventional radical polymerization reactions due to the slower RAFT process. Furthermore, by performing a sequential polymerization reaction involving the high-temperature reaction in the bulk state and the low-temperature reaction in the FAs, stereoblock copolymers of poly(vinyl alcohol) could be obtained.

New Complex Crystals of Chiral Poly(L -lactic acid) and Different Tactic Poly(methyl methacrylate)

Based on evidence from differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and Fourier-transform infrared (FTIR) spectroscopy, a new stereocomplex crystal (DSC Tm = 175 °C, with WAXD 2θ = 10.0° and 12.5°) is proven for the first time between structurally dissimilar chiral poly(L-lactic acid) (PLLA) and syndiotactic poly(methyl methacrylate) (sPMMA). There is a strong complexing capacity only between low molecular weight PLLA and sPMMA, in miscible state, at specific weight fractions (70:30). The complexing capacity is more significant when the mixtures are melt-crystallized at Tc = 110 °C or lower, and the intensity of this complex can be further enhanced if it is annealed between 100 and 160 °C, below its Tm = 175 °C. The new complex crystal can be formed only between PLLA and sPMMA, but not with isotactic or atactic PMMA.

Semi-continuous Heterophase Polymerization of n-Butyl Methacrylate: Effect of Monomer Feeding Rate

The effect of monomer feeding rate on particle size, molar masses, glass transition and tacticity of poly(n-butyl methacrylate) (PBMA) nanoparticles synthesized by semi-continuous heterophase polymerization under monomer-starved conditions is reported. Three feed rates were examined. Highly monomer-starved conditions at the two slower addition rates were confirmed by the low amount of residual monomer throughout the reaction and by the fact that the instantaneous polymerization and feeding rates became similar at later stages of the reaction. Under these conditions, polymer particles in the nanometer range (30 to 35 nm) were obtained. Glass transition temperatures are substantially higher than those reported for commercial PBMA. Polymers tacticity was determined by 13C-NMR spectroscopy. NMR measurements confirm that the syndiotactic content of the PBMA synthesized here is larger than those of the commercial ones made by free-radical polymerization. Molar masses are much lower than those expected from termination by chain transfer to monomer, which is the typical termination mechanism in microemulsion polymerization.

Tacticity of poly(butyl-α-cyanoacrylate) chains in nanoparticles: NMR spectroscopy and DFT calculations

NMR spectroscopy and quantum chemical calculations were applied for structural characterization and determination of the preferred stereochemical sequence distribution of the monomer units in the homopolymer chains of poly(butyl-α-cyanoacrylate) nanoparticles. The stereochemical sequence distribution of the monomer units was defined by analysis of their high-resolution 1D 1H and 13C NMR and 2D J-resolved, 1H/13C HSQC and 1H/13C HMBC NMR spectra. The results were verified by employment of B3LYP/6-31G(d) calculations and are consistent with the preferred tendency of polymer chains of PBCN to adopt syndiotactic placements. The proton and carbon chemical shielding were calculated at BPW91/6-31+G(2d,p) level using the GIAO approach and B3LYP/6-31G(d) optimized geometry.

Controlling the tacticity in the polymerization of N-isopropylacrylamide: A computational study

In this study, the effect of alcohols as solvents on the kinetics and the tacticity of poly(N-Isopropylacrylamide) (PNIPAM) is investigated with a combined static and molecular dynamics set of computational tools. Classical molecular dynamics calculations have been carried out to determine the location of the solvent molecules in the proximity of the monomer and the dimer. A combined implicit/explicit solvent model was used for the evaluation of the kinetics of the dimeric polymer chains. Rate constants are calculated with the B3LYP/6-311 + G(d,p)//B3LYP/6-31 + G(d), BMK/6-311 + G(d,p)//B3LYP/6-31 + G(d), and MPWB1K/6-311 + G(d,p)//B3LYP/6-31 + G(d) methodologies via the standard transition state theory. We show that due to the proximity of the –NH and carbonyl groups on the syndiotactic propagating dimeric and trimeric chains, the alcohol can stabilize the corresponding transition states by forming a bridge between these functionalities and accelerate this path more than its isotactic counterpart. In agreement with experiment, the increase in the syndiotactic PNIPAM and the acceleration of the reaction in the presence of t-BuOH is predicted with all the DFT functionals utilized in this study.

Narrow size‐distribution poly(methyl methacrylate) nanoparticles made by semicontinuous heterophase polymerization

AbstractThe effect of surfactant (sodium dodecyl sulfate) concentration on particle size, molar masses, glass transition, and tacticity of poly(methyl methacrylate) (PMMA) nanoparticles synthesized by semicontinuous heterophase polymerization under monomer‐starved condition at constant monomer feeding rate is reported. Starved conditions are confirmed by the low amount of residual monomer throughout the reaction and by the fact that the instantaneous polymerization rate is similar to the feeding rate of monomer. Under these conditions, polymer particles in the nanometer range (20–30 nm) were obtained with narrow size distribution (1.07 < Dw/Dn < 1.18), depending of surfactant concentration. Final particle size diminishes as the surfactant concentration is increased. Glass transition temperatures and syndiotactic content (54%–59%) of the produced polymers are substantially higher than those reported for commercial and bulk‐made PMMA. Molar masses are much lower than those expected from termination by chain transfer to monomer, which is the typical termination mechanism in 0–1 emulsion and microemulsion polymerization of this monomer. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Effect of Metal Oxide Nanoparticles on the Mechanical Properties and Tacticity of Poly(Methyl Methacrylate)

AbstractNanoparticles were incorporated into poly(methyl methacrylate) matrix by the mean of in situ bulk polymerization. Particle chemistry, size, shape, and percent loading were experimental variables in the synthesis and mechanical properties were assessed, particularly impact resistance, which showed improvement at the optimal particle loading. In assessing the mechanisms of this improvement, the elongated shape of zinc oxide particles appears to promote crack deflection processes to introduce a pull-out mechanism similar to that observed in fiber composite systems. Raman spectroscopy was performed to examine the effect of polymer chain conformation and configuration with the addition of nanoparticles.

Effects of chain configuration on UCST behavior in blends of poly(L‐lactic acid) with tactic poly(methyl methacrylate)s

AbstractChain configuration influences phase behavior of blends of poly(methyl methacrylate) (PMMA) of different tactic configurations (syndiotacticity, isotacticity, or atacticity) with poly(L‐lactic acid) (PLLA). Blends system of sPMMA/PLLA is immiscible with an asymmetry‐shaped UCST at ∼250 °C. The phase behavior of the sPMMA/PLLA blend is similar to the aPMMA/PLLA blend that has been already proven in the previous work to exhibit similar UCST temperatures (230–250 °C) and asymmetry shapes in the UCST diagrams. On the other hand, the iPMMA/PLLA blend remains immiscible up to thermal degradation without showing any transition to UCST upon heating. The blend system with UCST, that is, sPMMA/PLLA, can be frozen in a state of miscibility by quenching to rapidly solidify from the homogeneous liquid at UCST, where the Tg‐composition relationship for the sPMMA/PLLA blend fits well with the Gordon‐Taylor Tg model with k = 0.15 and the blend's T leads to χ12 = −0.26 for the UCST‐quenched sPMMA/PLLA blend. Both parameters (k and χ) as characterized for the frozen miscible blend suggest a relatively weak interaction between the two constituents (sPMMA and PLLA) in the blends. The interaction strength is likely not strong enough to maintain a thermodynamic miscibility when the blend is at ambient temperature or any lower temperatures below UCST. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2355–2369, 2008

Isotactic poly(1‐hexene sulfone)s by stereoselective anionic ring‐opening polymerisation

AbstractBackground: Poly(olefin sulfone)s have been shown to form helical regions and to display main‐chain liquid‐crystalline behaviour in the bulk when they possess long side‐chains. It is believed that increasing the tacticity of the backbone could enhance their liquid‐crystalline behaviour. This study aims to produce tactic poly(olefin sulfone)s by stereospecific ring‐opening polymerisation.Results:T actic polythiranes were successfully obtained from a racemic mixture of thirane monomers using a series of chiral catalyst systems. The isotactic placement (Pi) was determined using 13C NMR analysis of carbons in or near the backbone. The polysulfides were oxidised by peracetic acid to their corresponding polysulfones. Powder X‐ray diffraction studies showed that the isotactic polysulfones formed more ordered structures than their atactic analogues, an effect attributed to a tightening of the helical pitch.Conclusions: Tactic poly(1‐hexene sulfone)s have been prepared for the first time by stereoselective ring‐opening polymerisation. Copyright © 2007 Society of Chemical Industry

Quantum-Chemical Study of the Effect of Triethylaluminum on the Chain-End Structure and Tacticity of Poly(N,N-dimethylacrylamide) with Lithium Counterion in THF

Quantum-chemical DFT calculations of the structures and relative stabilities of poly(N,N-dimethylacrylamide) anionic living chain ends with lithium counterion and their aggregates in THF, both in t...

Phase behavior of ternary polymer blends with favorable interactions

AbstractAtactic poly(methyl methacrylate) (aPMMA) and poly(vinyl pyrrolidone) (PVP) with a weight‐average molecular weight of 360,000 g/mol were found to be immiscible on the basis of preliminary studies. Poly(styrene‐co‐vinyl phenol) (MPS) with a certain concentration of vinyl phenol groups is known to be miscible with both aPMMA and PVP. Is it possible to homogenize an immiscible aPMMA/PVP pair by the addition of MPS? For this question to be answered, a ternary blend consisting of aPMMA, PVP, and MPS was prepared and measured calorimetrically. The role of MPS between aPMMA and PVP and the effects of different concentrations of vinyl phenol groups on the miscibility of the ternary blends were investigated. According to experimental results, increasing the vinyl phenol contents of MPS has an adverse effect on the miscibility of the ternary blends. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2064–2070, 2005