Cyclic poly(pyridine ether)s by the polycondensation of 2,6‐difluoropyridine with various diphenols

The bistrimethylsilyl derivatives of six different diphenols were polycondensed with 2,6‐difluoropyridine in N‐methylpyrrolidone in the presence of K₂CO₃. On the basis of previous studies, the reaction conditions were optimized for almost quantitative conversions. The feed ratio was systematically varied to optimize the molecular weight. A 2 mol % excess of 2,6‐difluoropyridine was needed to obtain maximum molecular weights. In the matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) mass spectra of the optimized polyethers, only cycles were found (detectable up to 5000 Da). Obviously, the relatively low molecular weights obtained under optimized conditions resulted from a limitation of the chain growth by cyclization, indicating a high cyclization tendency for poly(pyridine ether)s. The size exclusion chromatography measurements not only proved low molecular weights but also demonstrated the existence of bimodal mass distributions and high polydispersities. Protonation of the poly(pyridine ether)s required strong acids such as methane or trifluoromethane sulfonic acid. The solubilities of the neutral and protonated polyethers derived from bisphenol A were studied in various solvents. The MALDI‐TOF mass spectra proved that protonation at 20–25 °C did not cause cleavage of ether bonds. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4781–4789, 2005     

Tertiary amine catalyzed polymerizations of α‐amino acid N‐carboxyanhydrides: The role of cyclization

Sarcosine N‐carboxyanhydride, D,L ‐alanine N‐carboxyanhydride, D,L ‐phenylalanine N‐carboxyanhydride, and D,L ‐leucine N‐carboxyanhydride were polymerized with pyridine or N‐ethyldiisopropylamine as the catalyst. With pyridine, cyclic oligo‐ and polypeptides were obtained in addition to water‐initiated or water‐terminated chains. The cyclopeptides were the main products in the case of sarcosine N‐carboxyanhydride and D,L ‐phenylalanine N‐carboxyanhydride. The fraction of cycles was particularly high when N‐methylpyrrolidone was used as the reaction medium. These results suggested the existence of a pyridine‐catalyzed zwitterionic mechanism. However, cyclopeptides were also obtained with N‐ethyldiisopropylamine as the catalyst. In this case, N‐deprotonation of N‐carboxyanhydrides, followed by the formation of N‐acyl N‐carboxyanhydride chain ends, was the most likely initiation mechanism. Various chain‐growth mechanisms were examined. In the case of γ‐benzyl ester‐L ‐glutamate N‐carboxyanhydride, side reactions such as the formation of pyroglutamoyl end groups were detected. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4680–4695, 2006     

SnOct2-catalyzed ROPs of l-lactide initiated by acidic OH- compounds: Switching from ROP to polycondensation and cyclization

Ring-opening polymerizations (ROPs) of l -lactide are performed in bulk at 130°C with tin(II) 2-ethylhexanoate as catalyst and various phenols of different acidity as initiators. Crystalline polylactides having phenyl ester end groups are isolated, which are almost free of cyclics. The dispersities and molecular weights are higher than those obtained from alcohol-initiated ROPs under identical conditions. Polymerizations at 160°C yield higher molecular weights than expected from the monomer/initiator ratio and a considerable fraction of cycles. The fraction of cycles increases with higher reactivity of the ester end group indicating that the cycles are formed by end-to-end cyclization.     

Polymers of carbonic acid. XXIV. Photoreactive,nematic or cholesteric polycarbonates derived from hydroquinone-4-hydroxybenzoate 4,4′-dihydroxychalcone and isosorbide

Numerous polycarbonates were prepared by means of “diphosgene” in pyridine using hydroquinone 4-hydroxybenzoate (HQHB) as mesogenic diphenol. In addition to the homopolycarbonate, binary copolycarbonates of HQHB and 4,4′-dihydroxychalcone (DHC) with varying molar composition were prepared. A series of ternary copolycarbonates were obtained by incorporation of isosorbide. Furthermore, an alternating copolycarbonate of HQHB and isosorbide was synthesized. All polycarbonates were characterized by inherent viscosities, elemental analyses, IR-, ¹H-NMR, and ¹³C NMR spectroscopy, by WAXS powder patterns DSC measurements, and optical microscopy with crossed polarizers. The homopolycarbonate of HQHB and most binary copolycarbonates were semicrystalline materials forming an enantiotropic nematic melt. Particularly noteworthy is the finding that the alternating copolycarbonate of HQHB and isosorbide forms a broad cholesteric phase despite the unfavorable stereochemistry of isosorbide. The ternary copolycarbonates containing isosorbide formed a cholesteric melt and a Grandjean texture upon shearing. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 1611–1619, 1997     

Effect of supercritical carbon dioxide on the crystallization and melting behavior of linear bisphenol A polycarbonate

The crystallization and melting behavior of bisphenol A polycarbonate treated with supercritical carbon dioxide (CO₂) has been investigated with differential scanning calorimetry. Supercritical CO₂ depresses the crystallization temperature (T_c) of polycarbonate (PC). The lower melting point of PC crystals increase nonlinearly with increasing treatment temperature. This indicates that the depression of T_c is not a constant at the same pressure. T_c decreases faster at a higher treatment temperature than at a lower temperature. The leveling off of the depression in T_c at higher pressures is due to the antiplasticization effect of the hydrostatic pressure of CO₂. The melting curves of PC show two melting endotherms. The lower melting peak moves to a higher temperature with increasing treatment temperature, pressure, and time. The higher temperature peak moves toward a higher temperature as the treatment temperature is increased, whereas this peak is independent of the treatment pressure, time, and heating rate. The double melting peaks observed for PC can be attributed to the melting of crystals with different stability mechanisms. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 280–285, 2004     

Gas transport properties of polycarbonates and polysulfones with aromatic substitutions on the bisphenol connector group

The effect that substitution of aromatic groups on the bisphenol connector unit of bisphenol-A based polycarbonate and polysulfone materials has on their gas transport properties was assessed. Replacement of a methyl group by a phenyl ring (bisphenol acetophenone polycarbonate, PC-AP, and bisphenol acetophenone polysulfone, PSF-AP) gives a small increase in permeability coefficients with similar or slightly higher selectivity for all gases compared to bisphenol-A polycarbonate, PC, or polysulfone, PSF. Substitution of two locked phenyl rings (fluorene bisophenol polycarbonate, FBPC, and fluorene bisphenol polysulfone, FBPSF) in place of the methyl groups in the connector unit leads to permeability and solubility coeffcients that are about twice those observed for PC or PSF. Increases in permeability for the polycarbonate and polycarbonate and polysulfone materials with aromatic substitutions are related to their larger fractional free volume. FBPC and FBPSF have the largest fractional free volume and the largest permeability coefficients. Thermal measurements show that the fluorene based polycarbonate and polysulfone materials have the highest thermal and oxidative stability. Such aromatic substitutions can be useful for developing gas separation membranes to be used in harsh thermal or oxidative environments. © 1993 John Wiley & Sons, Inc.     

Determination of the equilibrium constant for the reaction between bisphenol A and diphenyl carbonate

Despite the industrial significance of poly(bisphenol A carbonate), there is a scarcity of open literature on the equilibrium of the melt‐phase process. In fact, the equilibrium constant (K_eq) for this reaction has never been measured directly. This article describes a process on the basis of NMR for the measurement of K_eq for the reaction between bisphenol A and diphenyl carbonate in the presence and absence of a catalyst. The apparent enthalpy and entropy were calculated using a van't Hoff plot. Decomposition of bisphenol A is a common side reaction in the melt‐phase reaction performed at high temperatures in the presence of catalyst. The effect of these side reactions on the K_eq in the presence of catalyst is determined. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 171–178, 2002     

Effect of supercritical carbon dioxide on the diffusion coefficient of phenol in poly(bisphenol A carbonate)

For some polymers, the rate of solid‐state polymerization (SSP) is higher with supercritical carbon dioxide (scCO₂) as the sweep gas than with atmospheric N₂. One explanation for this higher rate is that the diffusion coefficient of the condensate molecule is higher in the CO₂‐swollen polymer. To investigate this hypothesis, we measured the diffusion coefficient of phenol in poly(bisphenol A carbonate) (BPA‐PC) by carrying out SSP of this polymer under diffusion‐limited conditions. Under these conditions, the diffusion coefficient of the condensate molecule could be calculated from the profile of the molecular weight versus time. The phenol diffusivity was determined between 135 and 180 °C in the presence of N₂ at about 1 bar and in the presence of scCO₂ at about 138, 207, and 345 bar. The diffusion coefficient of phenol was up to 200% higher in scCO₂ than in N₂, depending on the temperature and CO₂ pressure. With both N₂ and scCO₂, the activation energy for phenol diffusion in BPA‐PC was larger than the activation energy for the reaction between hydroxyl and phenyl end groups that occurred during SSP of BPA‐PC. As a result, the overall SSP reaction shifted from diffusion control at low temperatures toward chemical‐reaction control at high temperatures. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1143–1156, 2003     

One-pot high-temperature synthesis of polyimides in molten benzoic acid: Kinetics of reactions modeling stages of polycondensation and cyclization

For aromatic and aliphatic diamines of significantly different basicities, the kinetics of acylation with phthalic anhydride in glacial acetic acid in the range 16–70°C and of imidization of corresponding bis(o-carboxyamides) in acetic acid at 140°C has been studied. The reactions under study model the stages of polycondensation and intramolecular cyclization, respectively, in the high-temperature catalytic synthesis of polyimides in molten benzoic acid. It has been established that the acylation of amino groups in acetic acid proceeds as a reversible reaction and is catalyzed by the acidic medium. The kinetic and thermodynamic parameters of the above-mentioned model reactions have been determined, and the effect of the chemical structure of diamines on these parameters has been assessed. On the basis of the experimental data obtained for the model reactions, it is inferred that, in the synthesis of polyimides in benzoic acid, the overall rate of the process is determined by the rate of the intramolecular cyclization. A low sensitivity of the cyclization reaction to a change in the structure of the starting diamines explains why high-molecular-mass polyimides can be prepared at comparable rates under these conditions from both high-and low-basicity diamines.     

Hyperbranched polyesters of 3,5‐diacetoxybenzoic acid: A reinvestigation

3,5‐Diacetoxybenzoic acid was polycondensed at temperatures in the range of 200–250 °C either in the absence of a catalyst or with addition of MgO or SnCl₂. The highest molecular weight was obtained in the absence of a catalyst. The matrix‐assisted laser desorption/ionization time‐of‐flight mass spectra revealed the formation of cyclic hyperbranched polyesters. The content of polyesters with cyclic core increased with higher conversions, and thus, with higher molecular weights. Furthermore, a loss of acetyl groups was found to be a significant side reaction. The same side reactions were found when trimethylsilyl 3,5‐bisacetoxybenzoate was polycondensed at 280 or 310 °C. Model reactions concerning the deacetylation mechanism were performed and the results are discussed. Size exclusion chromatography measurements in two different solvents proved that the high‐molecular‐weight fraction is not the result of aggregation via hydrogen bonds. Yet, the nature of the solvent, the profile of the columns, and the character of the detector had a significant influence on the shape of the elution curves and on the apparent molecular weights. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3751–3760, 2004     

Structure of poly(L‐lactic acid)s prepared by the dehydropolycondensation of L‐lactic acid with organotin catalysts

The synthesis of low‐molecular‐weight (weight‐average molecular weight < 45,000 g/mol) lactic acid polymers through the dehydropolycondensation of L ‐lactic acid was investigated. Polymerizations were carried out in solution with solvents (xylene, mesitylene, and decalin), without a solvent using different Lewis acid catalysts (tetraphenyl tin and tetra‐n‐butyldichlorodistannoxane), and at three different polymerization temperatures (143, 165, and 190 °C). The products were characterized with differential scanning calorimetry, size exclusion chromatography, vapor pressure osmometry, ¹³C NMR, and matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF). The resulting polymers contained less than 1 mol % lactide, as shown by NMR. The number‐average molecular weights were calculated from the ratio of the area peaks of ester carbonyl and carboxylic acid end groups via ¹³C NMR. The stereosequences were analyzed by ¹³C NMR spectroscopy on the basis of triad effects. Tetraphenyl tin was an effective transesterification catalyst, and the randomization of the stereosequence at 190 °C was observed. In contrast, the distannoxane catalyst caused comparatively less transesterification reaction, and the randomization of the stereosequences was slow even at 190 °C. The L ‐lactic acid and D ‐lactic acid isomers were added to the polymer chain in a small, blocky fashion. The MALDI‐TOF spectra of poly(L ‐lactic acid) (PLA) chains doped with Na⁺ and K⁺ cations showed that the PLA chains had the expected end groups. The MALDI‐TOF analysis also enabled the simultaneous detection of the cyclic oligomers of PLA present in these samples, and this led to the full structural characterization of the molecular species in PLA. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2164–2177, 2005     

Formation of linear and cyclic polyoxetanes in the cationic ring‐opening polymerization of 3‐allyloxymethyl‐3‐ethyloxetane and subsequent postpolymerization modification of poly(3‐allyloxymethyl‐3‐ethyloxetane)

The synthesis of 3‐allyloxymethyl‐3‐ethyloxetane (AllylEHO) and its polymerization with BF₃ × Et₂O is described in this study. Size exclusion chromatography (SEC) and membrane osmometry are used for the determination of molecular weights of the obtained products, ranging from M_n,SEC = 41,500‐131,500 g/mol. ¹H NMR spectroscopy, SEC, as well as MALDI‐TOF MS reveal the formation of cyclic tetramer beside low, but detectable concentrations of larger cyclic oligomers as by‐products during the polymerization process. These results help to understand mechanistically why attempts for a controlled homopolymerization of AllylEHO fail and why a controlled homopolymerization of oxetanes has not been described so far in the literature. Additionally, the high versatility of allyl‐functional polyoxetane for postpolymerization modification is proven by thiol–ene reactions with 3‐mercaptopropionic acid and N‐acetyl‐L ‐cysteine methyl ester. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Influence of light intensity on the photodegradation of bisphenol A polycarbonate

The influence of light intensity on the photodegradation rate and photodegradation mechanisms of an unstabilized BPA-PC film was studied by irradiating the BPA-PC samples with a wavelength distribution comparable to terrestrial sunlight and varying irradiation intensities. The highest intensity used was comparable to maximum summer irradiance in Miami. The degradation rates and mechanisms were determined using IR, UV and fluorescence spectroscopy. A linear relationship between the irradiation intensity and the photodegradation, according to the reciprocity law, was found. It was also found that the irradiation intensity has no influence on the overall degradation mechanisms.     

Spontaneous formation of polypeptides in the interfacial thin films of amphiphilic amino acid esters: acceleration of the polycondensation and control of the structure of resultant polymers

For the purpose of elucidating the effects of molecular arrangements on the reaction rates and the structure of products, polycondensation of long-chain esters of alpha-amino acids in the monolayer on the water surface and the LB multilayers deposited on CaF2 plates were investigated by monitoring changes of the IR spectra. Spontaneous formation of the polypeptides occurs in the mono- and multilayers at room temperature without any catalyst. The rates of polycondensation in the monolayers are markedly influenced by the degree of molecular packing. Maximum polymerizability is obtained in the vicinity of the transition region from expanded to condensed films. The rates of polycondensation in the LB films are much higher than those in the bulk solids and the molten states. The polycondensation seems to be accelerated by regular arrangements of the monomer molecules in the LB films, where the functional groups are concentrated and situated more effectively for the reaction than in the bulk states. However, the polycondensation rates in the LB films are considerably slower when compared with those in the monolayers on the water surface kept at the optimum area or surface pressure, because the molecules in the LB films deposited under high compression are packed more closely than the optimum condition. Thus, suitably close packing of the monomer molecules, retaining a particular orientation together with some conformational freedom in the monolayer, is most favorable for the polycondensation. Two probable mechanisms for the polycondensation in the Y-type multilayers have been proposed. In the assembly of head-to-head double layers of the monomer molecules, the interlayer reaction propagates by sewing up the functional groups facing each other in the adjacent layers, and the polypeptide of a helical structure or random coil can be obtained. In contrast, for the alternating assembly of the amino acid ester and non-polymerizable octadecyl acetate, the polycondensation should proceed only in each single layer (intralayer reaction) and the polypeptide of the extended beta-form can be formed. In the case of dioctadecyl glutamate LB films, as well as the monolayer on water, the resultant polypeptide is the comb-like polymer with unreacted long-alkyl ester groups as side chains and abundant in the beta-form, indicating the dominant intralayer reaction. On the other hand, in the Y-type multilayer of the equimolar mixture of dioctadecyl glutamate (with two ester groups) and octadecyl ester of lysine (with two amino groups), both of the intra- and interlayer reactions occur effectively, resulting in a two-dimensional network structure of the polypeptide. In conclusion, not only the rate of polycondensation but also the higher-order structure of the resultant polypeptides can be controlled by organized arrangements of the monomer molecules in the interfacial thin films.     

Effect of a Set of Acids and Polymerization Conditions on the Architecture of Polycarbonates Obtained via Ring Opening Polymerization

Polycarbonate‐based polymers with a well‐defined architecture have become interesting materials due to their large range of applications. Ring opening polymerization (ROP) has been largely applied to make branched polycarbonates. The polymer architectures obtained via this method are strictly related with the polymerization mechanisms involved which depend on the polymerization conditions chosen. Hereby, we evaluate the catalytic activity of three acids, fumaric, trifluoroacetic, and methanesulfonic on the Cationic ROP of trimethylene carbonate (TMC) over a trifunctional initiator, trimethylol propane (TMP), under different reaction conditions. In‐detail characterization of the polymers showed the co‐existence of two polymerization mechanisms: the activated monomer (AM), which produces a tri‐armed branched polycarbonate with inclusion of the TMP initiator (TMP‐PTMC), and a combined AM/Activated Chain End (ACE) mechanism, which produces a linear polycarbonate (L‐PTMC). Such mixtures were identified for nearly all the reaction variables investigated, together with other side reactions. Upon optimization of the synthesis, the polymerizations in toluene with TFA at 35 °C and equimolar acid/initiator ratio were optimal, avoiding side reactions, but still resulting in a polymer mixture composed of ～69% TMP‐PTMC and 31% of a polycarbonate linear polymer. The occurrence of such mixed polymer architectures is commonly overlooked in literature regarding CROP of branched polycarbonates. We demonstrate the importance of performing a full characterization for a successful detection of polymer mixtures having different (number of) end‐functionalities, which are critical for further use in advanced applications, such as in the biomedical or pharmaceutical filed. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1502–1511     

乳酸直接聚合的热力学、动力学研究

以乳酸单体为原料，在无催化剂存在和常压下直接缩聚合成聚乳酸，并研究了乳酸直接聚合的动力学，结合其热力学的分析，指出提高真空度脱除水分、控制合适的温度、选择合适的催化剂是获得高分子量聚乳酸的关键，为乳酸地接聚合工艺提供了理论参考。     

A novel nanoporous structure on the surface of bubbles in polycarbonate foams

We found that novel nanoporous structures such as ellipsoidal‐ or string‐shaped nanopores were formed on the surface of the spherical bubbles in polycarbonate foams by annealing CO₂‐saturated polycarbonate. The pores were surrounded by polycarbonate nanofibrils, and birefringence with spherical symmetry was seen on the surface of the spherical bubbles. Thus, the formation of such a characteristic structure might be attributed to orientation‐induced crystallization of CO₂‐saturated polycarbonate on the surface of the bubbles and the exclusion of CO₂ from the fibrillar crystals thus obtained. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 843–846, 2008     

Formation of poly(ethylene phosphates) in polycondensation of H3PO4 with ethylene glycol. Kinetic and mechanistic study

Conditions of synthesis of poly(ethylene phosphates) in reaction of H₃PO₄ with HOCH₂CH₂OH (EG), the actual path of polycondensation, and structure of the obtained polymers (mostly oligomers) and kinetics of reaction are described. Preliminary kinetic information, based on the comparison of the MALDI‐TOF‐ms and ³¹P{¹H} NMR spectra as a function of conversion is given as well. Because of the dealkylation process fragments derived from di‐ and triethylene glycols are also present in the repeating units. Structures of the end groups (? CH₂CH₂OH or ? OP(O)(OH)₂) depend on the starting ratio of [EG]₀/[H₃PO₄]₀, although even at the excess of EG the acidic end groups prevail because of the dealkylation process. In MALDI‐TOF‐ms products with P_n equal up to 21 have been observed. The average polymerization degrees (P_n) are lower and have been calculated from the proportion of the end groups. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 830–843, 2008