Gel Permeation Chromatography Data Research Articles

Mechanism of Activation of a Hafnium Pyridyl−Amide Olefin Polymerization Catalyst: Ligand Modification by Monomer

We have investigated the olefin polymerization mechanism of hafnium catalysts supported by a pyridyl-amide ligand with an ortho-metalated naphthyl group. Ethylene-alpha-olefin copolymers from these catalysts have broad molecular weight distributions that can be fit to a bimodal distribution. We propose a unique mechanism to explain this behavior involving monomer modification of the catalyst, which generates multiple catalyst species when multiple monomers are present. More specifically, we present evidence that the hafnium alkyl cation initially undergoes monomer insertion into the Hf-naphthyl bond, which permanently modifies the ligand to generate new highly active olefin polymerization catalysts. Under ethylene/octene copolymerization conditions, a plurality of new catalysts is formed in relative proportion to the respective monomer concentrations. Due to the asymmetry of the metal complex, two "ethylene-inserted" and eight "octene-inserted" isomers are possible, but it is a useful approximation to consider only one of each in the polymerization behavior. Consequently, gel permeation chromatography data for the polymers can be fit to a bimodal distribution having a continuous shift from a predominantly low molecular weight fraction to predominantly higher molecular weight fraction as [octene]/[ethylene] is increased. Theoretical calculations show that such insertions into the Hf-aryl bond have lower barriers than corresponding insertions into the Hf-alkyl bond. The driving forces for this insertion into the Hf-aryl bond include elimination of an eclipsing H-H interaction and formation of a stabilizing Hf-arene interaction. These new "monomer-inserted catalysts" have no beta-agostic interaction, very weak olefin binding, and olefin-insertion transition states which differ on the two sides by more than 4 kcal/mol. Thus, the barrier to site epimerization is very low and high polymerization rates are possible even when the chain wags prior to every insertion. Experimental evidence for aryl-insertion products is obtained from reactions of ethylene (13C2H4 NMR studies) or 4-methyl-1-pentene (4M1P) using relatively low monomer/catalyst ratios. Quantitative generation of monomer-inserted products is complicated by slow initiation kinetics followed by fast polymerization kinetics. However, NMR evidence for reaction with 13C2H4 was observed in situ at low temperature, and the attachment of monomer to ligand was confirmed by GC/MS and 13C NMR after quenching. Furthermore, a 4M1P-appended ligand was isolated from a polymerization reaction (50:1 monomer:catalyst) by column chromatography followed by multiple recrystallizations. One isomer was characterized by X-ray crystallography, which unequivocally shows a 4-methylpentyl substituent at the 2-position of the naphthyl, consistent with 1,2-insertion into the Hf-aryl bond. NMR suggests a second diastereomer (not isolated) is formed from a 1,2-insertion of opposite stereoselectivity.

Copolymerization of styrene and maleic anhydride in supercritical carbon dioxide

AbstractThe copolymerization of styrene (St) and maleic anhydride (MA) was carried out in supercritical carbon dioxide (SC CO2). It was found that St and MA are easy to copolymerize in SC CO2 and the conversion can reach 97%, and that very fine and dry powders are obtained. The products were characterized using Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). GPC data showed that the molecular weight of the copolymer reach 3.62 × 104 g mol−1. Scanning electron microphotographs showed the minimum particle size of the product is about 200 nm. DSC measurements indicated that the glass transition temperature (Tg) of the copolymer increases with increasing the MA content in the copolymer. TGA curve showed that the copolymers were decomposed at about 350°C. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Synthesis, characterization, and in vitro drug‐release properties of 2‐hydroxyethyl methacrylate copolymers

Abstract2‐Hydroxyethyl methacrylate was copolymerized with acrylamide, N‐vinyl‐2‐pyrrolidone, and n‐butyl methacrylate by free‐radical solution polymerization with α,α′‐azobisisobutyronitrile as an initiator at 70 ± 1°C. The average molecular weights and molar compositions of the resultant copolymers were determined with gel permeation chromatography and 1H‐NMR spectroscopy data, respectively. Diclofenac or 2‐[(2,6‐dichlorophenyl)amino]benzene acetic acid, a nonsteroidal anti‐inflammatory drug, was chemically attached to the copolymers by transesterification reaction in the presence of N,N′‐dicyclohexylcarbodiimide to give macromolecular prodrugs. All the synthesized polymers were characterized with Fourier transform infrared, 1H‐, and 13C‐NMR spectroscopy techniques. The polymer–drug conjugates were hydrolyzed in cellophane member dialysis bags containing aqueous buffered solutions (pH 8) at 37°C, and the hydrolysis solutions were detected by UV spectrophotometer at selected intervals. The results showed that the drug could be released by selective hydrolysis of the ester bond from the side chain of the drug moiety. The release profiles of the drug indicated that the hydrolytic behavior of polymeric prodrugs strongly depends on the hydrophilicity of the polymer. The results suggest that the synthesized copolymers could be useful carriers for the release of diclofenac in controlled‐release systems. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2403–2409, 2007

Characterization of a slowly degrading biodegradable polyesterurethane for tissue engineering scaffolds

The purpose of this research was to develop and characterize a novel, slowly degrading polyester-urethane. In this study, a polyester-urethane with a crystalline segment of poly((R)-3-hydroxybutyric acid)-diol linked by a diisocyanate to an amorphous segment of poly(epsilon-caprolactone-co-glycolide)-diol was synthesized. Porous and nonporous scaffolds were processed using electrospinning and solvent casting respectively. The morphology, pore size, and filament diameter of the mesh and film were characterized using scanning electron microscopy (SEM). The thermal properties were examined using differential scanning calorimetry (DSC). A degradation study was initiated to characterize the change in mechanical properties, molecular weight, and surface morphology over 12 months using tensile testing, gel permeation chromatography (GPC), and SEM respectively. Concomitantly, cell morphology and viability on these variants were investigated using fibroblasts. The mechanical test data indicated a gradual decrease in the ultimate tensile strength and strain to break while the modulus of elasticity remained stable. GPC data suggested a slow decrease in the molecular weight while SEM examination revealed changed surface morphologies. The in vitro studies implied that the novel polyester-urethane was not cytotoxic and that the mesh was a more favorable scaffold towards cell viability. The summation of these results suggests that this polyester-urethane has the potential for tissue engineering applications.

Transesterification and polymerization reactions of aliphatic polyesters in supercritical CO 2 fluids without the presence of a catalyst

The reactions or stability of two crystalline polycaprolactones (PCL) and one amorphous poly( d, l-lactic acid) ( dl-PLA) in supercritical CO 2 fluid (scCO 2) at 35 °C and 34.5 MPa without the presence of a catalyst were investigated using gel permeation chromatography (GPC), nuclear magnetic resonance spectroscopy (NMR), and differential scanning calorimetry (DSC). The two PCLs having 80,000 and 10,000 in number average molecular weights ( M ¯ n ), denoted by PCL8 and PCL1, respectively, and dl-PLA having 74,000 in M ¯ n were studied. Carbon 13 NMR data found these polyesters no formation of carbonate linkages after treatments in scCO 2, indicating that the CO 2 did not copolymerize with the PCLs and dl-PLA. GPC data found that the molecular weights of PCL8/PCL1 blends considerably varied with the treatment time while the components of the blends negligibly changed throughout 6 h of treatments in scCO 2. These GPC data suggested that transesterification reactions occurred between PCL8 and PCL1. DSC data found that PCL8 co-crystallized with PCL1 and the treatments enhanced the melting temperatures and crystallinities of the blends. GPC data also found that the molecular weights of dl-PLA and PCL8/ dl-PLA blends considerably varied with the treatment time in scCO 2. From the variations of molecular weights, it could be suggested that transesterifications and polymerizations both occurred in the PCL8/ dl-PLA blends, with the polymerizations predominating over the transesterifications. This predomination in polymerizations lowered with increasing PCL8 content in the blends. DSC data found that the amorphous dl-PLA could be induced to crystallize by scCO 2, giving a heating endotherm at near 122 °C. Following scCO 2 treatments, enhancements of the melting temperature and crystallinity of PCL8 in the PCL8/ dl-PLA blends were found, although these enhancements fluctuated with the treatment time.

Degradation of trans‐polyisoprene over time following the analysis of root fillings removed during conventional retreatment

To evaluate in vivo degradation of root filling materials over time. Thirty-six root filled teeth with or without periapical lesions were selected. Teeth with poor coronal restoration were not included. The teeth had been root filled 3-30 years previous and were scheduled for conventional retreatment. The association of root canal treatment, age, periapical lesion and root filling degradation was investigated. The filling material was removed from the root canal using files and no solvent. Trans-1,4-polyisoprene was isolated through solubilization of root filling remnants in chloroform followed by filtration and centrifugation. Gel permeation chromatography (GPC) and infrared spectroscopy (FT-IR) were utilized to study the occurrence and degree of degradation. The GPC and FT-IR data were collected for each sample and analysed statistically using the Kruskal-Wallis test. Degradation of trans-1,4-polyisoprene was a slow process. The process was identified as an oxidation reaction through the production of carboxyl and hydroxyl groups. Compared with the control group, significant molar mass decrease was noted after 15 years (P = 0.0146) in teeth with no periapical lesions. However, in teeth associated with periapical lesions the number of years for significant degradation was reduced to 5 (P = 0.0009). Polyisoprene degrades inside root canals as an oxidative process. The presence of periapical lesions was associated with a more rapid onset of degradation.

Synthesis and matrix‐assisted laser desorption/ionization time‐of‐flight characterization of bisphenol‐A copolyformals containing nickel(II)/Schiff base, eicosane and 2‐butene units in the main chain

Some bisphenol-A copolyformals, containing in the main chain different amounts of a Ni-diimine nonlinear optical (NLO)-chromophore, eicosane and/or 2-butene units, were synthesized by condensation reaction between dibromomethane and suitable mixtures of Ni(II)/Schiff base complex, 1,20-di(bisphenol-A)ether-eicosane and/or 1,4-di(bisphenol-A)ether-2-butene. Structural composition and thermal properties of polymeric materials were inferred by analyses by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS), nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and thermogravimetry (TG). MALDI-TOF data show that both Ni-diimine and unsaturated units are present in the copolyformals with a homogeneous arrangement in all the polymer mass range (GPC data). It has also been ascertained that the glass transition temperature (Tg) of the copolymers changes as a consequence of the abundance of aliphatic units in the macromolecules (DSC data). Cross-linking experiments of the copolymer at temperatures near the Tg value and under UV irradiation were also performed.

Determination of the epoxide equivalent weight (EEW) of epoxy resins with different chemical structure and functionality using GPC and 1H-NMR

A method for the determination and quantification of epoxide equivalent weights (EEW) and average functionality ( F ¯ n ) of different liquid and solid epoxy resins using gel permeation chromatography (GPC) and 1H-NMR spectrometry is described. Good correlations between GPC and 1H-NMR data were obtained regarding the estimation of the oligomers content within the epoxy resin compositions. Dependencies of average functionality against the polymerization degree and average-number molecular mass were pointed out and, subsequently, the EEW was calculated and compared with chemical analysis and suppliers data for all the epoxy resins studied.

Grafting of maleic anhydride onto styrene–butadiene–styrene triblock copolymer using supercritical CO2 as a swelling agent

Grafting of maleic anhydride (MA) onto styrene–butadiene–styrene triblock copolymer (SBS) was carried out by free radical polymerization using supercritical carbon dioxide (SC CO2) as a solvent of MA and swelling agent of SBS. The effect of various factors such as monomer concentration, initiator concentration, SC CO2 pressure, and reaction time on grafting ratio was studied. SBS and the product (SBS-g-MA) were characterized by Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). GPC data showed that the molecular weight of SBS-g-MA is bigger than that of SBS. DSC testing indicated that the glass transition temperature (Tg) of SBS-g-MA is higher than that of SBS. By SEM photo, we can observe that some particles which contain more oxygen atom grew out from the surface of SBS-g-MA when grafting ratio reached at 5.6%, and the amount and diameter of particles increased with increasing of grafting ratio. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4425–4429, 2006

Radical cyclopolymerization of a divinylbenzyl- p- tert-butylcalix[4]arene derivative

The synthesis and characterization of a new homopolymer (poly 1), obtained in the course of the radical polymerization of 25,27-bis-(4-vinyl-benzyloxy)-26,28-dihydroxy- p-tert-butylcalix[4]arene ( 1), is described. Homopolymerization of 1 in THF, using BPO or thermal initiation, afforded soluble polymers in good isolated yields (60–90%). Gel permeation chromatography (GPC) profiles showed unimodal distributions for all the analyzed polymers, which is indicative that chain branching reactions did not occur to a major extent. Molecular weights ( M n ) ranging from 30,000 to 60,000 g mol −1 were reached within a 8 h period, when the reactions were conducted at 0.06–0.5 mol% of BPO or thermally initiated, showing relatively narrow polydispersity indexes (1.5–2.0). The structure of the polymers was deduced upon analysis of their 1H NMR and FT-IR spectra, which, in conjunction with GPC and solubility data led to their formulation as cyclopolymers.

Free‐radical polymerization of styrene with p‐nitrobenzyl triphenyl phosphonium ylide as an initiator

AbstractThe free‐radical polymerization of styrene with p‐nitrobenzyl triphenyl phosphonium ylide as an initiator in dioxane at 80 ± 1 °C in a dilatometer under a nitrogen atmosphere for 150 min resulted in a syndiotactic polymer, as evidenced by IR, 1H NMR, and 13C NMR spectroscopy. A 1H NMR spectrum showed methylene protons as triplets; 13C NMR signals of the phenyl ipso carbons were used for the determination of the tacticity. The system followed ideal kinetics. Gel permeation chromatography data were used evaluate the weight‐average molecular weight. The overall activation energy was 47 kJ/mol. Electron spin resonance spectroscopy confirmed the initiation by the phenyl radical obtained by the dissociation of the ylide and the free‐radical mode of polymerization. Differential scanning calorimetry studies showed the glass‐transition temperature of the polymer to be 342 K. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6524–6533, 2005

Single-Molecule AFM Study of Polystyrene Grafted at Gold Surfaces

ABSTRACT Single-molecule studies under poor solvent conditions show that atomic force microscopy (AFM) measurements of contour lengths of polystyrene 12K and 17K relate well to gel permeation chromatography (GPC) data when grafting concentrations are low (not greater than 0.7 mM). Contact angles and ellipsometry have been used to characterize the surfaces and indicate low grafting densities at this grafting concentration. Persistence lengths (p) of different length polystyrene chains, when fitted to a WLC (worm-like chain) model, show values of p = 0.23 nm±0.10 nm and p = 0.25 nm±0.13 nm, for 12 K and 17 K polystyrene, respectively, when the persistence lengths are fitted to log-normal distributions. These values are close to the expected theoretical value of 0.23 nm and further confirm that mostly single molecules were studied on these polystyrene surfaces. Higher grafting concentrations (≥1 mM) resulted in pulling multiple molecules.

Peracetylated cellulose: end group modification and structural analysis by means of1H-NMR spectroscopy

A homologous series of partially hydrolyzed celluloses (level-off-DP cellulose) with weight-average molecular weight (DP w ) < 150 were peracetylated and characterized by 1H-NMR spectroscopy. The results demonstrate the utility of 1H-NMR spectroscopy to assign the chemical shifts of all end groups of the peracetylated cellulose. On the one hand, the chemical shifts of all methine and methylene protons of the non-reducing terminal end group (TEG) as well as the α- and β-anomer of the reducing end group (REG) could be determined by two-dimensional NMR techniques (COSY-DQF) and by selective excitation of isolated proton signals (1D-TOCSY) of these end groups. On the other hand, the spectral resolution was high enough to determine the number-average molecular weight (DP n ) of peracetylated level-off-DP cellulose (LODP cellulose acetates) as shown in comparison with the data of gel permeation chromatography (GPC). This molecular weight determination of cellulose using end group analysis by means of 1H-NMR spectroscopy was demonstrated for the first time. Furthermore, a specific modification of hydroxyls in end groups could be exemplified in case of 1-OH-deacetylation of the REG of peracetylated LODP cellulose.

Analytical and computational techniques for exogenous depolymerization of xenobiotic polymers

We analyze a mathematical model for exogenous depolymerization of xenobiotic polymers. We derive a sufficient condition for the solvability of its inverse problem to determine degradation rates. We also introduce a numerical technique to solve the inverse problem with an example to show how our numerical technique is applied to gel permeation chromatography data of polyethylene wax obtained before and after three-week cultivation of a fungus Aspergillus sp. AK-3. In addition, we present a numerical result which we obtained by applying the degradation rate based on the three week cultivation to simulate the transition of the weight distribution after cultivation of the fungus for five weeks.

Continuous ultrasonic devulcanization of unfilled butadiene rubber

AbstractThe devulcanization of sulfur‐cured unfilled butadiene rubber (BR) with a grooved‐barrel ultrasonic reactor under various processing conditions was carried out. The experiments indicated that BR had a narrow devulcanization window. Outside this window, significant degradation or no devulcanization occurred. Gel permeation chromatography (GPC) measurements were carried out with the sol part of virgin and devulcanized samples to study the breakdown of the polymeric chains. The GPC data showed a significant molecular weight reduction and a broadening of the molecular weight distribution upon devulcanization, indicating that the devulcanization and degradation of BR occurred simultaneously. The rheological properties showed that devulcanized BR was more elastic than the virgin gum. The vulcanizates of the blends of virgin and devulcanized BR showed a considerable enhancement of the mechanical properties. The thermal behaviors of the virgin and devulcanized BR were found to be different. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1166–1174, 2004

Block and star block copolymers by mechanism transformation X. Synthesis of poly(ethylene oxide) methyl ether/polystyrene/poly( l-lactide) ABC miktoarm star copolymers by combination of RAFT and ROP

Poly(ethylene oxide) methyl ether/polystyrene/poly( l-lactide) (MPEO/PSt/PLLA) ABC miktoarm star copolymers were synthesized by combination of reversible addition–fragmentation transfer (RAFT) polymerization and ring-opening polymerization (ROP) using bifunctional macro-transfer agent, MPEO with two terminal dithiobenzoate and hydroxyl groups. It was prepared by reaction of MPEO with maleic anhydride (MAh), subsequently reacted with dithiobenzoic acid and ethylene oxide. RAFT polymerization of St at 110 °C yielded block copolymer, MPEO- b-PSt [(MPEO)(PSt)CH 2OH], and then it was used to initiate the polymerization of l-lactide in the presence of Sn(OCt) 2 at 115 °C to produce ABC miktoarm star polymers, s-[(MPEO)(PSt)(PLLA)]. The structures of products obtained at each synthetic step were confirmed by NMR and gel permeation chromatography data.

Hop (Humulus lupulus L.) proanthocyanidins characterized by mass spectrometry, acid catalysis, and gel permeation chromatography.

Proanthocyanidins extracted from hops (Humulus lupulus L. cv. Willamette) were subjected to Sephadex LH-20 column chromatography using a step gradient of methanol, water, and acetone. The resulting fractions were analyzed by acid catalysis, electrospray ionization and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and gel permeation chromatography (GPC). The proanthocyanidins contained catechin and epicatechin as monomers and as terminal and extension units. Epigallocatechin was found as extension units. The mean degree of polymerization (mDP) of the crude proanthocyanidins was 7.8, but heptamers were the largest oligomers visible in mass spectra of the whole. In the last-eluted fraction (mDP = 22.2), polymers as large as 20-mers were detected by MALDI-TOF-MS, demonstrating the effectiveness of prior separation in improving MS detection. GPC data correlated well with acid catalysis results, confirming the presence of large polymers that were not detected by MS.

High‐density polyethylene modified by polydimethylsiloxane

High-density polyethylene (HDPE) was modified by the grafting of polydimethylsiloxane (PDMS) through a free-radical process, in a melt-mixer chamber, using dicumyl peroxide (DCP) as an initiator. The influence of PDMS (0.2–0.8 mol %) and peroxide (0.03–0.08 mol %) concentrations on the grafting, final torque, and melt flow rate (MFR) of copolymers were investigated using factorial planning. Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), gel permeation chromatography (GPC), MFR, and rheometry were used to characterize the copolymers obtained. Surface plots showed that higher degrees of grafted PDMS and higher final torques were obtained with increase in the PDMS amount at low DCP levels and with increase in the DCP amount at low PDMS levels. The peaks of fusion and crystallization of the copolymers showed no significant changes with respect to HDPE. Data of MFR and GPC suggested that crosslinking reactions and/or chain extension occurred concomitant with the grafting reactions. Copolymers with high grafting degrees showed high MFR and low dynamic shear viscosities in comparison with low grafting degree copolymers, which is probably due to the migration of the PDMS-containing copolymers on the surface. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3460–3467, 2001

High‐density polyethylene modified by polydimethylsiloxane

AbstractHigh‐density polyethylene (HDPE) was modified by the grafting of polydimethylsiloxane (PDMS) through a free‐radical process, in a melt‐mixer chamber, using dicumyl peroxide (DCP) as an initiator. The influence of PDMS (0.2–0.8 mol %) and peroxide (0.03–0.08 mol %) concentrations on the grafting, final torque, and melt flow rate (MFR) of copolymers were investigated using factorial planning. Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), gel permeation chromatography (GPC), MFR, and rheometry were used to characterize the copolymers obtained. Surface plots showed that higher degrees of grafted PDMS and higher final torques were obtained with increase in the PDMS amount at low DCP levels and with increase in the DCP amount at low PDMS levels. The peaks of fusion and crystallization of the copolymers showed no significant changes with respect to HDPE. Data of MFR and GPC suggested that crosslinking reactions and/or chain extension occurred concomitant with the grafting reactions. Copolymers with high grafting degrees showed high MFR and low dynamic shear viscosities in comparison with low grafting degree copolymers, which is probably due to the migration of the PDMS‐containing copolymers on the surface. © 2001 John Wiley &amp; Sons, Inc. J Appl Polym Sci 82: 3460–3467, 2001

Poly(Azomethine)s with Rotaxane Architecture Containing a Blocking Group in Every Structural Unit: Synthesis and Characterization

Poly(azomethine)s with rotaxane architecture containing a blocking group in every structural unit of the polymer chain have been synthesized by polycondensation of 3,6-diformyl-9-butyl carbazole with 1,6-hexamethylenediamine/α-or β-cyclodextrin adducts. As compared with the polymers obtained in the absence of cyclodextrins, these polymers are more hygroscopic and soluble in polar solvents such as dimethylformamide and dimethyl sulfoxide. The infrared, proton nuclear magnetic resonance, gel permeation chromatography and differential scanning calorimetry data support the presence of cyclodextrins in every structural unit of the polymer chain.