Mechanism Of Cross-linking Reaction Research Articles

Pervaporation separation of water/isopropanol mixtures through crosslinked carboxymethyl chitosan/polysulfone hollow‐fiber composite membranes

AbstractCarboxymethyl chitosan (CMCS)/polysulfone (PS) hollow‐fiber composite membranes were prepared through glutaraldehyde (GA) as the crosslinking agent and PS hollow‐fiber ultrafiltration membrane as the support. The permeation and separation characteristics for dehydration of isopropanol were investigated by the pervaporation method. Pure chitosan, carboxymethyl chitosan, and crosslinked carboxymethyl chitosan membranes were characterized by Fourier transform infrared (FT‐IR) spectroscopy and X‐ray diffraction (XRD) to study the crosslinking reaction mechanism and degree of crystallinity, respectively. The effects of feed composition, crosslinking agent, membrane thickness, and feed temperature on membrane performance were investigated. The results show that the crosslinked CMCS/PS hollow‐fiber composite membranes possess high selectivity and promising permeability. The permeation flux and separation factor for isopropanol/water is 38.6 g/m2h and 3238.5, using 87.5 wt % isopropanol concentration at 45°C, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1959–1965, 2007

Synthesis and characterization of superabsorbent hydrogels composites based on polysuccinimide

AbstractA kind of novel superabsorbent hydrogel with high swelling ratio property that could be used for the development of water absorbing resin, soil water retention agent, and chemical sand‐fixing material was synthesized in this study. The hydrogels were prepared by the crosslinking reaction of polysuccinimide (PSI). The relationships between swelling ratio and volume of solvent as well as the concentration of crosslinking agent were investigated in detail. Several composites, such as starch, carrageenan, and polyacrylamide, were added into hydrogels to enhance the swelling ratio. It was found that the swelling ratio was significantly increased, which the maximum water absorbency was enhanced 2.46 times when the composite polyacrylamide (PAM) was added compared to the control. The effects of ionic strength and sensitivity of pH on hydrogels were also studied. The modified hydrogels products with swelling ratio less sensitivity to the salinity as well as relative high swelling ration in salinity system were also obtained by adding PAM. Through the Fourier transform infrared spectroscopy (FTIR) characterizations, the crosslinking reaction mechanism and the structure of composite were proposed. In addition, the transmission electron microscopy (TEM) examinations showed that some composite materials elevated the physical crosslinked and connected channels density substantially. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 550–557, 2006

Superabsorbent hydrogel of acrylic acid/potassium acrylate copolymers by ultraviolet photopolymerization: Synthesis and properties

AbstractSynthesis of superabsorbent hydrogel (SAHG) of acrylic acid‐potassium acrylate copolymer by direct Ultraviolet (UV) photopolymerization is a new method. The effects of degree of neutralization of acrylic acid (AA), photoinitiators, crosslinking agents, and exposure time of UV light on water absorbent properties were investigated. The results showed that the water absorbency (Q) and the salt solution (NaCl, 0.9%) absorbency (Qs) of SAHG, based on Irgacure 651, are high, reaching about 1400 and 130 mL/g, respectively. UV absorption spectrum proved that peak of UV absorption of Irgacure 651 matched the UV light source we used. Among the crosslinking agents, N,N′‐methylene bisacrylamide is more efficient than others, because of its very small content and high Q. 13C NMR spectrometry was used to identify the mechanism of crosslinking reaction through esterification of hydroxyethyl acrylate (HEA) and 2‐hydroxypropyl acrylate (HPA) with carboxylic acid group in acrylic acid‐ammonium acrylate copolymerization, but efficiency of crosslinking reaction by esterification was lower than that of copolymerization of vinyl group in crosslinking agent. The Q of acrylic acid‐potassium acrylate copolymer of SAHG reaches 1592 mL/g under the following conditions: degree of neutralization of acrylic acid is 80%, content of Irgacure 651 is 0.25 wt %, content of HEA is 0.2 wt %, and exposure time is 10 min. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1181–1187, 2006

Poly(dimethylsiloxane) coatings for controlled drug release. III. Drug release profiles and swelling properties of the free‐standing films

AbstractPrevious reports in this series have described the preparation of stable poly(dimethylsiloxane) (PDMS) latices suitable for spray‐coating of drug tablets, as well as the mechanism of associated crosslinking reactions in PDMS emulsions. In the present investigation, in vitro evaluations were performed to study the effects of the amount of channeling agents, the addition of colloidal silica, and the pH of the dissolution media used. The study involved hydrochlorothiazide (as a marker drug) released from compressed tablets, which had been spray coated using PDMS latices with various polyethylene glycol (PEG) loadings as channeling agents. The dissolution results showed that coated tablets containing up to 25% (w/w dps) PEG could have constant release rates. Higher amounts of PEG resulted in nonlinear release patterns. The addition of colloidal silica decreased the rates of drug release. The pH of dissolution media affected the structures of the exposed PDMS films. Swelling tests were carried out to determine water uptake. Scanning electron microscopy and density measurements showed that the films obtained after soaking in higher‐pH media were more condensed, with corresponding changes in drug‐release rates. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 494–501, 2005

Rheokinetics of the cross-linking of melt polyethylene initiated by peroxide

The cross-linking reaction of polyethylene/peroxide system was studied at elevated temperature. The dynamic modulus evolution was monitored within linear viscoelastic regime by parallel plate rheometer. Different strains were chosen to represent different flow fields. Based on the mechanism of cross-linking reaction, a new expression relating recombination rate constant with rheological conversion was derived. The experimental and calculated results showed that the strain had substantial effects on the cross-linking reaction kinetics. Consequently a formula reflecting the effect of the complex viscosity on recombination rate constant was found, which was further integrated into the classical Arrhenius kinetic equation. POLYM. ENG. SCI., 45:560–567, 2005. © 2005 Society of Plastics Engineers

Effect of functional crosslinking agents on preparation and swelling properties of dextran hydrogels

Crosslinking reactions of Dextran (Dx) ( M n of 2.0×10 6 g mol −1) with some selective Cl-, and P-containing functional monomers such as epichlorohydrin (ECH) and phosphorus oxychloride (POC1 3) were carried out in the basic aqueous solutions (2.8 N NaOH) at 40 °C. The optimum conditions of the effective swelling and crosslinking for the each system studied were found in nickel (NiCl 2·6H 2O) solution. The percent swelling, equilibrium swelling, initial rate of swelling, swelling rate of constant, equilibrium water content, and diffusion type and constant values were evaluated for Dx/crosslinker (CL) systems at 1 mg/100 ml nickel (NiCl 2·6H 2O) solution. A substantial difference of these parameters observed for the various Dx/CL systems was explained by the effect of nature of crosslinking agents on the mechanism of crosslinking and swelling processes. It was shown that S eq values increase depending on the nature of CLs in the following order: ECH<POCl 3. General scheme and proposed mechanism of crosslinking reactions in the Dx/CL systems were also described.

Preparation conditions and swelling equilibria of dextran hydrogels prepared by some crosslinking agents

Crosslinking reactions of Dextran (Dx) ( M n of 2.0×10 6 g mol −1) with some selective Cl-, P- and N-containing functional monomers such as epichlorohydrin (ECH), phosphorus oxychloride (POC1 3) and N, N′-methylenebisacrylamide (MBAM) were carried out in the basic aqueous solutions (2.8 N NaOH) at 25–50 °C. The optimum conditions of the effective swelling and crosslinking for the each system studied were found in copper (CuCl 2·2H 2O) solution. The percent swelling, equilibrium swelling, initial rate of swelling, swelling rate of constant, equilibrium water content, and diffusion type and constant values were evaluated for Dx/crosslinker (CL) systems at 1 mg/100 ml copper (CuCl 2·2H 2O) solution. A substantial difference of these parameters observed for the various Dx/CL systems was explained by the effect of nature of crosslinking agents on the mechanism of crosslinking and swelling processes. It was shown that S eq and M c values increase depending on the nature of CLs in the following order: ECH>MBAM>POCl 3. General scheme and proposed mechanism of crosslinking reactions in the Dx/CL systems were also described.

Selection of quaternary ammonium groups for optimizing properties of water‐soluble, photosensitive phenolic resins and study of their postcuring mechanism

AbstractThis article deals with the selection of quaternary ammonium groups for synthesis of water‐soluble, photosensitive phenolic resins, containing acrylate and different quaternary ammonium salt groups (AQSPRs), via ring‐opening reactions of epoxy phenolic resin (EPR) with acrylic acid and with different tertiary amine‐protonic acid salts. Conversion of epoxy groups, solubility, photosensitive properties, and thermal decomposition of the different AQSPRs were compared. Modification of AQSPR with methanol solution of KOH to form phenolic resin containing both quaternary ammonium hydroxide groups and acrylate groups (AQHPR) was also studied. Characterization by IR spectrum, DSC, and thermal gravimetric analysis was carried out. The results showed that in the synthesis of AQSPRs containing different quaternary ammonium salt groups, the efficiency of ring‐opening reaction of epoxy phenolic resin with tertiary amine salt in terms of conversion of epoxy groups decreases in the following order: for the tertiary amine, N,N‐dimethyl benzylamine (DMBA) &gt; triethylamine (TEA) &gt; trimethylamine (TMA) &gt; N,N‐dimethyl aniline (DMA) &gt; triethanolamine (TENA) &gt; tri(n‐butylamine) (TBA); for the protonic acid, HCl &gt; HBr &gt; HCOOH &gt; HI &gt; NaHSO3 &gt; Cl3CCOOH &gt; HClO4 &gt; HBF4. All the AQSPRs except that from HClO4 can be dissolved in water, methanol, DMF, or DMSO. The gel content formed during UV exposure decreases in the following order of acids used in forming quaternary ammonium salt groups: HCl &gt; HCOOH &gt; NaHSO3 &gt; Cl3CCOOH; or decreases in the following order of tertiary amines or hydrohalic acids used in forming the quaternary ammonium groups: TMA. &gt; TEA &gt; DBMA; HCl &gt; HBr &gt; HI. During thermal decomposition of EPR with about half epoxy groups of EPR ring‐opened with tertiary amine salt at 160°C for 0.5 h, water‐insoluble product was formed. The insoluble content and the % decrease of epoxy groups or halide ions increase in the following order: TMA &lt; TEA &lt; DMBA; HCl &lt; HBr &lt; HI. The % decrease of epoxy groups for the insoluble residue is nearly equal to the % decrease of halide ions. A crosslinking reaction mechanism occurred in the thermal decomposition was thus proposed. During the modification of AQSPR with KOH, conversion of quaternary ammonium chloride groups can reach above 90%. The decomposition temperature of the quaternary ammonium groups was lowered from 204 to 120°C after modification of AQSPR with KOH. The photosensitive properties of the resin after modification became lower. It is better to react DMBA · HCl with EPR so as to obtain a product with higher conversion of epoxy groups, good water solubility, moderate photosensitivity, lower decomposition temperature, and better postcuring. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2914–2922, 2004

A polymeric photobase generator containing oxime–urethane groups: Crosslinking reaction and application to negative photoresist

AbstractA polymeric photobase generator containing oxime–urethane groups was prepared by copolymerization of methyl methacrylate and methacryloxyethyl benzophenoneoxime urethane, and its photo and thermal crosslinking reaction after irradiation was examined from the measurement of UV and IR absorption spectral changes, insoluble fraction, and molecular weight changes. The photo‐crosslinking reaction of the copolymer film was more efficient when irradiations were carried out with 310 nm UV light in the presence of benzophenone than with 254 nm UV light without the addition of benzophenone. The crosslinking reaction increased after postexposure baking (PEB), and this thermal crosslinking reaction mechanism was studied from the identification of the photolysis products of a model compound, benzophenoneoxime phenylurethane, by a high‐performance liquid chromatography. The results indicate that the thermal crosslinking reaction of the copolymer after PEB is due to the formation of urea‐type chemical bonds. Resist properties of the copolymer were examined from the measurement of normalized thickness and micropattern development. A negative tone image with a resolution of 2 μm was obtained with this copolymer, having a sensitivity (D) of 1200 mJ/cm2 and contrast (γn) of 1.31, when irradiation was carried out with 310 nm UV light in the presence of benzophenone following chloroform development. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 975–984, 2004

Crosslinking reaction of polyacrylate soap-free hydrosol with zirconium chelate

The crosslinking reaction mechanism of polyacrylate copolymer (monomer mass ratio of methyl methacrylate/butyl acrylate/acrylic acid = 48.77:46.69:4.54) hydrosol with triethanolamine chelate of zirconium isopropyloxide was studied with IR spectroscopy, NMR spectroscopy, differential scanning calorimetry, and ultraviolet spectroscopy. The instrumental analyses on the copolymer mixture heated at different temperatures were investigated, and we proved that the crosslinking adduct had amino groups that were provided by the chelate. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3605–3609, 2004

Hyperbranched macromolecules through donor‐acceptor type copolymerization of allyl–vinylene bifunctional monomers

AbstractPresent review is an attempt to generalize and systematize the results accumulated in synthesis of cyclolinear and hyperbranched reactive macromolecules via radical alternating copolymerization of various bifunctional monomers containing donor and acceptor type double bonds. Synthesis of hyperbranched reactive macromolecules was carried out using complex‐radical cyclocopolymerization of donor‐acceptor type bifunctional monomers such as monoallyl ester of maleic acid (MAM), allyl acrylate (AA), allyl methacrylate (AM), allyl trans‐cinnamate (AC), methylallylmaleate (MeAM), methylallylfumarate (MAF) and allyl‐α‐(N‐maleimido)acetate (AMI), and maleic anhydride (MA) and styrene (St) as typical acceptor and donor comonomers, respectively. The kinetic parameters of these reactions, constants of cyclization, complex‐formation and copolymerization, as well as the ratios of chain growth rates for the participation of monomeric charge transfer complexes and free monomers, were all determined. It was demonstrated that in the studied systems, copolymerizations predominantly proceed according to alternating mechanism with formation of macromolecules having cyclolinear structure in the steady‐state and hyperbranched structure in the high conversion conditions. It was shown that formation of linear and hyperbranched macromolecules containing allyl or vinylene groups in the side chain occurs selectively carry out and depends on the nature of used comonomer. General schemes and proposed mechanism of hyperbranching and crosslinking reactions were also described. Some useful properties of synthesized reactive copolymers were discussed.© 2002 Society of Chemical Industry.

Effect of the coupling agent's reactivity on the shrinkability of the blends consisting of grafted low‐density polyethylene and ethylene acrylic elastomer

AbstractBlends prepared by melt mixing of thermoplastic elastomer have gained considerable attention in recent years from a heat shrinkability point of view. Our present study deals with the measurement of heat shrinkability of the maleic anhydride grafted low‐density polyethylene and ethylene acrylic elastomer. Two samples have been prepared to study the effect of coupling agent's reactivity on the shrinkability of the blends. The coupling agents used are 4,4′‐diamino diphenyl sulphone, and 4,4′‐diamino diphenyl methane. Shrinkability was measured at room temperature, 120 °C, 150 °C, and 180 °C. Shrinkability is found to be greater in high temperature stretched sample rather than that of room temperature stretched sample. It is observed that reactivity as well as heat shrinkability is more when 4,4′‐diamino diphenyl methane is used as a coupling agent. The mechanism of interchain crosslinking reaction has been confirmed by IR spectroscopy. Differential scanning calorimetry was performed to study the thermal stability of the sample. Copyright © 2002 John Wiley &amp; Sons, Ltd.

A New Method for Estimating the Cross-Linking Reaction during the Pyrolysis of Brown Coal.

Suppression of the cross-linking reaction at low temperature is a key factor to increase total volatiles during the pyrolysis of brown coals. To do so, it is essential to clarify the mechanism and kinetics of such cross-linking reactions. We prepared coal samples having similar monomer units but different macro-molecular structures through liquid phase oxidation of an Australian brown coal. The oxidized coal samples were heated from 303 to 1173 K at the rate of 20 K/min in a TG-MS analyzer and were pyrolyzed. It was found that the formation rates of inorganic gases, H2O, CO and CO2, were significantly dependent on the amount of oxygen functional groups in the coals. Oxidized coals having a large amount of COOH groups decomposed at lower temperatures and distinct peaks of the formation rates of inorganic gases appeared at around 673 K. We proposed six kinds of cross-linking reactions based on the types of hydrogen bonding formed between the functional groups, and estimated the change in the extent of each reaction with increasing temperature from the formation rates of the inorganic gases measured. The hydrogen bonded COOH–COOH and COOH–OH decomposed at 500–600 K and 600–700 K, respectively, to form the inorganic gases and cross-linked products such as anhydrides, ether, and ester. These cross-linked products decomposed further into CO and CO2 at temperatures above 800 K. Based on the above analysis and the 13C-NMR measurement of the pyrolyzed chars, it was clarified that the number of cross-links was equal to the number of H2O molecules formed and that the CO and CO2 formation rates were closely related to the H2O formation rate.

Effect of the coupling agent's reactivity on the shrinkability of the blends consisting of grafted poly (ethylene vinyl acetate) and polyurethane elastomer

Blend prepared by the melt mixing of thermoplastic elastomer has gained considerable attention in recent years from heat shrinkability point of view. Present study deals with the measurement of the heat shrinkability of the maleic anhydride grafted poly (ethylene vinyl acetate) and polyurethane elastomer blend. Two samples have been prepared to study the effect of the coupling agent's reactivity on the shrinkability of the blend. The used coupling agents are 4,4′ diamino diphenyl sulphone (DDS) and 4,4′ diamino diphenyl methane (MDA). It is revealed that heat shrinkability increases as the reactivity of the coupling agent increases. The reactivity as well as the shrinkability is more when the used coupling agent is 4,4′ diamino diphenyl methane. The mechanism of interchain crosslinking reaction is confirmed by IR spectroscopy.

Crosslinking of dextran with some selective Cl-, P- and N-containing functional substances in aqueous solutions

Crosslinking reactions of Dextran (Dx) ( M n of 5.0×10 5 and 2.0×10 6 g mol −1) with some selective Cl-, P- and N-containing functional monomers such as epichlorohydrin (ECH), phosphorus oxychloride (POC1 3) and N, N′-methylenebisacrylamide (MBAM) as well as with 60Co-γ-irradiation were carried out in the basic aqueous solutions (2.8 N NaOH) at 25–50°C. The optimum conditions of the effective swelling and crosslinking for the each system studied were found. Some swelling parameters such as initial rate of swelling ( R S, %min −1), crosslink density ( ρ, g ml −1), equilibrium swelling ( S eq, %) and average molecular weight of crosslinked fragments ( M c, g mol −1) for the Dx/crosslinker (CL) systems studied were determined by using of kinetic methods and well known Flory-Rehner equation. A substantial difference of these parameters observed for the various Dx/CL systems was explained by the effect of nature and concentration of crosslinking agents on the mechanism of crosslinking and swelling processes. It was shown that R S and ρ values increase while the S eq and M c values decrease depending on the nature of CLs in the following raw: γ-ray-ECH-MBAM-POCl 3. Concentration of MBAM does not influence the initial rate of swelling while effect of CL concentration is strongly advented in the Dx/ECH and Dx/POC1 3 systems. Effectiveness of CLs is also confirmed by TGA analysis of crosslinking polymers. The formation of unsaturated crosslinked fragments, which are easily polymerized or photocyclodimerized as affected of u.v.-irradiation, was confirmed by Fourier transform infrared (FT-IR) spectroscopy. General scheme and proposed mechanism of crosslinking reactions in the Dx/CL systems was also described.

Effect of interchain crosslinking on the shrinkability of the blends consisting of grafted low-density polyethylene and carboxylated nitrile rubber

Blend prepared by melt mixing of thermoplastic elastomer has gained considerable attention in recent years. The present study deals with the measurement of heat shrinkability of the blends of the low-density polyethylene (LDPE) and carboxylated nitrile rubber (XNBR). Two samples have been prepared for this purpose. In the first sample, LDPE is blended with XNBR, while in the second sample, glycidyl acrylate (GA) grafted LDPE is blended with XNBR. It is revealed that interchain crosslinking in the grafted LDPE and XNBR improves the heat shrinkability. Here, crosslinked point between rubber and plastic molecules serve as memory point during the recovery process, which is responsible for the improvement of heat shrinkability. High temperature performance of the sample (as far as shrinkability is concerned) depends on interchain crosslinking. The mechanism of interchain crosslinking reaction is confirmed by infrared (IR) spectroscopic analysis.

Electron spin resonance study on chemical crosslinking reaction mechanisms of polyethylene using a chemical agent. V. Comparison with polypropylene and ethylene‐propylene copolymer

We studied the chemical reaction process of polypropylene (PP), ethylene-propylene copolymer (EPM), and ethylene-propylene-diene copolymer (EPDM) crosslinking induced by dicumyl peroxide (DCP) using electron spin resonance (ESR). Free radicals appeared at an elevated temperature of around 120 °C and the behavior and kinetics of the reaction process were observed at 180 °C. The radical species detected in PP were alkyl type radicals, formed by the abstraction of hydrogen atoms from the tertiary carbon of polymer chains. For EPDM containing a diene component, the radicals were trapped at double bonds in this diene component to form allyl radicals. The resolutions of these spectra were extremely clear; hence, isotropic spectra of these polymer radicals were obtained. We measured the ESR at high temperatures and confirmed that the process of crosslinking induced by DCP was a free radical reaction. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3383–3389, 2000

Free-Radical Cross-Linking Polymerization of Diallyl Terephthalate in the Presence of Microgel-like Poly(allyl methacrylate) Microspheres

The cross-linking polymerization of diallyl terephthalate (DAT) was explored in detail in the presence of microgel-like poly(allyl methacrylate) (PAMA microsphere) as an extension of our continuing studies concerned with the elucidation of cross-linking reaction mechanism and the control of network formation in the free-radical cross-linking polymerization of multivinyl compounds. Thus, PAMA microspheres were prepared by the emulsion polymerization of AMA as reactive cross-linked polymer microspheres having abundant pendant allyl groups. The polymerization of DAT in the presence of microsphere was conducted in bulk using BPO as initiator at 80 °C. The gel-point conversion became higher with an increase in the feed amount of microsphere. No gel effect was observed. Variation of RI-monitored SEC curves with conversion suggested the copolymerization of the pendant allyl groups present on the surface of microsphere with DAT; this was also supported by light scattering and 1H NMR spectroscopy. The swelling ratios of the gels in the presence of microsphere were not high even just beyond the gel points, suggesting a preferential incorporation of microsphere into the gel. These are discussed mechanistically.

Synthesis and ring-opening reactions of tetraphenyl-substituted bisoxazoline monomers for preparing new thermosets and composites

To develop new monomers for formulating thermosetting composites, three new tetraphenyl-substituted bisoxazoline monomers were synthesized via the direct reaction of 2-(diphenylmethyl)oxazoline with bromoalkyls, using tert-butyllithium. The structures were confirmed by Fourier transform infrared, nuclear magnetic resonance and mass spectrometry. These bisoxazolines have different melting points with varied molecular chain flexibility. They functioned well as crosslinkers when heated with phenolic resins or poly(acrylic acid), providing a path to new thermosetting materials with controlled glass transition temperature. To clarify the crosslinking reaction mechanism, the model reactions of these monomers with phenol and acetic acid were investigated. Copyright © 2000 John Wiley & Sons, Ltd.

Electron spin resonance study on chemical crosslinking reaction mechanisms of polyethylene using a chemical agent. V. Comparison with polypropylene and ethylene-propylene copolymer

We studied the chemical reaction process of polypropylene (PP), ethylene-propylene copolymer (EPM), and ethylene-propylene-diene copolymer (EPDM) crosslinking induced by dicumyl peroxide (DCP) using electron spin resonance (ESR). Free radicals appeared at an elevated temperature of around 120 °C and the behavior and kinetics of the reaction process were observed at 180 °C. The radical species detected in PP were alkyl type radicals, formed by the abstraction of hydrogen atoms from the tertiary carbon of polymer chains. For EPDM containing a diene component, the radicals were trapped at double bonds in this diene component to form allyl radicals. The resolutions of these spectra were extremely clear; hence, isotropic spectra of these polymer radicals were obtained. We measured the ESR at high temperatures and confirmed that the process of crosslinking induced by DCP was a free radical reaction. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3383–3389, 2000