PCL Macromonomers Research Articles

Shape-memory balloon offering simultaneous thermo/chemotherapies to improve anti-osteosarcoma efficacy.

This paper proposes a shape-memory balloon (SMB) to improve bone cement injection efficiency and postoperative thermo/chemotherapy for bone tumors. The SMB consists of biodegradable poly(ε-caprolactone) (PCL), an anticancer drug (doxorubicin, DOX), and heat-generating magnetic nanoparticles (MNPs). The balloon shape is fabricated in a mold by crosslinking PCL macromonomers with DOX and MNPs. The mechanical properties and shape-transition temperature (approximately 40 °C) of the SMB are modulated by adjusting the molecular weight of PCL and the crosslinking density. This allows safe inflation at the affected site with a 400% expansion rate by simple blow molding. The expanded shape is temporarily memorized at 37 °C, and the computed tomography image shows that the bone cement is successfully injected without extra pressure or leakage. The SMB releases DOX for over 4 weeks, allowing a prolonged effect at the local site. The local dosing is constant as the medication is continuously released, demonstrating an ON-OFF switchable heating/cooling response to alternating magnetic field irradiation. In vitro cytotoxic studies have demonstrated that heat generation/drug release and only drug release from the balloon kill approximately 99% and 60% of human osteosarcoma cells, respectively. The proposed SMB is promising in postoperative local thermo/chemotherapy for bone tumors.

Macromolecular Brushes Based on Poly(L-Lactide) and Poly(ε-Caprolactone) Single and Double Macromonomers via ROMP. Synthesis, Characterization and Thermal Properties.

Single and double poly(L-lactide) (PLLA) and poly(ε-caprolactone) (PCL) macromonomers having a norbornenyl polymerizable group were prepared by conventional Ring Opening Polymerization (ROP). These macromonomers were further subjected to ring opening metathesis polymerization (ROMP) reactions in order to produce double polymer brushes consisting of PLLA or PCL side chains on a polynorbornene (PNBE) backbone. Statistical or block ring opening metathesis copolymerization of the PLLA and PCL macromonomers afforded the corresponding random and block double brushes. Sequential ROMP of the single PLLA, PCL and PLLA macromonomers resulted in the synthesis of the corresponding triblock copolymer brush. The molecular characteristics of the macromolecular brushes were obtained by 1H-NMR spectroscopy and Size Exclusion Chromatography. The thermal properties of the samples were studied by thermogravimetric analysis, TGA, Differential Thermogravimetry, DTG and Differential Scanning Calorimetry, DSC.

Reduction-responsive amphiphilic star copolymers with long-chain hyperbranched poly(ε-caprolactone) core and disulfide bonds for trigger release of anticancer drugs

In this contribution, the reduction-responsive star copolymers with long-chain hyperbranched poly(ε-caprolactone) (PCL) (HyperMacs) core and disulfide bonds were synthesized via Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. The HyperMacs core was constructed from disulfide-containing AB2-type PCL macromonomers, which possesses length-adjustable chain segments between branching points, large cavities, low degree of crystallinity, and reduction-responsivity. After grafted with poly(ethylene glycol), the reduction-responsive star copolymers can self-assemble into micelles in aqueous solution. The obtained micelles exhibited much lower critical micelle concentration (CMC) than their linear analogues. The reduction-responsivity from disulfide bonds makes them a promising carrier candidate for trigger release of anticancer drugs. The in vitro release results confirmed that their doxorubicin (DOX)-loaded micelles exhibited desirable reduction-triggered release performance. The cellular proliferation inhibition against HepG2 cells demonstrated that the DOX-loaded micelles showed a comparable anticancer activity with free DOX. Therefore, it can be expected that the reduction-sensitive micelles may serve as smart vehicles for intracellular delivery of anti-cancer drugs in tumour therapy.

Magnetic-Responsive Microparticles that Switch Shape at 37 °C

Shape-memory polymers have seen tremendous research efforts driven by the need for better drug carries and biomedical devices. In contrast to these advancements, fabrication of shape-memory particles which actuate at body temperature remains scarce. We developed a shape-memory microparticle system with dynamically tunable shapes under physiological temperature. Temperature-responsive poly(ε-caprolactone) (PCL) microparticles were successfully prepared by an in situ oil-in-water (o/w) emulsion polymerization technique using linear telechelic and tetra-branched PCL macromonomers. By optimizing the mixing ratios of branched PCL macromonomers, the crystal-amorphous transition temperature was adjusted to the biological relevant temperature. The particles with a disk-like temporal shape were achieved by compression. The shape recovery from the disk to spherical shape was also realized at 37 °C. We also incorporated magnetic nanoparticles within the PCL microparticles, which can be remote-controllable by a magnet, in such a way that they can be actuated and manipulated in a controlled way.

End-Group Evaluation of HEMA Initiated Poly(ε-caprolactone) Macromonomers via Enzymatic Ring-Opening Polymerization

Poly(ε-caprolactone) (PCL) macromonomers comprising acrylate end-functionality were synthesized via enzymatic ring-opening polymerization (eROP) by utilizing commercially availableCandida antarcticaLipase B (CALB), Novozyme-435. 2-Hydroxyethyl methacrylate (HEMA) was purposed to be the nucleophilic initiator in eROP. The side reactions generated due to the cleavage of ester bonds in HEMA and the growing polymer chains were investigated through altering polymerization period, initiator concentration, temperature, and enzyme concentration.1H NMR evaluations showed that minimum quantities of side reactions were in lower temperatures, initiator concentration, enzyme concentration, and lower monomer conversions. Gel permeation chromatography (GPC) results revealed that lower polydispersity along with number-average molecular weight of end-functionalized PCL macromonomers was obtained depending on higher initiator/monomer ratios, lower temperature (60°C), enzyme concentration (100 mg), and/or polymerization time (2 h). Furthermore, 0.1 HEMA/ε-caprolactone (CL) ratio had higher molecular weight than 0.5 HEMA/CL ratio, while keeping a close value of methacrylate transfer, total methacrylate end-groups, and lower polyester transfer.

The taming of the cell: shape-memory nanopatterns direct cell orientation.

We report here that the direction of aligned cells on nanopatterns can be tuned to a perpendicular direction without use of any biochemical reagents. This was enabled by shape-memory activation of nanopatterns that transition from a memorized temporal pattern to the original permanent pattern by heating. The thermally induced shape-memory nanopatterns were prepared by chemically crosslinking semi-crystalline poly(ε-caprolactone) (PCL) in a mold to show shape-memory effects over its melting temperature (Tm = 33°C). Permanent surface patterns were first generated by crosslinking the PCL macromonomers in a mold, and temporary surface patterns were then embossed onto the permanent patterns. The temporary surface patterns could be easily triggered to transition quickly to the permanent surface patterns by a 37°C heat treatment, while surface wettability was independent of temperature. To investigate the role of dynamic and reversible surface nanopatterns on cell alignment on the PCL films before and after a topographic transition, NIH 3T3 fibroblasts were seeded on fibronectin-coated PCL films with a temporary grooved topography (grooves with a height of 300 nm and width of 2 μm were spaced 9 μm apart). Interestingly, cells did not change their direction immediately after the surface transition. However, cell alignment was gradually lost with time, and finally cells realigned parallel to the permanent grooves that emerged. The addition of a cytoskeletal inhibitor prevented realignment. These results clearly indicate that cells can sense dynamic changes in the surrounding environments and spontaneously adapt to a new environment by remodeling their cytoskeleton. These findings will serve as the basis for new development of spatiotemporal tunable materials to direct cell fate.

Temperature-Responsive Poly(ε-caprolactone) Cell Culture Platform with Dynamically Tunable Nano-Roughness and Elasticity for Control of Myoblast Morphology

We developed a dynamic cell culture platform with dynamically tunable nano-roughness and elasticity. Temperature-responsive poly(ɛ-caprolactone) (PCL) films were successfully prepared by crosslinking linear and tetra-branched PCL macromonomers. By optimizing the mixing ratios, the crystal-amorphous transition temperature (Tm) of the crosslinked film was adjusted to the biological relevant temperature (~33 °C). While the crosslinked films are relatively stiff (50 MPa) below the Tm, they suddenly become soft (1 MPa) above the Tm. Correspondingly, roughness of the surface was decreased from 63.4–12.4 nm. It is noted that the surface wettability was independent of temperature. To investigate the role of dynamic surface roughness and elasticity on cell adhesion, cells were seeded on PCL films at 32 °C. Interestingly, spread myoblasts on the film became rounded when temperature was suddenly increased to 37 °C, while significant changes in cell morphology were not observed for fibroblasts. These results indicate that cells can sense dynamic changes in the surrounding environment but the sensitivity depends on cell types.

Synthesis and polymerization of cyclobutenyl-functionalized polylactide and polycaprolactone: a consecutive ROP/ROMP route towards poly(1,4-butadiene)-g-polyesters

Efficient preparation of cyclobutenyl end-functionalized polyester macromonomers bearing polylactide (PLA) or poly(ε-caprolactone) (PCL) arms was achieved by organocatalyzed ring-opening polymerization (ROP) of L-lactide or ε-caprolactone in the presence of cis-3,4-bis(hydroxymethyl)cyclobutene or cis-4-benzyloxymethyl-3-hydroxymethylcyclobutene acting as an initiator. Cyclobutenyl end-functionalized PLA and PCL macromonomers having one or two arms were obtained in high yields with excellent control over molecular weights (up to 11 000 g mol−1) and dispersity (PDI < 1.25) by organocatalyzed ROP using 4-(N,N-dimethylamino)pyridine (DMAP) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), respectively. Ring-opening metathesis polymerization (ROMP) of the macromonomers using ruthenium-based Grubbs' second generation catalyst afforded well-defined polybutadiene-g-polyester copolymers having an exclusively linear polybutadiene backbone with a strictly 1,4-type microstructure, with molecular weights ranging from 20 000 to 170 000 g mol−1 and low dispersities (PDI ≤ 1.30). The products resulting from this consecutive ROP/ROMP route represent the first examples of poly(1,4-butadiene)-g-polyesters through the macromonomer route.

Rewritable and shape-memory soft matter with dynamically tunable microchannel geometry in a biological temperature range

Here we present on-demand switchable microchip materials that display potent rewritable and shape-memory properties which are shown to contribute to fluidic control as pumps and valves. Semi-crystalline poly(e-caprolactone) (PCL) was chemically crosslinked to show shape-memory effects over its melting temperature (Tm) because the crosslinking points set the permanent shape and the crystalline domains serve as thermally reversible mobile phase. The Tm was adjusted to nearly biologically relevant temperatures by crosslinking two and four branched PCL macromonomers with different ratios. The Tm decreased proportionally with increasing four branched PCL content because an increase in crosslinking density imposes restrictions on chain mobility and reduces the crystallization. The sample with 50/50 wt% mixing ratio of two- and four-PCL had a Tm around 33 °C. Permanent surface patterns were first generated by crosslinking the macromonomers in a mold, and temporary surface patterns were then embossed onto the permanent patterns. From the cross-sectional profiles, almost 100% recovery of the permanent pattern was successfully achieved after shape-memory transition. The effects of dynamic geometric changes of the shape-memory channels on the microfluidic flow were also investigated and shape-memory channel closing was achieved by the application of heat. The proposed system can be potentially applied as a new class of microfluidic control techniques, which enables portable microfluidic based diagnostic tools for biomedical applications and environmental monitoring allowing on-site analysis.

Poly(vinyl pyrrolidone‐co‐vinyl acetate)‐graft‐poly(ε‐caprolactone) as a compatibilizer for cellulose acetate/poly(ε‐caprolactone) blends

AbstractPoly(vinyl pyrrolidone‐co‐vinyl acetate)‐graft‐poly(ε‐caprolactone) (PVPVAc‐g‐PCL) was synthesized by radical copolymerization of N‐vinyl‐2‐pyrrolidone (VP)/vinyl acetate (VAc) comonomer and PCL macromonomer containing a reactive 2‐hydroxyethyl methacrylate terminal. The graft copolymer was designed in order to improve the interfacial adhesiveness of an immiscible blend system composed of cellulose acetate/poly(ε‐caprolactone) (CA/PCL). Adequate selections of preparation conditions led to successful acquisition of a series of graft copolymer samples with different values of molecular weight ( $M_{n}^{{\rm PVPVA} {{\rm c} - g - {\rm PCL}}}$), number of grafts (n), and segmental molecular weight of PVPVAc between adjacent grafts (Mn (between grafts)). Differential scanning calorimetry measurements gave a still immiscible indication for all of the ternary blends of CA/PCL/PVPVAc‐g‐PCL (72 : 18 : 10 in weight) that were prepared by using any of the copolymer samples as a compatibilizer. However, the incorporation enabled the CA/PCL (4 : 1) blend to be easily melt‐molded to give a visually homogeneous film sheet. This compatibilizing effect was found to be drastically enhanced when PVPVAc‐g‐PCLs of higher $M_{ n}^{{\rm PVPVA}{\rm c} - g - {\rm PCL}}$ and Mn (between grafts) and lower n were employed. Scanning electron microscopy revealed that a uniform dispersion of the respective ingredients in the ternary blends was attainable with an assurance of the mixing scale of several hundreds of nanometers. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

New Amphiphilic Glycopolymers Based on a Polycaprolactone‐maleic anhydride Copolymer Backbone: Characterization by 15N NMR and Application to Colloidal Stabilization of Nanoparticles

Abstract8‐Amino‐3,6‐dioxaoctyl α‐D‐mannopyranoside and 8‐amino‐3,6‐dioxaoctyl β‐D‐galactopyranoside were prepared for coupling with a copolymer made from PCL macromonomers and MA. The new glycosyl compounds as well as PCL macromonomers were synthesized by appropriate techniques and fully characterized. The grafting of the latter with the functionalized copolymer was ascertained by 15N NMR experiments. The glycopolymers thus obtained were used in conjunction with Pluronic® F‐68 for the stabilization of PCL nanoparticles prepared by the emulsification‐diffusion technique. The stabilization of these nanoparticles upon storage and after centrifugation was investigated as well.magnified image

Amphiphilic polymer brushes with alternating PCL and PEO grafts through radical copolymerization of styrenic and maleimidic macromonomers

Synthesis of a novel amphiphilic polymer brush with a sequence of alternating poly(ɛ-caprolactone) (PCL) and poly(ethylene oxide) (PEO) side chains through grafting-through approach is reported. This strategy consists of two steps: first, a PCL macromonomer carrying maleimide functional group (MI-PCL) and a PEO macromonomer with vinylbenzyl group (St-PEO) were prepared; then a conventional radical copolymerization of the MI-PCL and the St-PEO macromonomers was carried out to obtain the proto-type polymer brush. The reactivity ratios of two macromonomers were studied and the alternating sequence structure was proved.

Structure and Properties of Poly(ε‐caprolactone) Networks with Modulated Water Uptake

AbstractSummary: A PCL macromonomer was obtained by the reaction of PCL diol with methacrylic anhydride. The effective incorporation of the polymerizable end groups was assessed by FT‐IR and 1H NMR spectroscopy. PCL networks were then prepared by photopolymerization of the PCL macromonomer. Furthermore, the macromonomer was copolymerized with HEA, with the aim of tailoring the hydrophilicity of the system. A set of hydrophilic semicrystalline copolymer networks were obtained. The phase microstructure of the new system and the network architecture was investigated by DSC, IR, DMS, TG, dielectric spectroscopy and water sorption studies. The presence of the hydrophilic units in the system prevented PCL crystallization on cooling; yet there was no effect on the glass transition process. The copolymer networks showed microphase separation and the α relaxation of the HEA units moved to lower temperatures as the amount of PCL in the system increased.Ideal structure, compatible with the experimental results, for the hydrophilized poly(ε‐caprolactone) networks with modulated water uptake.magnified imageIdeal structure, compatible with the experimental results, for the hydrophilized poly(ε‐caprolactone) networks with modulated water uptake.

Synthesis and characterization of naphthoxazine functional poly(ε-caprolactone)

A novel naphthoxazine ring-containing poly(ε-caprolactone) (PCL) was synthesized and characterized. For this purpose, first hydroxyl functional naphthoxazine, namely 2-(1 H-naphtho[1,2-e][1,3]oxazin-2-yl)-ethanol (N-a-OH) was prepared by the reaction of 2-naphthol, ethanolamine and paraformaldehyde either in bulk or in dioxane as solvent at 110 °C. Subsequently, N-a-OH was used as the coinitiator for the stannous-2-ethylhexanoate (Sn(Oct) 2) catalyzed living ring-opening polymerization of ε-caprolactone. The GPC, IR, 1H NMR, UV, and fluorescence spectroscopic studies revealed that low-polydispersity PCL with naphthoxazine functionality at the end of the chain was obtained. The resulting PCL macromonomer undergoes thermal curing in the presence of low molar mass benzoxazine (P-a) at various temperatures with the formation of thermosets having PCL segments.

Layered Morphology of Poly(phenylene)s in Thin Films Induced by Substitution of Well-Defined Poly(ε-caprolactone) Side Chains

Poly(e-caprolactone) (PCL)-based macromonomers and the corresponding substituted polyphenylene polymers have been synthesized in various chemical structures. The effect of chemical structure and the crystallization of PCL on the resulting morphology in thin films have been investigated. PCL macromonomers containing the central 2,5-dibromo-1,4-phenylene moiety were synthesized by ring-opening polymerization (ROP). Poly(phenylene) polymers were then synthesized by cross-coupling of the PCL macromonomers in Yamamato polycondensation reactions. Thin films of macromonomers and polymers were prepared by spin-coating on silicon substrates, and the resulting morphology in thin films was characterized by atomic force microscopy (AFM). Substitution of semicrystalline PCL side chains to the rigid poly(phenylene) backbone induced layered morphology in thin films. Our results indicate that increasing backbone rigidity causes grafted PCL side blocks to crystallize in better-defined layered structures parallel to the un...

Synthesis of Amphiphilic Poly((R,S)-β-malic acid)-graft-poly(ε-caprolactone): “Grafting From” and “Grafting Through” Approaches

The synthesis of novel amphiphilic graft copolyesters based on hydrophobic poly(ε-caprolactone) (PCL) segments grafted all along a hydrophilic poly((R,S)-β-malic acid) (PMLA) backbone is proposed. These PMLA-g-PCL graft copolymers have been prepared in a controlled way using either the “grafting through” or the “grafting from” technique. Both approaches involve two different well-controlled ring-opening polymerization (ROP) mechanisms: anionic ROP of β-lactones, i.e., benzyl or allyl β-malolactonate (MLABz or MLAAllyl), and living ROP of ε-caprolactone (CL) through a so-called coordination−insertion mechanism. The grafting through approach relies upon the anionic copolymerization of MLABz and ω-malolactonate−PCL macromonomer as initiated by potassium carboxylic acid salt in the presence of 1 equiv of 18-crown-6 ether. The second route follows a four-step synthesis involving the anionic ROP of MLABz and MLAAllyl, the conversion of allylic groups into pendant hydroxyl functions owing to a free-radical reaction with thioethanol in the presence of 2,2‘-azobis(2-methylpropionitrile), and then the CL polymerization as initiated from these pendant −OH groups after adequate activation into aluminum alkoxide active species. Whatever the synthetic approach, the final step consists of the deprotection of the MLABz repeating units along the backbone via a soft catalytic hydrogenation reaction. The amphiphilic character of the so-prepared graft copolyesters has been evidenced by some preliminary interfacial tension experiments using the pendant drop method.

Synthesis and characterization of poly(p‐phenylene)‐graft‐poly(ε‐caprolactone) copolymers by combined ring‐opening polymerization and cross‐coupling processes

Abstract2,5‐Dibromo‐1,4‐(dihydroxymethyl)benzene was used as initiator in ring‐opening polymerization of ε‐caprolactone in the presence of stannous octoate (Sn(Oct)2) catalyst. The resulting poly(ε‐caprolactone) (PCL) macromonomer, with a central 2,5‐dibromo‐1,4‐diphenylene group, was used in combination with 1,4‐dibromo‐2,5‐dimethylbenzene for a Suzuki coupling in the presence of Pd(PPh3)4 as catalyst or using the system NiCl2/bpy/PPh3/Zn for a Yamamoto‐type polymerization. The poly(p‐phenylenes) (PPP) obtained, with PCL side chains, have solubility properties similar to those of the starting macromonomer, ie soluble in common organic solvents at room temperature. The new polymers were characterized by 1H and 13C NMR and UV spectroscopy and also by GPC measurements. The thermal behaviour of the precursor PCL macromonomer and the final poly(p‐phenylene)‐graft‐poly(ε‐caprolactone) copolymers were investigated by thermogravimetric analysis and differential scanning calorimetry analyses and compared. Copyright © 2004 Society of Chemical Industry

Silyl ether-coupled poly(epsilon-caprolactone)s with stepwise hydrolytic degradation profiles.

Silyl ether-coupled poly(epsilon-caprolactone)s (PCLs) with stepwise degradation profiles were synthesized via the cross-dehydrocoupling polymerizations between 1,4-bis(dimethylsilyl)benzene (BDSB) and telechelic, diol-terminated PCL macromonomers. With the presence of 10 wt % palladium on activated carbon as the catalyst, the condensations between BDSB and diol-terminated PCL macromonomers having molecular weights of 1200, 2010, and 5500 g/mol were performed in toluene at 100 degrees C under argon. Hydrogen was eliminated as the condensate upon the formation of silyl ether bonds linking the PCL blocks, yielding within 24 h, silyl ether-coupled PCLs of molecular mass 7590, 29,900, and 29,500 g/mol, respectively. The characterization of each polymer included (1)H NMR, (13)C NMR, and (29)Si NMR spectroscopies, size exclusion chromatography (SEC), and differential scanning calorimetry. The hydrolytic degradation properties of the polymers in solution were studied, and the molecular weight reductions over time were monitored by SEC. The silyl ether linkages of the polymers underwent hydrolysis in the presence of mineral acids, whereas the PCL segments released from the cleavage of the labile silyl ether coupling unit did not undergo detectable molecular weight reduction over 15 days. In the presence of acetic acid, the silyl ether functionalities were cleaved with a half-life of 3 days; however, the PCL chain required reaction with trifluoroacetic acid to give a number-average molecular weight loss half-life of 4 days. The silyl ether-coupled PCLs underwent degradation in a gradient fashion, therefore, by a protocol that involved the addition of acetic acid for cleavage of the silyl ether functionalities, followed by further addition of trifluoroacetic acid to bring the hydrolysis of the silyl ether functionalities to completion and to trigger the degradation of PCL segments.

Emulsion-derived foams (PolyHIPEs) containing poly(epsilon-caprolactone) as matrixes for tissue engineering.

The preparation of PolyHIPE foams containing poly(epsilon-caprolactone) from macromonomers by free radical homo- or copolymerization is described. The macromonomers are synthesized from PCL diols and are polymerized in the continuous phase of high internal phase emulsions (HIPEs). Subsequent drying yields low-density foams with cell diameters of 5-100 microm. Foam morphology, as determined by scanning electron microscopy, depends on the type of diluent (styrene, methyl methacrylate, or toluene) added to the emulsion organic phase and on the PCL content. Increasing the latter increases the continuous phase viscosity to a point where emulsion formation is impeded. Foam swelling in toluene, 2-propanol, and water was investigated by solvent imbibition and increased with increasing solvent hydrophobicity. Furthermore, it was found generally to decrease with increasing PCL content, due to increasing cross-link density. Swelling generally increased when higher molar mass PCL macromonomer was used due to the formation of a less tightly cross-linked network. One type of foam sample was shown to support the growth of human fibroblasts over a period of 2.5 days.

Ring-opening metathesis polymerization of new ?-norbornenyl poly(?-caprolactone) macromonomers

Poly(ε-caprolactone) (PCL) macromonomers capped by a polymerizable norbornene end-group have been synthesized and (co)polymerized by ring-opening metathesis with formation of graft copolymers and polymacromonomers. α-Norbornenyl PCL macromonomers have been synthesized by ring opening polymerization (ROP) of ε-caprolactone (εCL) initiated by 2-diethylaluminoxymethyl-5-norbornene. Copolymerization of these PCL macromonomers with norbornene and polymerizable derivatives has been catalyzed by the [RuCl2(p-cymene)]2 PCy3/(trimethylsilyl)diazomethane complex yielding a series of poly(norbornene)-graft-poly(ε-caprolactone) copolymers. These new graft copolymers have been characterized by a set of analytical methods, i.e., SEC, 1H-NMR, FTIR, DSC, and TGA. Furthermore, PCL macromonomers have been polymerized into high molecular weight comb chains of narrow molecular weight distribution (Mw/Mn = 1.10) within high yields (90%). © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2447–2455, 1999