Living cationic polymerization of n‐butyl propenyl ether

Cationic polymerization of n‐butyl propenyl ether (BuPE; CH₃CH CHOBu, cis/trans = 64/36) was examined with the HCl–IBVE (isobutyl vinyl ether) adduct/ZnCl₂ initiating system at −15 ∼ −78 °C in nonpolar (hexane, toluene) and polar (dichloromethane) solvents, specifically focusing on the feasibility of its living polymerization. In contrast to alkyl vinyl ethers, the living nature of the growing species in the BuPE polymerization was sensitive to polymerization temperature and solvent. For example, living cationic polymerization of IBVE can be achieved even at 0 °C with HCl–IBVE/ZnCl₂, whereas for BuPE whose β‐methyl group may cause steric hindrance ideal living polymerization occurred only at −78 °C. Another interesting feature of this polymerization is that the polymerization rate in hexane is as large as in dichloromethane, much larger than in toluene. A new method in determining the ratio of the living growing ends to the deactivated ones was developed with a devised monomer‐addition experiments, in which IBVE that can be polymerized in a living fashion below 0 °C was added to the almost completely polymerized solution of BuPE. The amount of the deactivated chain ends became small in hexane even at −40 °C in contrast to other solvents. Thus hexane turned out an excellent solvent for living cationic polymerization of BuPE. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 229–236, 2000     

Ring‐opening polymerization of 1,3‐benzoxazines by p‐toluenesulfonates as thermally latent initiators

p‐Toluenesulfonic acid (TsOH) and several alkyl p‐toluenesulfonates, that is, methyl p‐toluenesulfonate (TsOMe), cyclohexyl p‐toluenesulfonate (TsOCH), and neopentyl p‐toluenesulfonate (TsONP), were evaluated as initiators for the ring‐opening polymerization of benzoxazines. TsOH and TsOMe were highly efficient initiators that induced the polymerization at 60 and 80 °C, respectively. In contrast, TsOCH and TsONP did not initiate the polymerization below 100 °C, while they induced the polymerization at elevated temperatures, 120 and 150 °C, respectively. When TsOCH was used as an initiator, the corresponding polymerization rate was comparable to that observed for the polymerization with using TsOH as an initiator. These results suggested that neutral TsOCH and TsONP can be regarded as “thermally latent initiators,” which underwent the thermal dissociation at the elevated temperatures to generate the corresponding alkyl cations and/or TsOH as the initiators of the polymerization. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Living cationic polymerization of amide‐functional vinyl ethers: Specific properties of SnCl4‐based initiating system

Living cationic copolymerization of amide‐functional vinyl ethers with isobutyl vinyl ether (IBVE) was achieved using SnCl₄ in the presence of ethyl acetate at 0 °C: the number–average molecular weight of the obtained polymers increased in direct proportion to the monomer conversion with relatively low polydispersity, and the amide‐functional monomer units were introduced almost quantitatively. To optimize the reaction conditions, cationic polymerization of IBVE in the presence of amide compounds, as a model reaction, was also examined using various Lewis acids in dichloromethane. The combination of SnCl₄ and ethyl acetate induced living cationic polymerization of IBVE at 0 °C when an amide compound, whose nitrogen is adjacent to a phenyl group, was used. The versatile performance of SnCl₄ especially for achieving living cationic polymerization of various polar functional monomers was demonstrated in this study as well as in our previous studies. Thus, the specific properties of the SnCl₄ initiating system are discussed by comparing with the Et_xAlCl_3?x systems from viewpoints of hard and soft acids and bases principle and computational chemistry. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6129–6141, 2008     

Reactions of vinyl ethers with 2‐cyano‐2‐propyl and benzoyloxy radicals

tert‐Butyl, cyclohexyl, n‐propyl, and n‐dodecyl vinyl ethers have been used as comonomers with styrene and methyl methacrylate using ¹³C‐enriched samples of azobis(isobutyronitrile) and benzoyl peroxide as initiators at 60°C. Examination by ¹³C‐NMR spectroscopy of either (¹³CH₃)₂C(CN) or Ph¹³COO end‐groups in the products has shown that the vinyl ethers have low reactivities toward the 2‐cyano‐2‐propyl radical but high reactivities toward the benzoyloxy radical. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 771–777, 1999     

Polymerization of o‐quinodimethanes. V. Ionic polymerization of o‐quinodimethanes generated by thermal isomerization of corresponding benzocyclobutenes

The ionic polymerization of substituted o‐quinodimethanes via thermal isomerization of benzocyclobutenes is described. In the cationic polymerizations of 1‐methoxy‐o‐quinodimethane in the presence of various cationic initiators at 110 °C for 12 h, chain transfer reactions also considerably underwent besides the polymerization. Meanwhile, cationic polymerizations of 1‐trimethylsilyloxy‐o‐quinodimethane under the same conditions gave good yields of the corresponding polymer. Anionic polymerizations of 1‐cyano‐o‐quinodimethane in the presence of anionic initiators such as n‐BuLi or t‐BuOK were performed at various temperatures for 12 h. Good yields of hexane‐insoluble polymer, which was produced by anionic polymerization of corresponding o‐quinodimethane as an intermediate, were obtained above 120 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 844–850, 2008     

Application of S‐alkylsulfonium salts of aromatic sulfides as new thermal latent cationic initiators

The cationic initiation activity of derivatives of S‐methylsulfonium salts of dibenzothiophene ( 3a ), diphenyl sulfide ( 4a ), thioanisole ( 4d ), and tetrahydrothiophene ( 5 ) was evaluated in the polymerization of glycidyl phenyl ether ( 1 ). These initiators were soluble in 1 and capable of initiating the cationic polymerization of 1 on heating, with the exception of methyltetrahydrothiophenium tetrafluoroborate ( 5 ; in the range of room temperature to 160 °C). Among them, methyldiphenylsulfonium tetrafluoroborate ( 4a ) showed a moderate thermal latency that brought about the polymerization of 1 efficiently at 160 °C but not below 80 °C. S‐Alkylsulfonium salts of aromatic sulfides such as phenoxathiin ( 6a ) and thianthrene ( 6b ) also were evaluated for their activity in the cationic polymerization of 1 , from which the thermal latent behavior of these salts also was confirmed (i.e., there was no reaction at 60 °C for 3 h, but there was a high enough conversion at 140 °C). Furthermore, the catalytic activity of S‐alkylsulfonium derivatives was controllable by both the property of the substituents on the aromatic rings and the character of the alkyl groups on the sulfur atom. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 18–27, 2000     

Ring opening polymerization of epoxides with urea‐derivatives of 4‐aminopyridine as thermally latent anionic initiator

In this study, a series of urea‐derivatives of 4‐aminopyridine (4AP) were evaluated as thermally latent initiators for the anionic ring‐opening polymerization of diglycidyl ether of bisphenol A (DGEBA). The urea‐derivatives were synthesized by the reactions of 4AP with the corresponding iso(thio)cyanates (phenyl isocyanate, tert‐butyl isocyanate, methylene diphenyl diisocyanate, and phenyl isothiocyanate). The ability of the urea‐derivatives as latent initiators was investigated with differential scanning calorimetry (DSC): Upon heating formulations comprised of DGEBA and the urea‐derivatives in a heating rate at 10 °C/min, the resulting DSC profiles indicated exothermic peaks to confirm that DGEBA underwent the polymerization efficiently. The corresponding DSC‐peak top temperatures (T_{peak top}) was higher than that observed for the formulation comprised of DGEBA and pristine 4AP, to clarify that the urea are useful initiators with thermal latency. A possible mechanism for the initiation step involves the thermal dissociation of the urea into 4AP and the corresponding isocyanates. 4AP thus generated readily initiated the ring‐opening polymerization of epoxide. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2518–2522     

Initiator effect on the cationic ring‐opening copolymerization of 2‐ethyl‐2‐oxazoline and 2‐phenyl‐2‐oxazoline

The aim of this research was to study the effect of the initiator on the resulting monomer distribution for the cationic ring‐opening copolymerization of 2‐ethyl‐2‐oxazoline (EtOx) and 2‐phenyl‐2‐oxazoline (PhOx). At first, kinetic studies were performed for the homopolymerizations of both monomers at 160 °C under microwave irradiation using four initiators. These initiators have the same benzyl‐initiating group but different leaving groups, Cl^?, Br^?, I^?, and OTs^?. The basicity of the leaving group affects the ratio of covalent and cationic propagating species and, thus, the polymerization rate. The observed differences in polymerization rates could be correlated to the concentration of cationic species in the polymerization mixture as determined by ¹H NMR spectroscopy. In a next‐step, polymerization kinetics were determined for the copolymerizations of EtOx and PhOx with these four initiators. The reactivity ratios for these copolymerizations were calculated from the polymerization rates obtained for the copolymerizations. This approach allows more accurate determination of the copolymerization parameters compared to conventional methods using the composition of single polymers. When benzyl chloride (BCl) was used as an initiator, no copolymers could be obtained because its reactivity is too low for the polymerization of PhOx. With decreasing basicity of the used counterions (Br^? > I^? > OTs^?), the reactivity ratios gradually changed from r_EtOx = 10.1 and r_PhOx = 0.30 to r_EtOx = 7.9 and r_PhOx = 0.18. However, the large difference in reactivity ratios will lead to the formation of quasi‐diblock copolymers in all cases. In conclusion, the used initiator does influence the monomer distribution in the copolymers, but for the investigated system the differences were so small that no difference in the resulting polymer properties is expected. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4804–4816, 2008     

Cationic polymerization of seven‐membered cyclic monothiocarbonate 1,3‐dioxepan‐2‐thione

The cationic ring‐opening polymerization of a seven‐membered cyclic monothiocarbonate, 1,3‐dioxepan‐2‐thione, produced a soluble polymer through the selective isomerization of thiocarbonyl to a carbonyl group {? [SC(C?O)O(CH₂)₄]_n? }. The molecular weights of the polymer could be controlled by the feed ratio of the monomer to the initiators or the conversion of the monomer during the polymerization, although some termination reactions occurred after the complete consumption of the monomer. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1014–1018, 2005     

Matrix‐assisted laser desorption ionization time of flight mass spectrometry analysis of living cationic polymerization of vinyl ethers. I. Optimization of measurement conditions for poly(isobutyl vinyl ether)

Matrix‐assisted laser desorption ionization time of flight mass spectrometry (MALDI‐TOF‐MS) was utilized for the analysis of polymers obtained by the living cationic polymerization of isobutyl vinyl ether (IBVE) with the HCl‐VE adduct/SnCl₄/n‐Bu₄NCl initiating system in CH₂Cl₂ at −78 °C. Under optimized analysis conditions, well‐resolved spectra were obtained for samples with number‐average molecular weights of ≤10⁴ with the use of 1,8‐dihydroxy‐9(10H)‐anthracenone (dithranol) as a matrix and sodium trifluoroacetate as an added salt. The MS spectra showed only one series of peaks separated exactly by the mass of the IBVE. The observed mass of each peak was in good agreement with the theoretical one, which possesses one initiator fragment at the α end and one methoxy group originated from quenching with methanol at the ω end. Thus, detailed end group analysis is possible for poly(VE). © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4023–4031, 2000     

Cationic addition polymerization of 1,4‐dioxene using gacl3 as lewis acid catalyst: Long‐lived species‐mediated polymerization

Effective cationic addition polymerization of 1,4‐dioxene, a six‐membered cyclic olefin with two oxygen atoms adjacent to the double bond, was performed using a simple metal halide catalyst system in dichloromethane. The polymerization was controlled when the reaction was conducted using GaCl₃ in conjunction with an isobutyl vinyl ether–HCl adduct as a cationogen at –78°C to give polymers with predetermined molecular weights and relatively narrow molecular weight distributions. The long‐lived properties of the propagating species were further confirmed by a monomer addition experiment and the analyses of the product polymers by ¹H NMR and MALDI–TOF–MS. Although highly clean propagation proceeded, the apparent rate constant changed during the controlled cationic polymerization of 1,4‐dioxene. The reason for the change was discussed based on polymerization results under various conditions. The obtained poly(1,4‐dioxene) exhibited a very high glass transition temperature (T_g) of 217°C and unique solubility. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Synthesis and characterization of well‐defined [polystyrene‐b‐poly(2‐vinylpyridine)]n star‐block copolymers with poly(2‐vinylpyridine) corona blocks

This article describes the synthesis and characterization of [polystyrene‐b‐poly(2‐vinylpyridine)]_n star‐block copolymers with the poly(2‐vinylpyridine) blocks at the periphery. A two‐step living anionic polymerization method was used. Firstly, oligo(styryl)lithium grafted poly(divinylbenzene) cores were used as multifunctional initiators to initiate living anionic polymerization of styrene in benzene at room temperature. Secondly, vinylpyridine was polymerized at the periphery of these living (polystyrene)_n stars in tetrahydrofuran at ?78 °C. The resulting copolymers were characterized using size exclusion chromatography, multiangle laser light scattering, ¹H NMR, elemental analysis, and intrinsic viscosity measurements. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3949–3955, 2007     

Synthesis and radical polymerization properties of thermal radical initiators based on o‐imino‐isourea: The effect of the alkyl side chain on the radical initiation temperature

Four types of thermal radical initiators (TRIs) that are based on o‐imino‐isourea with cyclohexyl and isopropyl groups were successfully synthesized, namely, C‐HexDCC, DiiprDCC, C‐HexDIC, and DiiprDIC. The free radical polymerization and thermal properties of those synthesized TRIs were determined via differential scanning calorimetry (DSC) (using n‐butyl acrylate) and thermogravimetric analysis (TGA), respectively. The TRI derivatives showed peak temperatures (T_max) from 89 to 97 °C in n‐butyl acrylate, and DiiprDIC, with isopropyl groups on both sides of the N O group, showed the lowest peak temperatures. The rates of N O bond homolysis (k_d) of all the TRIs were calculated from their half‐lives determined using real‐time nuclear magnetic resonance (NMR) spectroscopy, and their theoretical bond dissociation energies (BDEs) were calculated using density functional theory (DFT) calculations. The free radical polymerization of n‐butyl acrylate using each TRI was efficiently determined from T_peak of the DSC curves; conversions depending on polymerization temperature (80, 90, and 100 °C) were monitored to observe kinetic information of TRIs during polymerization. Furthermore, to investigate the use of TRIs in curing, we applied them to an automotive clear coating system and monitored the real‐time evolution of the elastic modulus (G′) during thermal curing using a rheometer for representative DiiprDIC. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1749–1756     

Thermal radical initiator derivatives based on O‐imino‐isourea: Synthesis,polymerization, and characterization

The new thermal radical initiators (TRIs) with linear and cyclic type groups based on derivatives of O‐imino‐isourea have been designed and synthesized. The radical polymerization property of the synthesized TRI derivatives as a radical initiator in n‐butyl acrylate was monitored by differential scanning calorimetry analysis. TRI derivatives with linear type groups, such as 3‐PenDCC, 3‐HexDCC, and 4‐HepDCC, showed peak temperatures (T_peak) of 80–84 °C, whereas those with cyclic type groups, such as C‐PenDCC, C‐HexDCC, and C‐HepDCC, exhibited a wide T_peak distribution in the 74–87 °C range. The polymerization efficiency using new TRIs in n‐butyl acrylate was elaborately identified from the molecular weights and conversion obtained using gel permeation chromatography analysis and NMR spectroscopy. To consider their possible application to automotive clearcoats, the real‐time evolution of the rheological properties of clearcoat resins during the crosslinking process with newly synthesized TRI derivatives was measured, confirming the different crosslinking kinetics of TRI derivatives in real thermal curing process. The results were found to be well correlated with data from the radical polymerization experiments of TRIs. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3593–3600.     

Synthesis of novel tetrahydrofuran‐epichlorohydrin [poly(THF‐b‐ECH)] macromonomeric peroxy initiators by cationic copolymerization and the quantum chemically investigation of initiation system effects

Cationic polymerization of tetrahydrofuran (THF) and epichlorohydrin (ECH) was performed with peroxy initiators synthesized from bis (4,4′‐bromomethyl benzoyl peroxide (BBP) or bromomethyl benzoyl t‐butyl peroxy ester (t‐BuBP) and AgSbF₆ or ZnCl₂ system at 0 °C to obtain the poly(THF‐b‐ECH) macromonomeric peroxy initiators. Kinetic studies were accomplished for poly(THF‐b‐ECH) initiators. Poly(THF‐b‐ECH‐b‐MMA) and poly(THF‐b‐ECH‐b‐S) block copolymers were synthesized by bulk polymerization of methyl methacrylate (MMA) and styrene (S) with poly(THF‐b‐ECH) initiators. The quantum chemical calculations for the block copolymers, the initiating systems of the cationic polymerization of THF and ECH were achieved using HYPERCHEM 7.5 program. The optimized geometries of the polymers were investigated with the quantum chemical calculations. Poly(THF‐b‐ECH) initiators having peroxygen groups were used for graft copolymerization of polybutadien (PBd) to obtain poly(THF‐b‐ECH‐g‐PBd) crosslinked graft copolymers. The graft copolymers were investigated by sol‐gel analysis. Swelling ratio values of the graft copolymers in CHCl₃ were calculated. The characterizations of the polymers were achieved by FTIR, ¹H NMR, GPC, SEM, TEM, and DSC techniques. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2896–2909, 2010     

Substituent effect on the cationic ring‐opening polymerization of acyloxymethyl five‐member cyclic dithiocarbonates

Five‐member cyclic dithiocarbonates were synthesized by the reactions of carbon disulfide with benzoic, p‐anisic, p‐chlorobenzoic, 1‐naphthalenecarboxylic, p‐nitrobenzoic, and p‐(tert‐butyl)benzoic glycidyl esters, and their cationic ring‐opening polymerizations were carried out with methyl trifluoromethane sulfonate and trifluoromethane sulfonic acid as initiators at room temperature to 80 °C. Polymers with number‐average molecular weights of 3400–24,900 were obtained in high yields, and their structures were estimated by NMR and IR spectroscopy. The monomers showed a clear difference in the polymerization rate according to the substituents. The rate of polymerization decreased in the order of p‐chlorobenzoic ≥ benzoic > 1‐naphthalenecarboxylic > p‐nitro‐benzoic > p‐tert‐butylbenzoic > p‐anisic. The data of the reaction kinetics, NMR studies, and molecular orbital calculations proved a plausible mechanism involving the participation of p‐substituted benzoyloxymethyl groups to stabilize the cationic propagating end. The polymers showed decomposition temperatures with 5% weight loss ranging from 200 to 260 °C. No glass‐transition temperatures for the polymers were observed below 200 °C by differential scanning calorimetry. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3967–3980, 2001     

Free‐radical crosslinking copolymerization of 2,2′‐azobis[N‐(2‐propenyl)‐2‐methylpropionamide] with vinyl benzoate resulting in polymeric azo initiators

2,2′‐Azobis[N‐(2‐propenyl)‐2‐methylpropionamide] (APMPA) with two carbon–carbon double bonds and an azo group was copolymerized with vinyl benzoate (VBz) at 60 °C, resulting in azo groups containing VBz/APMPA prepolymers and crosslinked polymers as soluble and insoluble polymeric azo initiators, respectively. The polymerization characteristics of APMPA as a novel diallyl monomer were clarified with the rate and degree of polymerization and the monomer reactivity ratios. The gelation behaviors in VBz/APMPA crosslinking copolymerizations were examined in detail with a comparison of the actual gel point and the theoretical gel point calculated according to Stockmayer's equation with the tentative assumption of equal reactivity for both vinyl groups belonging to VBz and APMPA. The effectiveness of the resulting branched or crosslinked poly(VBz‐co‐APMPA)s as soluble or insoluble polymeric azo initiators, respectively, at providing graft polymers through the cleavage of azo groups at an elevated temperature was examined by the polymerization of allyl benzoate at 120 °C. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 317–325, 2002     

Anionic polymerization of N,N‐dialkyl‐2‐methylenebut‐3‐enamide: Effect of alkyl substituents on polymerization behavior

Anionic polymerizations of three 1,3‐butadiene derivatives containing different N,N‐dialkyl amide functions, N,N‐diisopropylamide (DiPA), piperidineamide (PiA), and cis‐2,6‐dimethylpiperidineamide (DMPA) were performed under various conditions, and their polymerization behavior was compared with that of N,N‐diethylamide analogue (DEA), which was previously reported. When polymerization of DiPA was performed at ?78 °C with potassium counter ion, only trace amounts of oligomers were formed, whereas polymers with a narrow molecular weight distribution were obtained in moderate yield when DiPA was polymerized at 0 °C in the presence of LiCl. Decrease in molecular weight and broadening of molecular weight distribution were observed when polymerization was performed at a higher temperature of 20 °C, presumably because of the effect of ceiling temperature. In the case of DMPA, no polymer was formed at 0 °C and polymers with relatively broad molecular weight distributions (M_w/M_n = 1.2) were obtained at 20 °C. The polymerization rate of PiA was much faster than that of the other monomers, and poly(PiA) was obtained in high yield even at ?78 °C in 24 h. The microstructure of the resulting polymers were exclusively 1,4‐ for poly(DMPA), whereas 20–30% of the 1,2‐structure was contained in poly(DiPA) and poly(PiA). © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3714–3721, 2010     

Syntheses and properties of organosoluble polyimides based on 5,5′‐bis[4‐(4‐aminophenoxy)phenyl]‐4, 7‐methanohexahydroindan

A series of organosoluble aromatic polyimides (PIs) was synthesized from 5,5′‐bis[4‐(4‐aminophenoxy)phenyl]‐4,7‐methanohexahydroindan (3) and commercial available aromatic dianhydrides such as 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA), 4,4′‐oxydiphthalic anhydride (ODPA), 4,4′‐sulfonyl diphthalic anhydride (SDPA), or 2,2′‐bis(3,4‐dicarboxyphenyl) hexafluoropropanic dianhydride (6FDA). PIs (III_c–f), which were synthesized by direct polymerization in m‐cresol, had inherent viscosities of 0.83–1.05 dL/g. These polymers could easily be dissolved in N,N′‐dimethylacetamide (DMAc), N‐methyl‐2‐pyrrolidone (NMP), N,N‐dimethylformamide (DMF), pyridine, m‐cresol, and dichloromethane. Whereas copolymerization was proceeded with equivalent molar ratios of pyromellitic dianhydride (PMDA)/6FDA, 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (BTDA)/6FDA, or BTDA/SDPA, or ½ for PMDA/SDPA, copolyimides (co‐PIs), derived from 3 and mixed dianhydrides, were soluble in NMP. All the soluble PIs could form transparent, flexible, and tough films, and they showed amorphous characteristics. These films had tensile strengths of 88–111 MPa, elongations at break of 5–10% and initial moduli of 2.01–2.67 GPa. The glass transition temperatures of these polymers were in the range of 252–311°C. Except for III_e, the 10% weight loss temperatures (T_d) of PIs were above 500°C, and the amount of carbonized residues of the PIs at 800°C in nitrogen atmosphere were above 50%. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1681–1691, 1999