Triethylaluminum Research Articles

A new chromium-based catalyst coated with paraffin for ethylene oligomerization and the effect of chromium state on oligomerization selectivity

A new method is proposed in which a chromium-based catalyst system containing chromium(III) tris (2-ethylhexanoate) (Cr(EH) 3), 2,5-dimethylpyrrole (2,5-DMP), GeCl 4 and triethylaluminum (TEA) coated with solid paraffin is used for oligomerization of ethylene. GC-MS and 1 H -NMR analyses show that the coated catalyst system prepared via this approach is air-insensitive so it can be stored under ambient conditions. The selectivity for 1-hexene is up to 97.91 wt.%, and only slightly decreases when the catalyst system is exposed in air for 24 h; in contrast, the selectivity is only 35.6% for its non-coated counterpart. XPS reveals that an attachment of atmospheric oxygen to catalyst makes the chemical state of chromium change from Cr 3+ to Cr 6+. Such a change, in turn, reduces remarkably its selectivity for 1-hexene.

Improving the Performance of Heterogeneous Borane Cocatalysts by Pretreatment of the Silica Support with Alkylaluminum Compounds

The treatment of silica with alkylaluminum compounds, triisobutylaluminum (TIBA) and triethylaluminum (TEA), dramatically enhanced the cocatalytic performances of the [CPh 3 ] + [B(C 6 F 5 ) 3 -SiO 2 ] - . We measured the productivity and ethylene-consumption profiles for [CPh 3 ] + [B(C 6 F 5 ) 3 -SiO 2 ] - cocatalysts and Cp 2 ZrCl 2 / TIBA. Both of the treated cocatalyst systems improved the average molecular weight of the product when compared with the untreated cocatalyst system. The TIBA-treated cocatalyst provided a narrow molecular weight distribution while the TEA-treated cocatalyst system gave a broad distribution.

Asymmetric catalytic carbon–carbon bond formations in a fluorous biphasic system based on perfluoroalkyl-BINOLs

Optically active 1,1′-binaphthols (BINOLS) substituted at the 4,4′; 6,6′ and 4,4′,6,6′ positions with perfluoroalkyl groups have been synthesized. Asymmetric diethylzinc and triethyl aluminum addition to aryl aldehydes in a fluorous biphasic system catalyzed by these perfluoroalkyl-BINOL–titanium complexes have been accomplished with good enantiomeric excess obtained.

Formation, deactivation and transformation of stereospecific active sites on TiCl 4/dibutylphthalate/Mg(OEt) 2 catalyst induced by short time reaction with Al-alkyl cocatalyst

Formation, deactivation and transformation of stereospecific active sites on TiCl 4/dibutylphthalate (DBP)/Mg(OEt) 2 Ziegler–Natta catalyst induced by short time reaction with triethylaluminum (TEA) cocatalyst (with TEA pretreatment time from 0 to 600 s) were investigated by stopped-flow propene polymerization combined with temperature rising elution fractionation (TREF) and GPC methods. It was demonstrated that both formation and deactivation of active sites with broad multiplicity in isospecificity on the catalyst are slow reactions with an induction period of ca. 0.2 s. It was most important to find that the formation of active sites with the highest isospecificity strongly depends on the interaction between the catalyst and cocatalyst (up to 60 s of pretreatment) even in the presence of internal donor. This newly observed phenomenon (according to our knowledge) suggested that the transformation of monometallic active sites (aspecific or less isospecific) into bimetallic active sites (highly isospecific) through reversible complexing with TEA cocatalyst (or its reaction product diethylaluminum chloride (DEAC)) in Ziegler–Natta catalysts cannot be overlooked even in the presence of internal electron donor. The existence of –OC 2H 5 ligand in the catalyst most probably gave birth to a new group of active titanium species. The stability of active sites increases with increasing isospecificity in the early stage of pretreatment (up to 60 s of pretreatment). While all the active sites became relatively stable in the later stage of pretreatment (from 60 to 600 s of pretreatment). The extraction of internal donor DBP by TEA from the catalyst within the pretreatment procedure is found to initiate from 60 s of pretreatment resulting in slight transformation of isospecific active sites into aspecific sites.

Copolymerization of ethylene/propylene elastomer using high‐activity Ziegler–Natta catalyst system of MgCl2 (ethoxide type)/EB/PDMS/TiCl4/PMT

AbstractA high‐activity Ziegler–Natta catalyst of TiCl4‐supported MgCl2 (ethoxide type) was prepared. Ethyl benzoate (EB) and polydimethylsiloxane (PDMS) were used as internal donors, while p‐methyl toluate (MPT) and triethylaluminum (TEA) were used as the external donor and cocatalyst, respectively. Suspension copolymerization of ethylene–propylene (EPM) was carried out in n‐heptane using the catalyst system. The relative pressures of 1.5 : 1 and 2 : 1 atmosphere of propylene‐to‐ethylene (PP/PE) gave a polymer with elastomeric properties. The relative pressure of propylene‐to‐ethylene higher and lower than the above values, however, gave a homopolymer or copolymer without elastomeric properties. The effect of the TEA concentration, H2 concentration, temperature, and pressure of PP/PE on the yield of the polymer obtained was investigated. The effects of these factors on the glass transition temperature (Tg) and ethylene content of the polymer were studied. The content of ethylene in EPM was determined using the FTIR technique. The molar ratio of Al : MPT : Ti = 355 : 107 : 1 gave a polymer with an ethylene content of about 32%. However, a ratio lower than AL : MPT : Ti = 355 : 107 : 1 did not give a good elastomeric polymer. The highest productivity of the catalyst was obtained at a molar ratio of AL : MPT : Ti = 355 : 107 : 1. Increasing the temperature from 45 to 70°C caused a slight increase of Et % content and a decrease of the Tg of the copolymer obtained, while the highest productivity of the catalyst was obtained at 55°C with good elastomeric properties using an AL : MPT : Ti = 497 : 107 : 1 molar ratio. Increasing the pressure of propylene from 1.5 to 2.5 atm caused a sharp decrease in the Tg and ethylene content of the polymer. When the ethylene content in the copolymer was increased, the Tg value had a tendency to be closer to the polyethylene Tg. © 2002 John Wiley & Sons, Inc. J Appl Polym Sci 84: 785–790, 2002; DOI 10.1002/app.10330

Investigations on ternary metallocene-based catalyst systems

The influence of the cocatalyst triisobutyl aluminium (TiBA) on the two step reaction of the ternary activation of the metallocene precursor Ph2C(CpFlu)ZrCl2 with TiBA and the activator N,N-dimethylanilinium-tetrakis(pentafluorophenyl)borat (DMAB) was investigated by means of NMR spectroscopy. More than 5 equivalents of TiBA are required for a total conversion of Ph2C(CpFlu)ZrCl2. The reaction exclusiveley leads to the monoalkyl complex Ph2C(CpFlu)ZrClBui, independent of the Al / Zr ratio. The reaction of the Ph2C(CpFlu)ZrCl2 / TiBA mixtures, using 10, 20 and 50 equivalents of TiBA with DMAB, generates two cationic metallocene species. Using 100 equivalents of TiBA exclusiveley leads to the formation of the cationic heterodinuclear metallocene complex [Ph2C(CpFlu)Zr-(μ-H)-(μ-C4H7)-AlBu2i]+. In no case was the degradation or complexation of [B(C6F5)4]− detected, whereas the direct reaction between TiBA and DMAB gives AlBui3−x(C6F5)x compounds. A mechanism was postulated to explain the formation of [Ph2C(CpFlu)Zr-(μ-H)-(μ-C4H7)-AlBui2]+. The catalyst system Ph2C(CpFlu)ZrCl2 / TiBA / DMAB was used for continuous high pressure polymerizations of ethylene at 150 MPa and 180°C. To investigate the influence of additional TiBA and triethyl aluminium (TEA) in the reactor, their concentrations were varied over a wide range. Highest productivities and molecular weights were obtained with low concentrations of TiBA in the reactor. Up to a concentration of 30 molppm Al in the reactor, unimodal polymers were formed with Mw / Mn between 2 and 3. With higher aluminium concentrations, the products formed contained small amounts of waxes. This was due to oligomerization catalyzed by the aluminium alkyl compounds. Using the Schulz-Zimm distribution all MWDs were analyzed with regard to the amount of waxes produced by ethylene oligomerization and with regard to the influence of chain transfer reactions to the aluminium. The rate constants of chain transfer to aluminium in relation to the rate constants of insertion of ethylene were estimated.

Influence of Aluminium Alkyl Compounds on the High-Pressure Polymerization of Ethylene with Ternary Metallocene-Based Catalysts. Investigation of Chain Transfer to the Aluminium

The influence of aluminium alkyl compounds on metallocene-catalyzed high pressure polymerizations of ethylene has been investigated at 150 MPa and 180°C in a continuously operated autoclave. The catalysts were based on the metallocenes bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2) and diphenylmethylene (cyclopentadienylfluorenyl)zirconium dichloride (Ph2C-(CpFlu)ZrCl2), which were preactivated outside the reactor with triisobutylaluminium (TiBA) and N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate (DMAP, [PhNHMe2][B(C6F5)4]). The concentrations of triisobutylaluminium (TiBA) and triethylaluminium (TEA) in the reactor were varied over a wide range, using a separate dosing for these two aluminium alkyl compounds. Productivity and polymer properties strongly depended on the type and the concentration of the aluminium alkyl compound used. Highest productivities and molecular weights were obtained with low concentrations of TiBA in the reactor. Up to a concentration of 30 molppm Al in the reactor, unimodal polymers were formed with M̄w/M̄n between 2 and 3. With higher aluminium concentrations the products formed contained small amounts of waxes, due to oligomerization catalyzed by the aluminium alkyl compounds. The molecular weight distributions (MWDs) of these products could be described as a superimposition of two Schulz-Zimm distributions. All MWDs were analyzed with regard to the amount of waxes produced by ethylene oligomerization and with regard to the influence of chain transfer reactions to the aluminium. The rate constants of chain transfer to aluminium, in relation to the rate constants of insertion of ethylene, were estimated.

Influence of Aluminium Alkyl Compounds on the High-Pressure Polymerization of Ethylene with Ternary Metallocene-Based Catalysts. Investigation of Chain Transfer to the Aluminium

The influence of aluminium alkyl compounds on metallocene-catalyzed high pressure polymerizations of ethylene has been investigated at 150 MPa and 180°C in a continuously operated autoclave. The catalysts were based on the metallocenes bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2) and diphenylmethylene (cyclopentadienylfluorenyl)zirconium dichloride (Ph2C-(CpFlu)ZrCl2), which were preactivated outside the reactor with triisobutylaluminium (TiBA) and N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate (DMAP, [PhNHMe2][B(C6F5)4]). The concentrations of triisobutylaluminium (TiBA) and triethylaluminium (TEA) in the reactor were varied over a wide range, using a separate dosing for these two aluminium alkyl compounds. Productivity and polymer properties strongly depended on the type and the concentration of the aluminium alkyl compound used. Highest productivities and molecular weights were obtained with low concentrations of TiBA in the reactor. Up to a concentration of 30 molppm Al in the reactor, unimodal polymers were formed with Mw/Mn between 2 and 3. With higher aluminium concentrations the products formed contained small amounts of waxes, due to oligomerization catalyzed by the aluminium alkyl compounds. The molecular weight distributions (MWDs) of these products could be described as a superimposition of two Schulz-Zimm distributions. All MWDs were analyzed with regard to the amount of waxes produced by ethylene oligomerization and with regard to the influence of chain transfer reactions to the aluminium. The rate constants of chain transfer to aluminium, in relation to the rate constants of insertion of ethylene, were estimated.

MgCl 2/TiCl 4/AlEt 3 catalytic system for olefin polymerisation: a XPS study

A variety of techniques have been applied for 40 years in order to characterise Ziegler–Natta catalysts, however, few studies report the use of surface science technologies in an effort to understand surface structure, composition and chemical bonding in MgCl 2 supported catalytic system. In this paper, the synthesis and the characterisation of the MgCl 2/TiCl 4/AlEt 3 catalyst in controlled conditions is reported. Our aim is to understand (1) the activation process of the magnesium chloride support; (2) the interaction between the magnesium chloride support and TiCl 4; (3) the chemical structure of the titanium active centres formed by treatment of the procatalyst, MgCl 2/TiCl 4, with the co-catalyst, TEA (triethylaluminium ); (4) which Ti n+ species is active in ethene and propene polymerisations. These experiments were performed in conditions as similar, as possible, to the ones used for laboratory synthesis and polymerisation. The experiment consists of different steps: (1) preparation of δ-MgCl 2 under strictly inert atmosphere as previously reported [1]; (2) activation of δ-MgCl 2 in a UHV chamber by means of argon ion sputtering; (3) exposure to TiCl 4 vapours; (4) exposure to TEA. The surface composition was checked during the experimental steps by means of XPS technique.

Metallocene Derivatives as Active Supports of Olefin Polymerization Catalysts

A number of supported Ziegler-Natta catalysts built on magnesocene and manganocene based supports have been synthesized with the purpose to study the influence of both structure and composition of the supporting material on the catalytic performance. Thus, procatalysts containing molecular moieties of the type MCl(C 5 H 5 )/TiCl 4 , MCl(C 5 H 4 TiCl 3 ) (M = Mg, Mn), and MgCl(C 5 H 4 CH 2 CH 2 CH 2 C 5 H 4 TiCl 3 ) were prepared and tested in ethene and propene polymerization after activation with the cocatalyst methylalumoxane (MAO) and triethylaluminium (TEA).

Styrene polymerization with ternary neodymium-based catalyst system: effects of catalyst preparation procedures

The ternary catalyst system neodymium phosphate/di- n-butyl magnesium triethyl aluminum complex/hexamethyl phosphoramide (Nd(P 204) 3/MgBu 2·AlEt 3/HMPA) for the polymerization of styrene was examined. The effects of the addition order of each component, the aging time and temperature and the presence of a small amount of styrene during the preparation of the catalyst system on the activity of the catalyst system and the molecular weight of the resultant polymers were investigated. A stable and homogeneous catalyst system which produces polystyrene with narrow molecular weight distributions can be prepared by an alkylation reaction of Nd(P 204) 3 with MgBu 2·AlEt 3 at ambient temperature followed by the addition of HMPA. An introduction of a small amount of styrene during the preparation of the catalysts can further improve their stability.

High precision prepolymerization of propylene at extremely low reaction rates—kinetics and morphology

A one-liter slurry phase polymerization reactor was set-up to carry out catalytic polymerizations of propylene at low reaction rates. The catalyst system used in this work was a fourth generation Ziegler–Natta catalyst, with tri-ethyl aluminum as cocatalyst and di-cyclopentyl dimethoxy silane as external electron donor. The low reaction rates allowed us to systematically study the formed particles. Polymer powder was produced with a yield of prepolymerization (YPP) varying from 0.3 g of polypropylene (PP) per gram catalyst to 50 g PP per gram catalyst. Because of the well-defined polymerization conditions, the intra- and inter-particle morphologies can be studied in order to investigate the fragmentation of the catalyst. Cross-sectional SEM pictures show a decreasing size of fragments with increasing YPP. The fragmentation does not proceed as sometimes described in literature in layers, starting from the outer layer and advancing to the center of the particle, rather the fragments are initially well distributed throughout the particle. At higher values for YPP—above 2– 4 g PP per gram the size of the fragments continues to decrease with increasing YPP, but the fragments ‘drift’ to the outer portions of the particles. In addition to the morphological aspects, a precise study of the ‘early-stage’-polymerization kinetics was carried out using measurements of monomer pressure as a function of time. Up to a YPP of about 2– 4 g PP per gram catalyst, the reaction rate decreases strongly with increasing YPP, but above this value, reaction rate remains constant with increasing YPP. This effect is ascribed to a phase transition in the growing particle. Initially, the catalyst forms the continuous phase, within which the polymer is distributed. After the phase transition, the polymer forms the continuous phase in which catalyst fragments are distributed. This change causes a change in monomer concentration at the active sites, resulting in lower reaction rates.

The Mechanism of Cp2ZrCl2-Catalyzed Alkene Cycloalumination with Triethylaluminum to Give Alumacyclopentanes

The Mechanism of Cp2ZrCl2-Catalyzed Alkene Cycloalumination with Triethylaluminum to Give Alumacyclopentanes

One-step cyclopropanation of alkynes with diiodomethane and triethylaluminum

Cyclopropanation of mono- and dialkyl-substituted alkynes under the action of CH2I2 and Et3Al afforded the corresponding 1-alkyl- and 1,2-dialkyl-substituted 1,2-diethylcyclopropanes in 65—80% yields.

STRUCTURAL CHARACTERIZATION OF POLY(ϵ-CAPROLACTONE) AND POLY(ϵ-CAPROLACTONE-b-ISOBUTYLENE-b-ϵ-CAPROLACTONE) BLOCK COPOLYMERS BY MALDI-TOF MASS SPECTROMETRY

Poly(ϵ-caprolactone)(PC1) and PC1-polyisobutylene-PC1 (PC1-PIB-PC1) block copolymers were synthesized in anhydrous toluene by in situ conversion of 2-methyl-1-propanol (2M1P) and α,ω-dihydroxy PIB, respectively, to the corresponding aluminum alkoxide by reaction with a stoichiometric amount of triethylaluminum (TEA) followed by the addition of e-caprolactone. Structural characterization of 2M1P-initiated PCl by gel permeation chromatography (GPC) and matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) mass spectrometry (MS) demonstrated the presence of cyclic oligomers, which are produced by intramolecular transesterification reactions that become significant at high monomer conversions. A minor fraction of chains bearing carboxylic acid termini was also observed in the MALDI-TOF mass spectrum; however, carboxylic acid chain ends could not be detected by 13C NMR analysis. Thus, the likely origin of the carboxylic acid termini is fragmentation of the initiator residue from the chain end during MALDI-TOF analysis. For PCl-PIB-PCl block copolymers, two different α,ω-telechelic PIB diols were used as macroinitiators, one derived from allyl and one from isopropenyl terminated PIB. Terminal olefins were converted to primary alcohols via regioselective hydroboration followed by alkaline hydrogen peroxide oxidation. After reaction with ϵ-caprolactone, formation of a block copolymer was evidenced by a shift of the polymer peak to lower elution volume in GPC analysis. Block copolymers possessed molecular weight distributions ≤1.4, and molecular weights of the PCl blocks calculated from GPC were in excellent agreement with those found using MALDI-TOF MS. Structural analysis indicated that the PCl end blocks were severed from the crude block copolymer during MS analysis, for both allyl- and isopropenyl-derived materials. For allyl-derived materials the PCl blocks were found to predominantly carry a C2 residue at the point of detachment of the PIB block; however, the isopropenyl-derived block copolymers showed a complex mixture of different residues suggesting a complex fragmentation mechanism during loss of the PIB block.

Supported metallocenes using inorganic–organic hybrid xerogels

Four metallocene catalysts were prepared using two indenyl-containing aerogels as supports, treated under different conditions (triethylaluminum (TEA), methylaluminoxane (MAO) and butyllithium). The catalysts were characterized by complementary spectroscopic techniques: ultraviolet–visible spectroscopy (UV–Vis), X-ray photoelectronic spectroscopy (XPS), matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF-MS), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The highest metal content (1.32 mmol Zr/g-cat.) was achieved in the case of xerogel treated with TEA prior to metallocene synthesis. After MAO leaching, such content is reduced to 0.82 mmol-Zr/g-cat., exhibiting the highest activity in ethylene polymerization among the four developed catalysts. A relationship between catalyst activity and binding energy of Zr 3d 5/2 core level could be evidenced. The most active catalyst system showed a better Zr distribution, being the catalyst grain covered by aluminum cocatalyst. Depth profile monitoring by XPS measurements showed that, in fact, MAO leaches the zirconocene low binding energy species, being deposited in the uttermost surface of the grains.

Roles of chloro compound in homogeneous [Cr(2-ethylhexanoate) 3/2,5-dimethylpyrrole/triethylaluminum/chloro compound] catalyst system for ethylene trimerization

The effects of chloro compounds on the catalytic performance in ethylene trimerization reactions were studied using a homogeneous [Cr(2-ethylhexanoate) 3/2,5-dimethylpyrrole (DMP)/triethylaluminum (TEA)/chloro compound] catalyst system. Chloro compounds of different types showed a broad range of effects on the performance of the catalytic system. The chloro compounds of geminal chloro groups greatly improved the catalytic activity and 1-hexene selectivity of ethylene trimerization reaction. Some specific interaction modes of chloro groups with dimeric TEA and active Cr species in the catalytic cycle were proposed to explain the catalytic activity and 1-hexene selectivity improvement effects of chloro compound in the ethylene trimerization reaction.

Aluminum titanium nitride films grown with multiple precursors

Metallorganic chemical vapor deposition (MOCVD) of Al xTi 1−xN films was investigated at 200 to 400°C using terakis(dimethylamino)titanium (TDMAT), dimethylaluminum hydride (DMAH), triethylaluminum (TEA) and dimethylhydrazine (DMH). The film composition was examined by in-situ X-ray photoelectron spectroscopy. Al xTi 1−xN and Al xTi 1−xCN (CN designate to carbo-nitride) film growth was observed on SiO 2/Si(100) using various combinations of above precursors. With TDMAT and either Al precursor, the metal to C to N ratio is approximately constant at 1:1:1 for most conditions studied. Introducing DMH significantly lowers the carbon concentration from ∼33 to ∼10% and increases the N content from 33 to more than 50%. The chemical states of Ti, C and N in Al xTi 1−xCN and Al xTi 1−xN films are not identical. The Al chemical state in Al xTi 1−xCN films is nitride at low Al concentration, but increasingly more carbidic at high Al concentration. The Al content in the film is controlled by the ratio of partial pressures of the Al and Ti precursors in the gas phase. Using triethylaluminum (TEA) instead of DMAH does not introduce extra carbon into the film. A higher flow rate of TEA is needed, compared to that for DMAH, for the same Al/Ti ratio in the film. Finally, the step coverage of films grown at various temperatures was examined.

Modification of a Ziegler‐Natta catalyst with a metallocene catalyst and its olefin polymerization behavior

AbstractA Ziegler‐Natta catalyst was modified with a metallocene catalyst and its polymerization behavior was examined. In the modification of the TiCl4 catalyst supported on MgCl2 (MgCl2‐Ti) with a rac‐ethylenebis(indenyl)zirconium dichloride (rac‐Et(Ind)2ZrCl2, EIZ) catalyst, the obtained catalyst showed relatively low activity but produced high isotactic polypropylene. These results suggest that the EIZ catalyst might block a non‐isospecific site and modify a Ti‐active site to form highly isospecific sites. To combine two catalysts in olefin polymerization by catalyst transitioning methods, the sequential addition of catalysts and a co‐catalyst was tried. It was found that an alkylaluminum like triethylaluminum (TEA) can act as a deactivation agent for a metallocene catalyst. In ethylene polymerization, catalyst transitioning was accomplished with the sequential addition of bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2)/methylaluminoxane (MAO), TEA, and a titanium tetrachloride/vanadium oxytrichloride (TiCl4/VOCl3, Ti‐V) catalyst. Using this method, it was possible to control the molecular weight distribution (MWD) of polyethylene in a bimodal pattern. In the presence of hydrogen, polyethylene with a very broad MWD was obtained due to a different hydrogen effect on the Cp2ZrCl2 and Ti‐V catalyst. The obtained polyethylene with a broader MWD exhibited more apparent shear thinning.

Synthesis of dendritic-linear block copolymers by living ring-opening polymerization of lactones and lactides using dendritic initiators

Polyether dendrons have been successfully used as macroinitiators for the living ring-opening polymerization (ROP) of lactones and lactides. A hydroxyl group located at the focal point of dendrimers of different generations was transformed into diethyl aluminum alkoxides by reaction with triethyl aluminum. The dendritic aluminum alkoxides proved to be efficient macroinitiators for the living ROP of e-caprolactone (eCL), 1,4,8-trioxa(4,6)spiro[9]undecanone (TOSUO), D,L- and L,L-lactide. Formation of these block copolymers of unusual macromolecular architecture was supported by size exclusion chromatography and spectroscopy. The versatility of this synthetic approach allowed ω-functional dendrimer block-polyesters, such as macromonomer, and macromolecules with novel architectures, to be prepared. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1923–1930, 1999