Preparation and characterization of photoresponsive poly(methyl methacrylate) microspheres bearing phosphorylcholine-analogous and spirooxazine moieties

The photoresponsive copolymer microspheres [poly(MAIP-co-MMA)] were prepared from the emulsifier-free emulsion copolymerization of 2-[2-(methacryloyloxy) ethyldimethylammonio]-ethyl indolinonaphthooxazine phosphate (MAIP) and methyl methacrylate (MMA). From the kinetics of the copolymerization of MAIP and MMA, it was found that the initial rate of polymerization of MMA increased by the addition of a small amount of MAIP. From the X-ray photoelectron spectroscopy (XPS) measurements MAIP moiety was found to be located on the surface of a particle. The introduction of a MAIP moiety into poly(MMA) microspheres results in a decrease in the amount of bovine serum albumin (BSA) adsorbed. A photoresponsive adsorption of BSA on poly(MAIP-co-MMA) microspheres was observed with spirooxazine-merocyanine photoisomerization caused by UV irradiation.     

Hydrogel Nanocomposites: A Potential UV/Blue Light Filtering Material for Ophthalmic Lenses

Poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) (poly(HEMA-co-MMA)) and ZnS hydrogel nanocomposites were prepared and characterized. The chemical composition of the inorganic nanoparticles was confirmed by X-ray diffraction, and the homogeneity of their distribution within the hydrogel was assessed by transmission electron microscopy. The influence of the content of ZnS nanoparticles on the optical performances of the nanocomposites was investigated by UV-Vis spectroscopy. The ability of the hydrogel nanocomposites to filter the hazardous UV light and part of the blue light was reported, which makes them valuable candidates for ophthalmic lens application. In contrast to the optical properties, the thermo-mechanical properties of neat poly(HEMA-co-MMA) hydrogels were found to be largely independent of filling by ZnS nanoparticles (≤2 mg/ml co-monomer mixture). Finally, in vitro cell adhesion test with lens epithelial cells (LECs), extracted from porcine lens crystalline capsule, showed that ZnS had no deleterious effect on the biocompatibility of neat hydrogels, at least at low content.     

Thermally irreversible photoresponsive polymers: Synthesis and preliminary characterization of copolymers containing side-chain trans-cinnamamide chromophores

A novel photoresponsive polymer containing organic chromophores which possess thermal stability as well as substantial and reversible responsiveness upon external photostimulus has been developed. trans-2{4-[2-(Dimethylcarbamoyl)vinyl]phenoxy}ethyl methacrylate ( 1 ) was synthesized and copolymerized by free-radical methods with methyl methacrylate giving a copolymer poly[(methyl methacrylate)-co- 1 ] (poly(MMA-co- 1 )) ( 2 ) with cinnamamide moiety chromophores randomly attached as side groups. Photoisomerization and ultraviolet absorbance were used to examine the photostationary states and solubility changes upon photoirradiation. Poly[(methyl methacrylate)-co- 1 ] (90,7 : 9,3 mol%) with 9,3 mol-% cinnamamide content has better solubility in a polar solvent such as acetonitrile (theta temperature Θ_PMMA = 30°C) than in a nonpolar solvent such as carbon tetrachloride (Θ_PMMA = 27°C). Photoisomerization of the trans-cinnamamide to cis-cinnamamide units significantly shifts the cloud point of poly[(methyl methacrylate)-co- 1 ] (90,7 :9,3 mol-%) in carbon tetrachloride; this cloud point shift is reversible by photoisomerization back to the trans state. However, in a hydrogen-bonding solvent such as tert-butyl alcohol, while photoirradiation of poly[(methyl methacrylate)-co- 1 ] (90,7 :9,3 mol%) yields a significant shift in cloud point, this is not reversible by photoirradiation as hydrogen-bond formation between the tertiary amide and the hydroxylic solvent reduces the isomerization efficiency. Fatigue of the photoreversibility of the copolymer, which is not present in dilute solutions of the cinnamamide chromophore, is noted.     

Porosity determination of poly[(trimethylolpropane trimethacrylate)-co-(methyl methacrylate)] gels

In this paper we studied the influence of the copolymerization of trimethylolpropane trimethacrylate (TRIM) with methyl methacrylate (MMA) on the pore structure of the resulting gels. TRIM and MMA were suspension-copolymerized with toluene as pore-forming agent. All gels have bimodal pore-size distributions. The size distribution of large pores (r > 50 Å) of the solvent-free poly(TRIM-co-MMA) broadens when the TRIM/MMA ratio decreases. The size distribution of small pores (r < 50 Å) shows sharp maxima for pores with a radius of 20 Å. In the swollen state the separation range for the poly(TRIM-co-MMA) particles of polystyrene standards varies with the TRIM/MMA ratio. The results of the porosity measurements of the swollen gels show good agreement with the results obtained from the dry gels. The porosity of the dry gels was studied with scanning electron microscopy, nitrogen adsorption/desroption and mercury porosimetry. The pore-size distributions of the swollen poly(TRIM-co-MMA) gel particles were calculated from the size-exclusion chromatography (SEC) results.     

Emulsion copolymerization of 2-(acryloyloxy)ethylphosphoryl-choline with vinyl monomers and protein adsorption at resultant copolymer microspheres

A series of copolymer microspheres of 2-(acryloyloxy)ethylphosphorylcholine (APC) with comonomers (M) such as methyl (MMA), ethyl (EMA), butyl (BMA), hexyl methacrylate (HMA), and styrene (St), i. e. poly(APC-co-M) microspheres, were prepared by emulsifier-free emulsion copolymerization. From the kinetics of the copolymerization, it was found that the initial rate of polymerization of St increases in the presence of small amounts of APC. The diameters of poly-(APC-co-M) microspheres were much smaller than those of the corresponding homopolymer, poly(M). It was confirmed by X-ray photoelectron spectroscopy measurements that the APC moiety is concentrated on the surface of the particles. A series of poly(APC-co-M) microspheres was found to adsorb both bovine serum albumin (Alb) and human serum γ-globulin (Glo) less that the corresponding poly(M) microspheres as a control.     

Surface modified poly(methyl methacrylate) with 1-methyl- 2-methacrylamidoethyl phosphorylcholine moiety

A series of poly(methyl methacrylate) [poly(MMA)] microspheres covered with the 1-methyl-2-methacrylamidoethyl phosphorylcholine (MAPC) moiety, poly(MAPC-co-MMA), were prepared by emulsifier-free emulsion copolymerization of methyl methacrylate (MMA) and MAPC using potassium peroxodisulfate (KPS) or 2,2′-azobis[2-(imidazolin-2-yl)propane] dihydrochloride (ABIP) as initiators. The ζ-potentials of the particles are –72 to –26 mV and 0 to 27 mV for poly(MAPC-co-MMA) produced by KPS and ABIP, respectively. Poly(MAPC-co-MMA) suppresses the adsorption of albumin, γ-globulin, and fibrinogen more than poly(MMA) as the control. From XPS measurements the MAPC moiety and the fragments of the initiator are located on the surface of the polymer films prepared from poly(MAPC-co-MMA). Egg yolk lecithin adsorbs on the surface of the films, and an organized adsorption layer of lipid, i. e., a hydrogel layer with an analogous structure to biomembrane, is formed.     

Kinetics of the polymer-analogous reaction of poly[(methyl methacrylate)-co-(methacrylic acid)] with epichlorohydrin

Poly[(methyl methacrylate)-co-(glycidyl methacrylate)] (poly(MMA-co-GMA)) was obtained by polymer-analogous reaction of poly[(methyl methacrylate)-co-(methacrylic acid)] with an excess of epichlorohydrin (ECH) in the presence of a quaternary ammonium salt R₄NX as catalyst. The kinetics of the addition reaction and the consecutive transepoxidation reaction was studied at 50–90°C. The rate constant of the addition reaction is one order of magnitude higher with E₁ = 71 kJ · mol^?1 than that of the transepoxidation reaction with E₂ = 83 kJ · mol^?1. The rate constants rise linearly with increasing concentration of the catalyst R₄NX. The equilibrium constants K ≈ 2,3 of transepoxidation of glycidyl methacrylate (GMA) as well as of poly(MMA-co-GMA) both with 1,3-dichloropropane-2-ol were determined in butyl acetate and dimethylformamide as solvent at 80–100°C. During the reverse transepoxidation reaction of 3-chloro-2-hydroxypropyl methacrylate (CHPM) as well as of its MMA-copolymer with equimolar amounts of ECH, side products were formed from the beginning of the reaction and the equilibrium was not established. The addition of ECH to CHPM was observed as a side reaction.     

A Thermo-Sensitive NIPA-Based Co-Polymer and Monosize Polycationic Nanoparticle for Non-viral Gene Transfer to Smooth Muscle Cells

Primary smooth muscle cells (SMC) isolated from the aorta of fetal calf were transfected with a green fluorescent protein (GFP)-encoding plasmid DNA, which was carried by a water-soluble and temperature-sensitive N-isopropylacrylamide-based (NIPAAm-based)-co-polymer, either poly(N-isopropylacrylamide-co-2-methacryloamidohistidine) (poly(NIPAAm-co-MAH)) or monosized PEGylated nanoparticle poly(styrene/poly(ethylene glycol) ethyl ether methacrylate/N-(3-(dimethylamino)propyl) methacrylamide) (poly(St/PEG-EEM/DMAPM)). Poly(NIPAAm-co-MAH) co-polymer was synthesized by solution polymerization of n-isopropylacrylamide (NIPAAm) and 2-methacrylamidohistidine (MAH). Monosized cationic nanoparticles were produced by emulsifier-free emulsion polymerization of styrene, PEG ethyl ether methacrylate and N-[3-(dimethyl-amino) propyl] methacrylamide, in the presence of a cationic initiator, 2,2-azobis (2-methylpropionamidine) dihydrochloride. The structure of poly(St/PEG-EEM/DMAPM) and poly(NIPAAm-co-MAH) was confirmed by¹ H-NMR and FT-IR spectroscopy. Particle size/size distribution and surface charges of both carriers were measured by Zeta Sizer. The LCST behavior of poly(NIPAAm-co-MAH) co-polymer was followed spectrophotometrically. Poly(St/PEG-EEM/DMAPM) nanoparticles, with an average size of 78 nm and zeta potential of 54.4 mV, and an average size of 200 nm with a zeta potential of 54.2 mV, and poly(NIPAAm-co-MAH) were used in the transfection studies. The cytotoxicity of the vectors was tested using the MTT method. According to conditions for the transfection study (polymer/cell ratio and polymer–cell incubation period), cell loss was only 4 and 15% with poly(St/PEG-EEM/DMAPM) sized 78 and 200 nm, respectively. Poly(NIPAAm-co-MAH) cytotoxicity was insignificant. Poly(NIPAAm-co-MAH) uptake efficiency in SMCs was around 85%, but gene expression efficiency were low compared to poly(St/PEG-EEM/DMAPM)/pEGFP-N2 conjugates because of the low zeta potential of the co-polymer. Polymer uptake efficiencies of the nanoparticles were 90–95%. GFP expression efficiency was 68 and 64% after transfection with pEGFP-N2 conjugate with 78 and 200 nm sized poly(St/PEG-EEM/DMAPM) nanoparticles.     

Functional Surfaces Constructed with Hyperbranched Copolymers as Optical Imaging and Electrochemical Cell Sensing Platforms

In the present study, hyperbranched copolymers (HBCs), namely poly(methyl methacrylate) (PMMA)‐co‐poly(2‐hydroxyethylmethacrylate) and PMMA‐co‐poly(2‐dimethylamino ethyl methacrylate), are photochemically synthesized by self‐condensing vinyl polymerization of methyl methacrylate with the corresponding inimer using Type II photoinitiators. HBCs with different functional group and branching densities are used as surface coating materials in cellular adhesion and the respective electrochemical‐based studies. After the main surface characterization of the synthesized three HBCs with contact angle measurements and atomic force microscopy, HaCaT keratinocytes and human neuroglioblastoma (U‐87MG) cell lines to the surfaces are conducted. The adherence of cells is proven by both fluorescence cell imaging and electrochemical methods such as cyclic voltammetry and differential pulse voltammetry. The described strategy involving hyperbranched polymers offers great potential for fabricating various new surfaces in particular “on‐chip‐sensing” applications.     

Synthesis of Poly(ϵ‐caprolactone‐co‐methacrylic acid) Copolymer via Phosphazene‐Catalyzed Hybrid Copolymerization

Poly(?‐caprolactone‐co‐tert‐butyl methacrylate) (CL‐co‐BMA) random copolymer is synthesized via hybrid copolymerization with 1‐tert‐butyl‐4,4,4‐tris(dimethylamino)‐2,2‐bis[tris(dimethylamino)phophoranylidenamino]‐2Λ5,4Λ5‐catenadi(phosphazene) (t‐BuP₄) as the catalyst. The copolymer is hydrolyzed into poly(?‐caprolactone‐co‐methacrylic acid) (CL‐co‐MAA), a charged copolymer. Nuclear magnetic resonance, Fourier transform infrared spectroscopy, differential scanning calorimetry, and thermogravimetric analysis measurements indicate that cyclic ester and vinyl monomer form a random copolymer. The degradation of the copolymers has also been studied by use of quartz crystal microbalance with dissipation.     

Oxidative crosslinking polymerization of poly[(methyl methacrylate)-co-(2-(n-pyrrolyl)ethyl methacrylate)] with iron chloride in nitromethane

Poly[(methyl methacrylate)-co-(2-(N-pyrrolyl)ethyl methacrylate)] (PMMA-co-PEMA, 1) containing 0,7 to 7 mol-% pyrrolylethyl methacrylate (PEMA) units was crosslinked via oxidative polymerization with FeCl₃ in inert atmosphere. The properties of the products 2 depended on the amount of electroactive side groups and on the molecular weight of the precursor copolymers 1 as well as on the reaction conditions. Crosslinking between pendant pyrrole groups and an increase in the glass transition temperature in the oxidative coupling could be avoided using precursor copolymers 1 with low PEMA content (<1 mol-%).     

Preparation of Polyisobutene‐graft‐Poly(methyl methacrylate) and Polyisobutene‐graft‐Polystyrene with Different Compositions and Side Chain Architectures through Atom Transfer Radical Polymerization (ATRP)

Polyisobutene‐graft‐poly(methyl methacrylate) and polyisobutene‐graft‐polystyrene with controlled compositions and side chain architectures were prepared through atom transfer radical polymerization (ATRP). Poly[isobutene‐co‐(p‐methylstyrene)‐co‐(p‐bromomethylstyrene)] (PIB) was used as a macroinitiator in the presence of CuCl or CuBr as a catalyst and dNbpy as a ligand. The compositions were controlled by the conversion of the monomer with polymerization time. The molecular weight distributions of the side chains were controlled through ATRP in the presence/absence of a halogen exchange reaction. DSC and DMA measurements showed that graft copolymers have two glass transition temperatures suggesting microphase separated behavior, which was also confirmed by SAXS measurements. The phase and dynamic mechanical behaviors were strongly affected by the compositions and/or the side chain architectures. The properties of the graft copolymers were controlled in a wide range leading to toughened glassy polymers or elastomers.     

Functional Poly(Dimethyl Aminoethyl Methacrylate) by Combination of Radical Ring‐Opening Polymerization and Click Chemistry for Biomedical Applications

In this work, the macromolecular design and modular synthesis of degradable and biocompatible copolymers via radical polymerization and click chemistry is highlighted and the resulting systems are evaluated as gene delivery carriers. Poly(ethylene glycol) (PEG) grafted poly[2‐methylene‐1,3‐dioxepane (MDO)‐co‐propargyl acrylate (PA)‐co‐2‐(dimethyl aminoethyl methacrylate (DMAEMA)] (MPD) is synthesized using radical polymerization and azide‐alkyne click chemistry. The polymers are less cytotoxic and are able to condense plasmid DNA into nanosized particles. The low transfection efficiency of polyplexes in HepG2 cells is significantly improved by mixing Tat peptide with polyplexes.     

Synthesis of Block Graft Copolymer with Syndiospecific Living Polymerization of Styrene Derivatives by (Trimethyl)pentamethylcyclopentadienyltitanium/Tris(pentafluorophenyl)borane/Trioctylaluminium Catalytic System

Two kinds of syndiotactic AB type block copolymers were prepared, which were (1) poly(4‐methylstyrene)‐block‐polystyrene {Poly(4MS‐b‐S), (A: poly(4MS), B: polystyrene (S))}, (2) poly(4‐methylstyrene)‐block‐poly(styrene‐co‐3‐methylstyrene) {poly[4MS‐b‐(S‐co‐3MS)] (A: poly(4MS), B: styrene/3‐methylstyrene (3MS) copolymer)}. For the syntheses of these diblock copolymers, the living polymerization catalytic system composed of (trimethyl)pentamethylcyclopentadienyltitanium (Cp*TiMe₃) premixed with trioctylaluminium (AlOct₃), and tris(pentafluorophenyl)borane (B(C₆F₅)₃) was used at –25°C. Chlorination of the methyl groups of poly[4MS‐b‐(S‐co‐3MS)] was conducted by aqueous sodium hypochlorite (NaOCl) and phase‐transfer catalyst such as tetrabutylammonium hydrogensulfate (TBAHS). The novel tapered densely grafted diblock copolymer was synthesized with by coupling reaction of living poly(2‐vinyl pyridine)lithium (Poly(2VP)Li) with the partly chloromethylated poly[4MS‐b‐(S‐co‐3MS)].     

Synthèse et caractérisation de polystyrènes siliciés

Poly(p-trimethylsiloxystyrene) (1a) , poly[p-(tert-butyldimethylsiloxy)styrene] (1b) , poly[p-(trimethylsiloxy)-α-methylstyrene] (1c) , poly[p-(tert-butyldimethylsiloxy)-α-methylstyrene) (1d) and poly{p-[2-(tert-butyldimethylsiloxy)ethyl]styrene]} (1e) were prepared by free-radical or cationic polymerization of the corresponding monomers. Poly{[p-[2(trimethylsiloxy)ethyl]styrene]-co-[p-(2-hydroxyethyl)styrene]} (2a) was synthetized by anionic polymerization of the corresponding trimethylsilylated monomer, followed by acid hydrolysis of the resulting polymer. Poly{[p-[2-(trimethylsiloxy)ethyl]styrene]-co-[p-(tert-butoxycarbonyloxy)styrene]} (2b) were prepared by free-radical polymerization of the corresponding monomers.     

Preparation and characterization of brush-like PEGMA-graft-PDA coating and its application for protein separation by CE

In this paper, a novel brush-like copolymer consisting of poly(ethylene glycol) methyl ether methacrylate and 2-aminoethyl methacrylate (AEMA) named as poly(PEGMA₃₀₀-co-AEMA) was synthesized by atom transfer radical polymerization (ATRP), and then, poly(PEGMA₃₀₀-co-AEMA) copolymer was immobilized onto material surfaces through polydopamine (PDA)-anchored coating. The defined copolymer structure was characterized by nuclear magnetic resonance hydrogen spectroscopy (¹H NMR) and gel permeation chromatography (GPC). The chemical component and surface morphology of the brush-like copolymer-graft-PDA coating were studied by using X-ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM), respectively. The hydrophilicity of the brush-like copolymer-graft-PDA coating was investigated by using static water contact angle. The protein-resistant property of the brush-like copolymer-graft-PDA coating was investigated by using quartz crystal microbalance with dissipation (QCM-D), and finally the coating was applied to capillary inner surface for protein separation by capillary electrophoresis (CE).     

Reactive poly(2-vinyl-4,4-dimethyl-5-oxazoione) and poly[(2-vinyl-4,4-dimethyl-5-oxazolone)-co-(methyl methacrylate)]s. Synthesis,characterization and chemical modification with 4-methoxy-4′-(β-aminoethoxy)biphenyl

Poly(2-vinyl-4,4-dimethyl-5-oxazolone) ( P₀ ) and poly[(2-vinyl-4,4-dimethyl-5-oxazolone)-co-(methyl methacrylate)]s with increasing content of methyl methacrylate units ( P₁–P₄ ) were synthesized and characterized. NMR spectra were discussed in terms of monomer sequence distribution and tacticity effects. The reaction of 4-methoxy-4′-hydroxybiphenyl ( 1 ) with 2-ethyl-2-oxazoline was utilized to prepare 4-methoxy-4′-(β-aminoethoxy)biphenyl ( 3 ) through the intermediate 4-methoxy-4′-[(N-propanoyl)-β-aminoethoxy]biphenyl ( 2 ). The homopolymer P ₀ and two copolymers P ₂ and P ₃ were functionalized with 4-methoxybiphenyl side groups by reaction with 3 via a ring-opening process in N,N-dimethylformamide (DMF) or 1,2-dichloroethane. The resulting copolymers P₅–P₈ were characterized by ¹H and ¹³C NMR. The highest degree of functionalized units was obtained in DMF at 80°C.     

Study of the compatibility of poly[styrene-co-(cinnamic acid)]/poly[(ethyl methacrylate)-co-(2-dimethylaminoethyl methacrylate)] blends

Compatibilization of an immiscible polymer pair, polystyrene and poly(ethyl methacrylate), is achieved by introducing along the polymer chains cinnamic acid and 2-dimethylaminoethyl methacrylate groups, respectively. The miscibility behavior of a series of poly[styrene-co-(cinnamic acid)] (PSCA) copolymers containing 5, 8, and 23 mol-% of acidic units, with poly[(ethyl methacrylate)-co-(2-dimethylaminoethyl methacrylate)] (PEMADAE) was investigated by DSC and FTIR. Based on the single composition-dependent glass transition criterion, each PSCA copolymer is miscible with PEMADAE over the three blend compositions studied. The glass transition temperatures are higher than predicted according to the additivity principle. This indicates the occurrence of strong intermolecular interactions between the polymeric chains of the two components. The T_g-composition curves of the investigated systems are interpreted according to the Kwei and the Schneider approaches. The results of the FTIR study reveal that the changes detected in the carbonyl stretching frequency region are the consequence of hydrogen bonding between the carboxylic acid groups in PSCA and the carbonyl groups in PEMADAE.     

Role of hydrophobic interaction in the enantioselective solvolyses of amino acid p-nitrophenyl esters by optically active imidazole-containing polymers

Solvolytic reactions of N-benzyloxycarbonyl-D -phenylalanine p-nitrophenyl ester and N-benzyloxycarbonyl-L -phenylalanine p-nitrophenyl ester by optically active imidazole-containing polymers were performed at different temperatures and in aq. ethanol of different water contents. The catalyst polymers employed were homopolymers of N-methacryloyl-L -histidine ( 1a ) and N-methacryloyl-L -histidine methyl ester ( 1b ), as well as copolymers of 1a with dodecyl methacrylate (DMA) and 1b with DMA. In 30 vol. -% aqueous ethanol at pH 7,02 the homopolymers did not show any enantioselective catalysis. However, the copolymers did exhibit enantioselective catalysis, viz., k_cat(L )/k_cat(D ) = 1,25 for poly ( 1b -co-DMA) containing 5,7 mol-% of DMA. As the reaction temperature was lowered, the reaction rate increased and the enantioselectivity was enhanced (k_cat(L )/k_cat(D ) = 1,67 for poly ( 1b -co-DMA) at 10°C). When the ethanol content was decreased, enhanced reaction rates and enantioselectivity (k_cat(L )/k_cat(D ) = 1,65 for poly ( 1 b -co-DMA) in 20 vol.-% aqueous ethanol) were observed. From these results it is concluded that hydrophobic interaction plays an important role in the enantioselective catalysis of optically active imidazole-containing polymers.     

Synthesis of Poly(ethylene‐co‐butylene)‐block‐Poly(methyl methacrylate) by Atom Transfer Radical Polymerization: Determination of the Macroinitiator Conversion

The synthesis of poly(ethylene‐co‐butylene)‐block‐poly(methyl methacrylate) via atom transfer radical polymerization (ATRP) of methyl methacrylate onto a poly(ethylene‐co‐butylene) macroinitiator is described. Copper bromide (CuBr), copper chloride (CuCl) and copper thiocyanate (CuSCN) were applied as active ATRP catalysts with N‐pentyl‐2‐pyridylmethanimine as ligand, in order to study the effect of the copper counter ion on the rate of activation of the bromine‐functional polyolefin macroinitiator. The reaction products were subjected to normal phase gradient polymer elution chromatography (NP‐GPEC) in order to separate residual macroinitiator from the block copolymers. The amount of residual macroinitiator was monitored for each catalyst system by evaporative light scattering detection, where the chromatograms were normalized on a PS standard, which was added to the reaction mixture before polymerization. The rate of activation of the macroinitiator is strongly affected by the copper counter ion and increases in the order CuSCN < CuBr < CuCl. The high reproducibility of this observation is in contrast with the significant effect of polymerization conditions like e. g. catalyst solubility, presence of water and trace amounts of oxygen, on the overall rate of polymerization.