Monomer Pressure Research Articles

Synthesis and characterization of plasma-polymerized hexamethyldisiloxane films

The deposition rates of plasma-polymerized (pp-) films from hexamethyldisiloxane (HMDSO) monomer were determined by the quartz crystal microbalance technique. Using the several polymerization conditions for HMDSO, these were found that the deposition rates were proportional to the polymerization time. The deposition rate of the pp-HMDSO films increased with the increasing RF power. But for vapor pressure, the deposition rates increased with increasing the vapor pressure until 100 Pa; above it, deposition rates tended to decrease gradually. The maximum deposition rate of pp-HMDSO film was obtained 4.577 μg/min on 150 W of RF power at 100 Pa of monomer pressure.

Surface Modification of Aligned Carbon Nanotube Arrays for Electron Emitting Applications

We have previously developed a simple pyrolytic method for large-scale production of aligned carbon nanotube arrays perpendicular to the substrate. These aligned carbon nanotube arrays can be transferred onto various substrates of particular interest (e.g. polymer films for organic optoelectronic devices) in either a patterned or non-patterned fashion. The well-aligned structure is important, which not only allows us to prepare aligned coaxial nanowire of carbon nanotubes sheathed with polymers by electrochemical polymerization of conducting polymers on the individual nanotubes but also enables us to develop a facile approach for modification of carbon nanotube surfaces via plasma activation, followed by chemical reactions characteristic of the plasma-induced functionalities. These surface modification methods are particularly attractive, as they allow surface characteristics of the aligned carbon nanotubes to be tuned to meet specific requirements for particular applications while their alignment structure can be largely retained. The aligned carbon nanotubes with tunable surface characteristics thus prepared are of great significance to various practical applications, especially for the use of carbon nanotubes as electron emitters in flat panel displays. Particularly, we found that hexane-plasma coating reduced the turn-on electric field E to, coupled with a concomitant increase in the emission current at a constant V; the turn-on electric field decreased from E to = 2.5 V/μm, characteristic of the pristine aligned carbon nanotubes, to E to = 1.5 V/μm with a significantly increased emission current after the treatment of n-hexane plasma at 30 W, 250 KHz and under a monomer pressure of 0.65 Torr for 2 min. In this paper, we will discuss the effects of the surface modification on the electron-emitting properties of the aligned carbon nanotubes by presenting some examples from our recent work.

The Glass Transition Temperature of Polymer Melts

We develop an analytic theory to estimate the glass transition temperature T(g) of polymer melts as a function of the relative rigidities of the chain backbone and side groups, the monomer structure, pressure, and polymer mass. Our computations are based on an extension of the semiempirical Lindemann criterion of melting to locate T(g) and on the use of the advanced mean field lattice cluster theory (LCT) for treating the thermodynamics of systems containing structured monomer, semiflexible polymer chains. The Lindemann criterion is translated into a condition for T(g) by expressing this relation in terms of the specific volume, and this free volume condition is used to calculate T(g) from our thermodynamic theory. The mass dependence of T(g) is compared to that of other characteristic temperatures of glass-formation. These additional characteristic temperatures are determined from the temperature variation of the LCT configurational entropy, in conjunction with the Adam-Gibbs model for long wavelength structural relaxation. Our theory explains generally observed trends in the variation of T(g) with polymer microstructure, and we find that T(g) can be tuned either upward or downward by increasing the length of the side chains, depending on the relative rigidities of the side groups and the chain backbone. The elucidation of the molecular origins of T(g) in polymer liquids should be useful in designing and processing new synthetic materials and for understanding the dynamics and controlling the preservation of biological substances.

Surface Analysis of Plasma‐Deposited Polymer Films, 4

AbstractSummary: XPS and NEXAFS spectroscopy were used for the chemical characterization of pulsed plasma‐deposited ethylene films before and after exposure to ambient air. The influence of external plasma parameters on the spectroscopic results was investigated. Information on the chemical character of the plasma‐polymerized films, such as the regularity of the primary structure, the amount of long‐living radicals, and branching or cross‐linking or both was derived from this data. “Irregularity” and radical concentration for post‐plasma reactions of the plasma‐deposited films increased with the duty cycle or power. A decreased level of monomer fragmentation and, consequently, an increase in regularity is partially concluded from the experimental data when the monomer pressure is increased. The concentration of surface radicals available for post‐plasma reactions also increased with monomer pressure. The correlation found between the degree of the post‐plasma oxygen incorporation and the variation of the various external parameters agreed with earlier ToF‐SSIMS findings. magnified image

Polymerization of propylene using MgCl2 (ethoxide type)/TiCl4/diether heterogeneous Ziegler–Natta catalyst

AbstractHeterogeneous Ziegler–Natta catalyst of MgCl2 (ethoxide type)/TiCl4/diether was prepared. 2,2‐Diisobutyl‐1,3‐dimethoxy propane (DiBDMP), diether, was used as internal donor. Slurry polymerization of propylene was carried out using the catalyst in dry heptane while triethylaluminium (TEA) was used as co‐catalyst. The co‐catalyst effects, such as catalyst molar ratio, polymerization temperature, H2 pressure, external donor, triisobutylaluminium (TiBA) and monomer pressure, on the activity of the catalyst and isotacticity index (II) of the polymers obtained were studied. Rate of polymerization versus polymerization time is of a decay type with no acceleration period. There are an optimum Al/Ti molar ratio and temperature to obtain the highest activity of the catalyst. The maximum activity was obtained at 60 °C. Increasing the monomer pressure to 1 010 000 Pa linearly increased the activity of the catalyst. Addition of hydrogen to 151 500 Pa pressure increased activity of the catalyst from 2.25 to 5.45 kg polypropylene (PP) (g cat)−1 h−1 using 505 000 Pa pressure of monomer. The II decreased with increasing Al/Ti ratio, monomer pressure, hydrogen pressure and increased with increasing temperature to 60 °C, following with decrease as the temperature increases. Productivity of 11.55 kg (PP) (g cat)−1 h−1 was obtained at 1 010 000 Pa pressure of monomer and temperature of 60 °C. Addition of methyl p‐toluate (MPT) and dimethoxymethyl cyclohexyl silane (DMMCHS) as external donors decreased the activity of the catalyst sharply, while the II slightly increased. Some studies of the catalyst structure and morphology of the polymer were carried out using FTIR, X‐ray fluorescence, scanning electron microscopy and Brunauer–Emmett–Teller techniques. Copyright © 2005 Society of Chemical Industry

Bis(amido)cyclodiphosph(III)azane]dichlorozirconium Complexes for Ethene Polymerization

AbstractA series of new zirconium dichloride complexes bearing bulky cis‐bis(amino)cyclodiphosph(III)azane ligands, [(RN)2(tBuNP)2]ZrCl2 [R = phenyl (7), diphenylmethyl (8), 2,6‐diethylphenyl (9), 2,5‐di‐tert‐butylphenyl (10), and 2,6‐diisopropylphenyl (11)] were prepared in high yields by a two‐step synthetic route. The structure of cis‐[(Ph2CHN)(tBuNP)]2Zr(NMe2)2 (8a), determined by X‐ray analysis, shows that the zirconium atom has a distorted trigonal‐bipyramidal configuration consisting of four equidistant metal−amido nitrogen bonds and an additional coordination between the metal and a nitrogen atom from the cyclodiphosph(III)azane ring. After methylaluminoxane activation, these complexes exhibit moderate to high activities in ethene polymerization and produce high molar mass polyethene (Mw up to 1100 kg/mol). Polymerization experiments reveal that the behavior of bis(amido)cyclodiphosph(III)azane‐based ZrIV catalysts depends on the nature and steric bulkiness of the amido substituents, reaction temperature, MAO/zirconium ratio, as well as monomer pressure. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Surface Analysis of Plasma‐Deposited Polymer Films, 2

AbstractSummary: Chemical properties of plasma‐polymerized styrene films can be effectively controlled by the appropriate choice of external plasma parameters, such as duty cycle, power and monomer pressure. Chemical properties here are the regularity of the primary structure, the amount of long living radicals and branching or cross‐linking or both in plasma polymerized styrene films, which can be indirectly monitored by X‐ray photoelectron spectroscopy (XPS) and near edge X‐ray absorption fine structure (NEXAFS) spectroscopy. Principally, XPS and NEXAFS spectra of plasma‐polymerized styrene show all the spectroscopic fingerprints characteristic of atactic polystyrene indicating an inherent similarity of the respective primary chemical structures. There is an increase in irregularity and radical concentration for the plasma deposited films with increasing duty cycle or power. With an increase in monomer pressure, a decreased level of monomer fragmentation and, consequently, an increase in regularity must be concluded from the experimental data. The applied power regulates the fragmentation of the monomers as monitored by spectroscopic signals correlated to the aromatic rings. A trend in the extent of the post‐plasma oxygen incorporation according to the variation of the various external plasma parameters was deduced, which agrees with earlier ToF‐SIMS findings (Plasma Process. Polym., 2004, 1, 123).Total electron yield NEXAFS spectra of atactic polystyrene reference film (a) and a selected pulsed plasma‐polymerized styrene film (b).magnified imageTotal electron yield NEXAFS spectra of atactic polystyrene reference film (a) and a selected pulsed plasma‐polymerized styrene film (b).

PH Sensitive polypropylene porous membrane prepared by grafting acrylic acid in supercritical carbon dioxide

pH sensitive membrane was prepared by grafting acrylic acid (AA) on the porous polypropylene (PP) membrane using supercritical (SC) CO 2 as a solvent. The monomer (AA) and the initiator (benzyl peroxide, BPO) were impregnated into the PP substrate with the aid of SC CO 2, and were grafted onto the microporous PP substrate. The grafted membranes were characterized by Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), and the water permeability of the virgin and grafted membranes were determined at different pH values. It was demonstrated that the grafting degree (Dg) could be easily controlled by varying operating conditions, such as the monomer concentration, pressure, and temperature during the soaking process. The water permeation of the virgin membrane is nearly independent of pH. However, the water permeation of grafted membranes decreases dramatically with pH as the pH varies from 3 to 6 because the conformation of the PAA changes significantly with the pH of the contacting solution.

Amorphous fluorocarbon polymer (a-C:F) films obtained by plasma enhanced chemical vapor deposition from perfluoro-octane (C8F18) vapor I: Deposition, morphology, structural and chemical properties

The method of obtaining amorphous fluorocarbon polymer (a-C:F) films by plasma enhanced chemical vapor deposition in a capacitively coupled, 13.56 MHz reactor, from a new monomer, namely perfluoro-octane (C8F18) vapor, is presented. For monomer pressure ranging from 0.2 to 1 Torr and input power density from 0.15 to 0.85 W/cm3, the maximum deposition rate reached 300 nm/min, while 10% monomer dilution with argon led to a deposition rate of 200 nm/min. The film surface and bulk morphologies, chemical and structural compositions were investigated using scanning electron microscopy, x-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. It was revealed that the films have a dense and compact structure. The fluorine to carbon ratio (F/C) of the films was between 1.57 and 1.75, and the degree of cross-linking was between 55% and 58%. The relative amount of perfluoroalkyl (CF2) groups in the films was 29%. The FTIR spectra showed absorption bands corresponding to the different vibrational modes of CF, CF2, and CF3 moieties.

Polymerization of propylene using the high‐activity Ziegler–Natta catalyst system SiO2/MgCl2 (ethoxide type)/TiCl4/Di‐n‐butyl phthalate/triethylaluminum/dimethoxy methyl cyclohexyl silane

AbstractThe bisupported Ziegler–Natta catalyst system SiO2/MgCl2 (ethoxide type)/TiCl4/di‐n‐butyl phthalate/triethylaluminum (TEA)/dimethoxy methyl cyclohexyl silane (DMMCHS) was prepared. TEA and di‐n‐butyl phthalate were used as a cocatalyst and an internal donor, respectively. DMMCHS was used as an external donor. The slurry polymerization of propylene was studied with the catalyst system in n‐heptane from 45 to 70°C. The effects of the TEA and H2 concentrations, temperature, and monomer pressure on the polymerization were investigated. The optimum productivity was obtained at [Al]/[DMMCHS]/[Ti] = 61.7:6.2:1 (mol/mol/mol). The highest activity of the catalyst was obtained at 60°C. Increasing the H2 concentration to 100 mL/L increased the productivity of the catalyst, but a further increase in H2 reduced the activity of the catalyst. Increasing the propylene pressure from 1 to 7 bar significantly increased the polymer yield. The isotacticity index (II) decreased with increasing TEA, but the H2 concentration, temperature, and monomer pressure did not have a significant effect on the II value. The viscosity‐average molecular weight decreased with increasing temperature and with the addition of H2. Three catalysts with different Mg/Si molar ratios were studied under the optimum conditions. The catalyst with a Mg/Si molar ratio of approximately 0.93 showed the highest activity. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1177–1181, 2003

Catalytic Polymerization Of Ethylene, 1,4-Butadiene, and Ethylene Oxide with No Solvent: Heat Effects, Kinetics, and Mechanisms

The processes of solid polyethyleneoxide (PEO), polyethylene (PE), and trans-1,4-polybutadiene (PB) formation at solid catalysts in gas-solid systems with no solvent are studied. For a monomer pressure of 760 Torr, maximum rates of 1.5·10 m 7 , 1.2·10 m 5 , and 4·10 m 7 (mol/s cm 3 ) are fixed for C 2 H 4 O, C 2 H 4 , and 1,4-C 4 H 6 polymerization, respectively; therewith, potentialities of intensification of the reactions are not exhausted. The j H 298 0 values measured for the reaction (MON) (gas) =(1/n)(-MON-) n(solid) (MON is a monomer molecule) are m 107.5 - 2.5, m 112.5 - 2.5, and m 140.5 - 3.8 (kJ/mol) for PE, PB, and PEO, respectively.

Influences of copolymerization conditions on the structure and properties of isotactic polypropylene/ethylene–propylene random copolymer in situ blends

A spherical TiCl4/MgCl2-based catalyst was used in the synthesis of in situ isotactic polypropylene/ethylene–propylene random copolymer blends by propylene bulk polymerization and subsequent gas-phase copolymerization of ethylene with propylene. Different copolymerization conditions, such as the reaction time, monomer pressure, and composition, were investigated, and their influences on the structure and properties of the products were studied. Raising the monomer pressure was the most effective way of speeding up the copolymerization, but it caused more increases in the random copolymer than the block copolymer fractions. Increasing the ethylene content of the monomer feed also resulted in higher reaction rates and copolymer contents, but the ethylene contents of both the random and block copolymer fractions were also raised. In situ blends that contain more than 50 wt % copolymer were prepared. The mechanical properties of the blends, including the impact strength and flexural modulus, were regulated in a rather broad range with changes in the copolymerization conditions. The properties were highly dependent on the amount, distribution, and chain structure of the copolymer fractions. The impact strength was influenced by both the random copolymer and block copolymer portions in a complicated way, whereas the flexural modulus was mainly determined by the amount of random copolymer. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 445–453, 2002; DOI 10.1002/app.10415

Vaporization of LnCl 3 and thermochemistry of Ln 2Cl 6(g), Ln=Ce, Pr, Nd, Dy

The vaporization of LnCl 3, Ln=Ce, Pr, Nd, Dy, was investigated by Knudsen effusion mass spectrometry. The monomer and dimer partial pressures in the equilibrium vapor of the compounds were determined. Molecular parameters of LnCl 3(g) and Ln 2Cl 6(g) were determined theoretically and the thermodynamic functions of these species were computed applying statistical thermodynamics. The enthalpies of sublimation of LnCl 3(g) were evaluated according to the second- and third-law methods. Recommended values of Δ sub H o(298)=334.8±7.2 kJ mol −1 (CeCl 3), 323.3±3.5 kJ mol −1 (PrCl 3), 317.3±6.4 kJ mol −1 (NdCl 3), and 305.3±6.3 kJ mol −1 (DyCl 3), resulted for the sublimation enthalpies of gaseous species given in brackets by compilation of the present study with the data available in the literature. The enthalpies of sublimation Δ sub H o(298)=443.2±12.0 kJ mol −1 (Ce 2Cl 6), 418.7±12.1 kJ mol −1 (Pr 2Cl 6), 414.8±11.3 kJ mol −1 (Nd 2Cl 6), and 354.1±11.3 kJ mol −1 (Dy 2Cl 6), were obtained for dimeric species by the use of the third law method.

Influences of copolymerization conditions on the structure and properties of isotactic polypropylene/ethylene–propylene random copolymer in situ blends

AbstractA spherical TiCl4/MgCl2‐based catalyst was used in the synthesis of in situ isotactic polypropylene/ethylene–propylene random copolymer blends by propylene bulk polymerization and subsequent gas‐phase copolymerization of ethylene with propylene. Different copolymerization conditions, such as the reaction time, monomer pressure, and composition, were investigated, and their influences on the structure and properties of the products were studied. Raising the monomer pressure was the most effective way of speeding up the copolymerization, but it caused more increases in the random copolymer than the block copolymer fractions. Increasing the ethylene content of the monomer feed also resulted in higher reaction rates and copolymer contents, but the ethylene contents of both the random and block copolymer fractions were also raised. In situ blends that contain more than 50 wt % copolymer were prepared. The mechanical properties of the blends, including the impact strength and flexural modulus, were regulated in a rather broad range with changes in the copolymerization conditions. The properties were highly dependent on the amount, distribution, and chain structure of the copolymer fractions. The impact strength was influenced by both the random copolymer and block copolymer portions in a complicated way, whereas the flexural modulus was mainly determined by the amount of random copolymer. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 445–453, 2002; DOI 10.1002/app.10415

POLYETHYLENEOXIDE: KINETICS AND POLYMERIZATION HEAT MEASUREMENTS FOR THE PROCESS OF CATALYTIC POLYMERIZATION IN A SOLID-GAS SYSTEM FREE OF SOLVENT

The process of polymerization of ethylene-oxide (EO) vapor to solid polyethyleneoxide (PEO) at the dry KOH catalyst with no solvent is studied at 298 K and a monomer pressure up to 400 Torr. The maximum rate observed is about 1.5·10-7 mol/s per 1 cm3 of the catalyst at 760 Torr; therewith, potentialities of intensification of the process are not exhausted. It is found that, at any fixed degree of polymerization, the rate of the process is proportional to the monomer pressure. A phenomenon of gradual increasing in the rate of polymerization with the degree of polymerization is observed. The ΔH0298 value measured for the reaction n (EO) (gas) = (-EO-) n(solid) is equal to -140.5 ± 3.8 kJ/mol; by using this value, ΔH (liq. EO Π sol.. PEO) = -109.20 kJ/mol is computed. The calorimetric results and the nature of the phenomenon revealed are discussed.

Plasma etching and plasma polymerization coating of carbon fibers. Part 2. Characterization of plasma polymer coated carbon fibers

Unsized AS-4 carbon fibers were subjected to RF plasma etching and/or plasma polymerization coating in order to enhance their adhesion to vinyl ester resin. Ar, N2 and O2 were utilized for plasma etching, and acetylene, butadiene and acrylonitrile were used for plasma polymerization coating. Etching and coating conditions were optimized in terms of plasma power, treatment time, and gas (or monomer) pressure by measuring the interfacial adhesion strength. Interfacial adhesion was evaluated using micro-droplet specimens prepared with vinyl ester resin and plasma etched and/or plasma polymer coated carbon fibers. Surface modified fibers were characterized by SEM, XPS, FT-IR, α-Step, dynamic contact angle analyzer (DCA) and tensile strength measurements. Interfacial adhesion between plasma etched and/or plasma polymer coated carbon fibers and vinyl ester resin was reported previously (Part 1), and characterization results are discussed is this paper (Part 2). Gas plasma etching resulted in preferential etching of the fiber surface along the draw direction and decreased the tensile strength, while plasma polymer coatings altered neither the surface topography of fibers nor the tensile strength. Water contact angle decreased with plasma etching, as well as with acrylonitrile and acetylene plasma polymer coatings, but did not change with butadiene plasma polymer coating. FT-IR and XPS analyses revealed the presence of functional groups in plasma polymer coatings.

Lipase-catalyzed synthesis of aliphatic polyesters in supercritical carbon dioxide

AbstractSynthesis of aliphatic polyesters has been examined using Candidaantarctica lipase in supercritical carbon dioxide (scCO2). The enzymatic ring-opening polymerization of ε-caprolactone (ε-CL) proceeded in scCO2 to give a polymer with molecular weight higher than 104. Effects of reaction parameters such as the monomer concentration, enzyme amount and pressure have been systematically investigated. Copolymerization of ε-CL with 12-docecanolide afforded the random copolymer. The enzymatic polycondensation of divinyl adipate and 1,4-butanediol also proceeded in scCO2 to produce the corresponding polyester.

A New Class of Ethylene—Propylene—Diene Terpolymers Produced from Constrained Geometry Metallocene Catalysts

Abstract Until recently, ethylene/propylene/diene (EPDM) terpolymers have been manufactured exclusively using vanadium based Ziegler—Natta catalyst systems. The vanadium based catalysts are known to have both product and process limitations. These limitations are due to a complex combination of factors including monomer reactivity, catalyst activity, reactor temperatures and pressures. Single-site metallocene constrained geometry catalyst (CGC) technology enables the manufacture of EPDM terpolymers at higher reactor temperature and catalyst efficiency than standard Ziegler—Natta catalyst systems. The unique combination of monomer, catalyst and process technology enables the manufacture of highly crystalline EPDMs having ethylene contents over 90 wt %. This study investigated these new highly crystalline EPDM terpolymers and their differentiation from commercially available EPDMs. The results indicated that these new EPDMs had very high green strength and barrier properties, yet could be cured by conventional vulcanization techniques. The cure response of these EPDMs to either peroxide or sulfur increased with increasing ethylene content. Sulfur cured vulcanizates had much higher tensile strength than peroxide cured vulcanizates. These new EPDMs demonstrated utility as additives to enhance the abrasion, hot tear, and tensile properties of natural rubber compounds. Sponge compounds, having the unique combination of polyethylene physical properties and the utility of sulfur vulcanization were developed from these new polymers.

Bimodal Polyethylenes Obtained with a Dual-Site Metallocene Catalyst System. Effect of Trimethylaluminium Addition

Bimodal polyethylenes were obtained with the dual site Cp*2ZrCl2(1)/Et(IndH4)2ZrCl2(2) metallocene catalyst system with a mixture of methylaluminoxane (MAO) and trimethylaluminium (TMA) as the cocatalyst. Polymer properties can be controlled by the amount of TMA added, monomer pressure, polymerization temperature and the addition of hexene or hydrogen. TMA is suggested to be partly coordinated to the active sites, thereby enhancing termination (1), increasing comonomer incorporation (2), but also partially blocking coordination and chain transfer to hydrogen. For the ansa catalyst, hydrogen probably relieves dormant (β-agostic) sites.

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.