Low Monomer Conversion Research Articles

Role of surfactant in composite latex particle morphology

AbstractComposite particles were prepared by seeded emulsion polymerization at 70°C using K2S2O8 as initiator and two different nonionic surfactants. Monodisperse polystyrene latex particles were used as seed and methyl methacrylate was used as second‐stage monomer. When the surfactant, polyethylene oxide–propylene oxide (Pluronic F‐108), was used, the final particle morphology showed that the PMMA (core) was partially covered by polystyrene. However, when nonyphenol polyethylene oxide (Igepal Co‐990) was used as surfactant, one observed a reversed type of encapsulation (i.e., PS core is partially engulfed by PMMA). The interfacial tensions of the polymer phase against water containing the appropriate surfactant were measured by the drop‐volume method and used in a mathematical model based on thermodynamic analysis to predict the equilibrium particle morphology. The observed particle morphologies were found to differ from the predicted morphologies at low conversion of the second‐stage monomer but agreed with it at higher conversion.

Kinetics of the copolymerization of the donor-acceptor monomers

A general equation for the copolymerization rate at low monomer conversion has been proposed on the basis of theoretical considerations concerning the monomers'capability of forming intermolecular complexes. Modifications for the case of linear or quadratic termination and for the case of the bifunctional alkyl monomers have also been proposed. An algorithm for the solution of these equations has been prepared, and the values of the rate constants of elementary reactions of chain growth for the two systems have been determined

Asymmetric Membrane Formation in the Heterogeneous Polymerization of Methyl Methacrylate

AbstractA mathematical model is presented to describe the fundamental phenomena occurring in the vicinity of the interface between a polymerizing medium and a non-solvent. The model combines the principles of polymerization kinetics, mass transfer and polymer solution thermodynamics and is applied to the polymerization of methyl methacrylate with water. Numerical simulations indicate the development of large interface velocities and the formation of a dense polymer skin at very low monomer conversions in agreement with experimental observations.

Radical concentrations in free radical copolymerization of MMA/EGDMA

Radical concentration histories for the bulk free radical copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) initiated with 0.3 wt% 2,2'-azobis(2-methyl-propionitrile) (AIBN) at 70°C over the entire composition range were measured with an on-line electron spin resonance spectrometer. Radical concentrations of 10 −7−10 −3moll −1 were observed. At high EGDMA levels, the radical concentration increases monotonously from the onset of polymerization. Even in the glassy state, the radical concentrations continue to increase significantly. Once high concentrations of radicals which are chemically bound to the crosslinked network form, they are stable to temperature increases above the glass transition temperature. These stable radicals can propagate with a newly added monomer to the swollen gel. For low EGDMA levels (<25wt%) there exist four stages of radical concentration change with reaction time (monomer conversion): (1) the radical concentration remains relatively constant for low monomer conversions; (2) at some intermediate conversion there is a dramatic rise; (3) after reaching a peak concentration there is a small decrease; (4) the concentration then increases gradually and levels off. The dramatic rise in the second stage is in agreement with the hypothesis that the autoacceleration of reaction rate in a free radical polymerization is due to an increase in the radical concentration. An analysis of the radical concentration rise rates reveals that the use of the quasi-stationary state hypothesis (QSSH) at high levels of crosslinking is not valid. These radical concentration measurements coupled with the corresponding conversion rate data give a direct estimate of the propagation constant ( K p). The propagation constant was found to fall slightly at low conversions, but to fall dramatically at some high conversion where the conversion rate approaches zero. This explains why the polymerization rate of a bulk free radical polymerization run at T< T gp (the glass transition temperature of the polymer) falls to almost zero even though appreciable monomer and initiator still exist in the reacting mass. The fall in K p is in turn due to a radical trapping effect, i.e. the radical centres are surrounded by a solid environment consisting of polymeric segments. Using the assumption that bimolecular termination at high conversions is controlled by ‘propagation diffusion’, with K t∝ K p[M], the initiation efficiency ( f) and the termination rate constant ( K t) were also calculated from the data. The initiation efficiency was found to fall almost simultaneously with the propagation rate constant.

Kinetics of group transfer polymerization of tert‐butyl methacrylate in tetrahydrofuran

AbstractThe reaction kinetics for the group transfer polymerization (GTP) of tert‐butyl methacrylate (TBMA) using a silyl ketene acetal initiator and a nucleophilic catalyst are investigated. The reaction is shown to be of first order in both monomer and catalyst concentrations. The “livingness” of this system appears to be influenced by the reaction temperature. At temperatures above −20°C, deactivation is observed, with its severity increasing with increasing temperature. This deactivation is attributed to a depletion of catalyst by side reactions. It was demonstrated that reactivation is made possible by the addition of more catalyst. This result is in contrast to the anionic polymerization of TBMA, where no deactivation was observed even at ambient temperature. At temperatures below −20°C no deactivation is observed; however, at these temperatures, the reactions manifest induction periods with lengths increasing with decreasing temperature. The rate constants are lower than those for the GTP of methyl methacrylate (MMA) by a factor of 1,5 to 2. The following Arrhenius parameters were obtained for the propagation rate constants: activation energy, Ea = (19,1 ± 3) kJ/mol, preexponential factor, log10 A = (7,05 ± 0,3). These values are comparable with those obtained for MMA. The molecular weight distributions are similar to those obtained in the GTP of MMA, i.e. the ratio of weight‐to number‐average molecular weights is rather high for low monomer conversions and narrows to M̄w/M̄n ≥ 1,3 for full conversion. This is attributed to the rates of the catalyst exchange equilibrium.

ESR study of the polymerization of methyl methacrylate photoinhibited by butyl nitrite

The polymerization of methyl methacrylate in the presence of the photoinhibitor butyl nitrite is studied. The photoinhibitor does not affect the initiated polymerization, but under the action of ultraviolet light it generates spin traps, which interact with the macroradicals forming nitroxyl radicals, which can be recorded by ESR. It is shown that the main growth radical both a at low and high monomer conversions is the ∼CH 2C(CH 3)  COOCH 3 radical. The formation of radicals of ∼C(CH 3) (COOCH 3)  CH 2 is not observed.

Parametric sensitivity in tubular polymerization reactors

A recent technique for studying the parametric sensitivity of chemical reactors is applied to tubular chain homopolymerization reactors. The sensitivities of the temperature maxima with respect to various parameters of the model are computed. Using conditions typically encountered for high-pressure polyethylene systems, it is found that the temperature sensitivities with respect to all the nine parameters have their maxima at approximately the same value of the feed initiator concentration, thus leading to a generalized sensitivity-based constraint for design. It is also found that, under usual conditions of operation, no significant design constraints on the feed temperature are indicated. Detailed sensitivity plots are presented, which could be used to obtain “safe” operating conditions. The effects of changing the most important parameters, the dimensionless heat of reaction and the dimensionless activation energy (ϵ), on the sensitivity envelope are also investigated. Our studies reveal that better estimates of ϵ than are available presently are required. Sensitivities of the number-average chain length maxima with respect to the same nine parameters are also computed. Under conditions where the steady-state hypothesis applies, estimates of these sensitivities can be obtained analytically. However, for the usual values of the parameters, and close to “sensitive” values of the feed initiator concentration, this hypothesis does not apply, and the chain length sensitivities need to be obtained numerically. In the absence of the gel effect, chain length sensitivities do not usually provide design constraints because of the very low monomer conversions encountered.

Free radical polymerization of p-methyl styrene

An experimental investigation is reported of the free-radical synthesis kinetics of poly( p-methyl styrene) in cyclohexane at low monomer conversion in the temperature range 50°–70°C using AIBN initiator. The p-methyl styrene monomer contained greater than 97% paraisomer. Isothermal conversion/time curves were obtained using glass ampoule reactors and gravimetry and molecular weight distribution and weight-average molecular weights were measured by s.e.c. and low angle laser light scattering photometry ( LALLSP). Transfer to small molecules (monomer, cyclohexane, —) was found to be small or negligible with virtually all of the polymer chains formed by termination by combination. Predicted and measured weight-average molecular weights are in good agreement for polymer synthesized at low conversions.

Synthesis and Characterization of Poly(vinyl Chloride-g-α-Methylstyrene)- A New Thermoplastic Resin. 2

Abstract α-Methylstyrene was grafted on PVC using alkylaluminum Et2AlCl, Et3Al, and Me3Al coinitiators and CH2Cl2 as solvent or suspending agent. Grafting was shown to be feasible using both solution and suspension techniques. High grafting efficiency and low monomer conversion were obtained using Et3Al and Me3Al, while low grafting efficiency but high conversion could be obtained using Et3AlCl coinitiator. This paper discusses the synthesis, characterization and some mechanical properties of α-methyl-styrene grafted PVC.

Electropolymerization : direct film formation on metal substrates-I: Kinetics and mechanism

Attempts have been made to produce, in situ, polymer films on tinplate cathodes by the electrolysis of conducting solutions of vinyl monomers for use in the can-lacquering industry. Study of a range of vinyl monomers revealed that film formation occurs at low monomer conversion only in the electrolysis of acrylonitrile and methacrylonitrile in NN'-dimethyl formamide. The highest rates of film formation were obtained by constant current electrolysis when tetraethyl ammonium p-toluene sulphonate (McKee Salt) was used as electrolyte. The rate of film formation increases with monomer concentration to a maximum and then falls rapidly. Chain propagation occurs by an anionic mechanism with ion pair formation favoured at high monomer concentrations. The physical properties of the coloured films produced rarely approach those required industrially and the method does not represent an alternative approach to the lacquering of food and beverage cans.

High performance gel permeation chromatography of polystyrene

Microparticulate crosslinked polystyrene packings in short columns have been investigated with high performance liquid chromatography instrumentation. Reliable molecular weight data for six polystyrene standards having narrow molecular weight distributions and for a polystyrene having a broad distribution have been obtained by optimizing the injection procedure, using a constant flow pump, and incorporating an internal standard into each injected solution. Experimental determinations of the dependence of the polydispersity for polystyrene standards on eluent flow rate and polymer diffusion coefficient were in agreement with a relation predicted from theoretical considerations of chromotogram broadening. Because of the dependence of chromatogram broadening on polystyrene molecular weight, high efficiency separations for high polymers were only obtained at low eluent flow rates. For low polymers, high efficiency separations may be performed at fast eluent flow rates. It was concluded that accurate molecular weight distributions can only be determined from chromatograms obtained at low eluent flow rates, which was supported by experimental measurements of polydispersity on polystyrene sample prepared by a radical polymerization at low monomer conversion. A differential weight distribution calculated from an experimental chromatogram for the polydisperse polystyrene determined at the lowest eluent flow rate (0.1 cm 3min −1) was compared with distributions predicted theoretically for polystyrenes prepared by radical polymerization. It was concluded that the experimental distribution contained a small contribution from chromatogram broadening and that most of the radicals in the polymerization of styrene terminated by combination.

Cationic polymerization of α‐methylstyrene from polydienes. I. Synthesis and characterization of poly(butadiene‐g‐α‐methylstyrene) copolymers

AbstractThe preparation of poly(butadiene‐g‐α‐methylstyrene) copolymers was investigated with three different alkylaluminum coinitiators. The alkylaluminum compounds in conjunction with polybutadiene which contained a low concentration of labile chlorine atoms initiated the polymerization of α‐methylstyrene to produce graft copolymers. Trimethylaluminum gave higher grafting efficiencies than diethylaluminum chloride at comparable monomer conversions. Triethylaluminum produced only very low monomer conversions (&lt;5%), even at long reaction times, and for this reason was not studied extensively. The number of grafts per polybutadiene backbone was determined for a number of copolymers and found to increase slightly as the allylic chlorine concentration in the polybutadiene backbone was increased. In all cases, however, only a low percentage of the available labile chlorine sites along the polybutadiene backbone resulted in grafted α‐methylstyrene side chains. The addition of small quantities of water to the polymerization solvent greatly enhanced the grafting rate and ultimate monomer conversion during the synthesis of these poly(butadiene‐g‐α‐methylstyrene) copolymers. The mechanistic role of water during these grafting reactions is unknown at the present time.

Kinetic Study of Cationic Polymerization of α-Methylstyrene Initiated by BuOTICl3In Dlchloromethane Solution

Abstract The cationic polymerization of α-methylstyrene Initiated by n-BuOTiCl3 has been studied at -70° C in dlchloromethane solution by using a calorlmetric technique. Polymerizations were performed under high vacuum either in dry conditions for which low monomer conversions were observed (4-12%), or In the presence of added cocatalyst (H2O or HCl). In these last cases, yields were quantitative, and it was shown that polymerization rate was proportional to added water concentration and first order with respect to catalyst and to monomer. A kinetic scheme is proposed, based on a monomer-Independent initiation step and on a unimolecular termination process. At -70° C, the initiation rate is higher than termination rate during the whole course of the polymerization, and the concentration of active centers increases continuously. The following rate constants were found at -70°C: ki. = 17 ± 6, k = 2.2 ± 1.1 ± 104,ktrm = 30 ± 15 liter/mole-Sec, and kt =0 54 ± 0 05 sec−1. At -50 and -30° C, the concentration of active centers goes through a maximum during the polymerization and incomplete monomer conversions were observed, showing that all the catalyst is consumed. The different rate constants were tentatively estimated at these temperatures by using a simulation method, and this led to a negative value of ca. -7 kcal/mole for the apparent activation energy for propagation, and to a value of ∼ 5 kcal/ mole for Ei. The observation of a negative (Ep)app might be explained either by a shift of the dissociation equilibrium of the growing ends or by a solvation process of these growing ends by monomer prior to the propagation step.

Cationic polymerization of siloxanes kinetically controlled oligomerization in hexamethylcyclotrisiloxane – linear dimethylsiloxane systems

AbstractThe kinetically controlled distribution of linear oligomers in the cationic solution polymerization of hexamethylcyclotrisiloxane (D3) in the presence of hexamethyldisiloxane (MM), octamethyltrisiloxane (MDM), and decamethyltetrasiloxane (MD2M) as molecular chain stoppers was investigated by gas chromatography. At an early stage of the process the distribution was practically unaffected by consecutive reactions involving original stable products of oligomerization, which renders the distribution an important tool in the studies of the mechanism of cationic polymerization of siloxanes. Triple pattern distribution (the excess of every third homologue of the MDnM series) is observed even at nearly full monomer conversion. The distribution of this excess oligomers at low monomer conversion is highly specific as the amounts of each subsequent homologue is decreased by a constant factor. The factor is related to the relative reactivity of the siloxane bonds in the chain stopper and in the monomer. In principle, the results agree with the chain mechanism of polymerization involving kinetic chain transfer to the molecular chain stopper.Studies of the kinetically controlled distribution of linear oligomers in the oligomerization involving a chain stopper of unsymmetrical structure with respect to the reacting siloxane bond allow to determine the specificity of the combination of stopper fragments with a linear molecule. Low specificity showed by MDM constitutes evidence against the mechanism involving a silicenium ion intermediate.