Methyl methacrylate limiting conversion

Bulk Polymerization. This is the method of choice for the manufacture of poly(methyl methacrylate) sheets, rods, and tubes, and molding and extmsion compounds. In methyl methacrylate bulk polymerization, an auto acceleration is observed beginning at 20—50% conversion. At this point, there is also a corresponding increase in the molecular weight of the polymer formed. This acceleration, which continues up to high conversion, is known as the Trommsdorff effect, and is attributed to the increase in viscosity of the mixture to such an extent that the diffusion rate, and therefore the termination reaction of the growing radicals, is reduced. This reduced termination rate ultimately results in a polymerization rate that is limited only by the diffusion rate of the monomer. Detailed kinetic data on the bulk polymerization of methyl methacrylate can be found in Reference 42. 

In the literature there is only one serious attempt to develop a detailed mechanistic model of free radical polymerization at high conversions (l. > ) This model after Cardenas and 0 Driscoll is discussed in some detail pointing out its important limitations. The present authors then describe the development of a semi-empirical model based on the free volume theory and show that this model adequately accounts for chain entanglements and glassy-state transition in bulk and solution polymerization of methyl methacrylate over wide ranges of temperature and solvent concentration. 

Although the potassium superoxide route can be universally applied to various alkyl methacrylates, it is experimentally more difficult than simple acid hydrolysis. In addition, limited yields do not permit well-defined hydrophobic-hydrophilic blocks. On the other hand, acid catalyzed hydrolysis is limited to only a few esters such as TBMA, but yields of carboxylate are quantitative. Hydrolysis attempts of poly(methyl methacrylate) (PMMA) and poly(isopropyl methacrylate) (PIPMA) do not yield an observable amount of conversion to the carboxylic acid under the established conditions for poly(t-butyl methacrylate) (PTBMA). This allows for selective hydrolysis of all-acrylic block copolymers. 

Schultz (7) has studied the methyl methacrylate polymerization, which is interesting to compare with effects noted in the poly (vinyl chloride)-styrene graft polymerization. When his polymerizations were carried out well below the glassy transition temperature, the conversions reached limiting values. Monomer present in the system functioned as a plasticizing agent, allowing polymerization to occur up to the point... 

The available data from emulsion polymerization systems have been obtained almost exclusively through manual, off-line analysis of monomer conversion, emulsifier concentration, particle size, molecular weight, etc. For batch systems this results in a large expenditure of time in order to sample with sufficient frequency to accurately observe the system kinetics. In continuous systems a large number of samples are required to observe interesting system dynamics such as multiple steady states or limit cycles. In addition, feedback control of any process variable other than temperature or pressure is impossible without specialized on-line sensors. This note describes the initial stages of development of two such sensors, (one for the monitoring of reactor conversion and the other for the continuous measurement of surface tension), and their implementation as part of a computer data acquisition system for the emulsion polymerization of methyl methacrylate. 

To synthesize water-soluble or swellable copolymers, inverse heterophase polymerization processes are of special interest. The inverse macroemulsion polymerization is only reported for the copolymerization of two hydrophilic monomers. Hernandez-Barajas and Hunkeler [62] investigated the copolymerization of AAm with quaternary ammonium cationic monomers in the presence of block copoly-meric surfactants by batch and semi-batch inverse emulsion copolymerization. Glukhikh et al. [63] reported the copolymerization of AAm and methacrylic acid using an inverse emulsion system. Amphiphilic copolymers from inverse systems are also successfully obtained in microemulsion polymerization. For example, Vaskova et al. [64-66] copolymerized the hydrophilic AAm with more hydrophobic methyl methacrylate (MMA) or styrene in a water-in-oil microemulsion initiated by radical initiators with different solubilities in water. However, not only copolymer, but also homopolymer was formed. The total conversion of MMA was rather limited (<10%) and the composition of the copolymer was almost independent of the comonomer ratio. This was probably due to a constant molar ratio of the monomers in the water phase or at the interface as the possible locus of polymerization. Also, in the case of styrene copolymerizing with AAm, the molar fraction of AAm in homopolymer compared to copolymer is about 45-55 wt% [67], which is still too high for a meaningful technical application. 

Continuons emulsion polymerization is one of the few chemical processes in which major design considerations require the use of dynamic or unsteady-state models of the process. This need arises because of important problems associated with sustained oscillations or limit cycles in conversion, particle number and size, and molecular weight. These oscillations can occur in almost all commercial continuous emulsion polymerization processes such as styrene (Brooks et cl., 1978), styrene-butadiene and vinyl acetate (Greene et cl., 1976 Kiparissides et cl., 1980a), methyl methacrylate, and chloropene. In addition to the undesirable variations in the polymer and particle properties that will occur, these oscillations can lead to emulsifier concentrations too low to cover adequately the polymer particles, with the result that excessive agglomeration and fouling can occur. Furthermore, excursions to high conversions in polymer like vinyl acetate... 

Esterification is finally an equilibrium reaction (35 per cent methyl methacrylate), which can be continued to completion by removing one or both of the products obtained as soon as they are formed. It takes place preferably in the liquid phase, in the presence of sulfuric acid or cation exchange resins as a catalyst, with a slight excess of methanol (1.2/1 in mol), at temperatures (110 to 115°Q apd pressures (30 to 50 kPa absolute) designed to limit polymerization reactions. The addition of an inhibitor (such as hydro-quinone) is also practised. With residence time of about 1 h. once-through conversion is total and the molar yield is close to 99 per cent. 

The mechanism by which monomer is transported between miniemulsion droplets and from these to polymer particles was the focus of a study using styrene and methyl methacrylate as comonomers [7]. The SLS/HD surGactant/cosurfactant system was used in this work. Several approaches were applied in an attempt to determine the diffusive mass-transfer coefficients of the monomeis as well as the contribution of monomer droplef polymer particle collision to the monomer transport process. Mass transfer was shown not to be a limitation in miniemulsion polymerization (i.e. equilibrium was satisfied under normal conditions). However, the thermodynamic model developed previously [6] predicted the continued existence of monomer droplets until the end of a miniemulsion polymerization provided that not all droplets were nucleated. Experimentally though, nucleation has been shown to end well before complete conversion which implies another route for monomer droplet disappearance, namely, collision. 

Bon and coworkers carried out a study on the fate of the nanoparticles throughout solids-stabilized emulsion polymerization [119], A quantitative method based on disk centrifugation was developed to monitor the amount of nanoparticles present in the water phase in solids-stabilized emulsion polymerizations of vinyl acetate, methyl methacrylate, and butyl acrylate. The concentration profile of nanoparticles in the water phase as a function of monomer conversion agreed with theoretical models developed for the packing densities in these systems [120]. Noteworthy was that in the case of silica-nanoparticle-stabilized emulsion polymerization of vinyl acetate, the event of late-stage limited coalescence, leading to small armored non-spherical clusters, could be predicted and explained on the basis of the concentration profiles and particle size measurements. Adjusting the amount of silica nanoparticles prevented this phenomenon. 

Using variable power of microwave irradiation, Boey et al. showed that bulk polymerization of methyl methacrylate (MMA) was faster by ca. 130-150% compared with the conventional methods (Fig. 4) [35]. Also, the limiting conversion... 

Radical polymerization is initiated by a free radical, which subsequently adds to a vinyl or diene monomer to produce a propagating radical. To obtain information about the structure and concentrations of initiating and propagating radicals in radical polymerizations, use of ESR spectroscopy has called the interest of physical or polymer chemists. However, ESR measurements on these radicals in solution poly merizati on were found to be difficult, except for the case where polymers precipitated, because otherwise the concentrations of the radicals were too low. Thus, these measurements had to be limited to polymerization systems in highly viscous solutions or in the solid state, where the disappearance of free radicals by bimolecular reactions is suppressed. Bresler et al. -i7) succeeded for the first time in obtaining ESR spectra of free radicals which were produced in homogeneous bulk polymerization of methyl methacrylate (MMA), methyl acrylate (MA) and vinyl acetate (VAc) at conversions of 50-60% (in the gel state). 

Pistoia has used electrochemically generated nitrate radicals to effect the bulk polymerization of acrylonitrile the system shows a remarkable postpolymerization effect which is affected by such factors as the anode material, current, temperature, stirring, electrolysis time, and HNO, concentration. Radical occlusion phenoma and the formation of oligomers limit the monomer to >oly-mer conversion. Pistoia has also reported the polymerization of acrylonitrile by the oxidation of sulphuric acid at the anode and has extended the work to the anodic polymerization of methyl methacrylate in methanol-sulphuric acid... 

Several groups have also reported the synthesis of polystyrene [RAN 07], poly(methyl methacrylate) [VEE 11] and poly(styrene-o/t-maleic anhydride) [WU 03] via photoinitiated RAFT polymerization. However, this method exhibits some limitations such as low conversions even at long... 

The difficulty results, in part, from the fact that only a small fraction of the chemical bonds, generally less than one in a thousand, are involved in me-chanochemical processes. The concentration of connecting units is therefore at the detection limit and below for traditional analytical methods such as conventional nuclear magnetic resonance and infrared spectroscopy. The sensitivity can, of course, be enhanced by techniques such as cumulative, multiple scans, Fourier transform analysis, and difference techniques for detection to one part in ten thousand and better. It may yet be difficult to determine whether polymers are linked by chemical bonds or whether they are simply intimate mixtures. For this distinction, other tests can be of value. For example, the difference between blocks and blends for ethylene-propylene polymer systems has been distinguished by thermal analysis [5]. In many cases, simple extraction tests can distinguish between copolymers and blends. For example, for rubber milled into polystyrene, the fraction of extractable rubber is a measure of mechanochemistry. Conversely, only the rubber in this system is readily cross-linked by benzoyl peroxide after which free polystyrene may be conveniently extracted [6]. In another case, homopolymers of styrene and methyl methacrylate can be separated cleanly from each other and from their copolymers by fractional precipitation [7]. The success of such processes, of course, depends on both the compositions and molecular weights involved. 

For the polymerization of tert-butyl acrylate, the monomer consumption followed the first-order kinetics, while that of MMA could be described with a kinetics model that includes the persistent radical effect. The control over the reaction could be preserved for monomer conversions of up to 90%, and poly(methyl methacrylate) s (PMMAs) with narrow molecular weight distributions (PDI below 1.3) were obtained. Conventional experiments with an oil bath showed a limited reproducibility and furthermore failed to yield polymers with similar narrow molecular weight distributions (for high conversions). This observation was refereed to the superiority of the uniform, noncontact, and internal heating mode of micro-wave irradiation. 

Montaudo [11, 13, 14] also described a new method for fully characterising copolymers, which is based on off-line SEC-NMR and SEC-MALDI. It was applied to the analysis of random copolymers reacted at high conversions. The method involves fractionation of the copolymers by size exclusion chromatography, analysis of the fractions by NMR and MALDI mass spectroscopy and derivation of bivariate distribution of composition of the fractions. These copolymers include copolymers containing units of methyl methacrylate, butyl acrylate, styrene and maleic anhydride. Perspectives and limitations of the technique are also considered. 

Methyl methacrylate Figure 4 presents values obtained by relaxation experiments for k tor methyl methacrylate at 50 C at Wp values in the range 0.3 - 0.95 (7). Also presented in this figure are the theoretical estimates for the upper and lower bounds for residual termination. It should first be noted that the absolute magnitude of the measured values of k (lO -lO dm mol S ) is several orders of magnitude smaller than the literature value (- 10 dm3 mol- s- reported for low conversions (11). In the Wp range studied, it seems reasonable to infer that residual termination is the major radical annihiliation mechanism operative. The flexible limit for residual termination seems to be the more appropriate in these systems, at least at the lower values of Wp studied. This limit seems physically reasonable. 

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