Polymerization conversion monitoring

For the purposes of conversion monitoring of emulsion polymerization, we have found the DMA40D with a precision of +1 x 10 g/cm capable of resolving monomer conversion to +0.2% in the absence of thermostating and sampling errors. On-line, in the presence of such possible errors, a resolution of at least +0.5% can be expected. Care must be taken to ensure a representative sample and good temperature control of the sample stream before introducing it into the instrument. Some example results with this instrument are presented below. 

Real Time (RT-) FTIR spectroscopy pormits not only to follow quantitatively the polymerization by monitoring the disappearance of the IR absorption characteristic of the polymerizable reactive groups (acrylates, methacrylates, epoxy rings, vinyl ether double bonds, thiol groups etc.) but also to determine at any moment the actual degree of conversion and hence the residual imreacted groups content. This analytical method has proved extremely valuable for measuring the polymerization rates and quantum yields of reactions that develop in the millisecond time scale. 

Other non-invasive (e.g., calorimetry and ultrasounds) and invasive (e.g., densimetry) techniques can also be used for monitoring of monomer concentration in homopolymerization reactions. However, in multimonomer formulations the individual monomer concentrations cannot be obtained with these techniques, meaning that a state estimator is required [66, 67, 69]. The use of reaction calorimetry is appealing because the hardware is very cheap and, when coupled with a state estimator, provides good estimation of the monomer concentration. The performance of calorimetry was compared with that of the Raman spectroscopy in emulsion polymerization to monitor overall and individual monomer conversions [68]. Calorimetry was as good as FT-Raman spectroscopy when monomer concentrations in the reactor were relatively high, but the performance of calorimetry was poorer when monomer concentrations were low. 

Cherfi, A. and G Fevotte, On-Line Conversion Monitoring of the Solution Polymerization of Methyl Methacrylate Using Near-Infrared Spectroscopy. MocromoZ. Chem. Phys., 2002. 203 1188-1193. 

Cherfi A, Fevotte G. On-line conversion monitoring of the solution polymerization of methyl methacrylate using near-infrared spectroscopy. Macromol Chem Phys 2002 203 1188-1193. 

Alhamad B, Romagnoh JA, Gomes VG. Inferential conversion monitoring in emulsion co-polymerization through calorimetric measurements. Int J Chem React Eng 2006 4 A7,1-10. 

Striking support of this contention is found in recent data of Castro (16) shown in Figure 14. In this experiment, the polymerization (60-156) has been carried out in a cone-and-plate viscometer (Rheometrics Mechanical Spectrometer) and viscosity of the reaction medium monitored continuously as a function of reaction time. As can be seen, the viscosity appears to become infinite at a reaction time corresponding to about 60% conversion. This suggests network formation, but the chemistry precludes non-linear polymerization. Also observed in the same conversion range is very striking transition of the reaction medium from clear to opaque. 

Bauer et al. describe the use of a noncontact probe coupled by fiber optics to an FT-Raman system to measure the percentage of dry extractibles and styrene monomer in a styrene/butadiene latex emulsion polymerization reaction using PLS models [201]. Elizalde et al. have examined the use of Raman spectroscopy to monitor the emulsion polymerization of n-butyl acrylate with methyl methacrylate under starved, or low monomer [202], and with high soUds-content [203] conditions. In both cases, models could be built to predict multiple properties, including solids content, residual monomer, and cumulative copolymer composition. Another study compared reaction calorimetry and Raman spectroscopy for monitoring n-butyl acrylate/methyl methacrylate and for vinyl acetate/butyl acrylate, under conditions of normal and instantaneous conversion [204], Both techniques performed well for normal conversion conditions and for overall conversion estimate, but Raman spectroscopy was better at estimating free monomer concentration and instantaneous conversion rate. However, the authors also point out that in certain situations, alternative techniques such as calorimetry can be cheaper, faster, and often easier to maintain accurate models for than Raman spectroscopy, hi a subsequent article, Elizalde et al. found that updating calibration models after... 

Monitoring monomer conversion during emulsion polymerization... 

Chabot et al. at Atofina Chemicals (King of Prussia, PA, USA) used in-line NIR to monitor monomer conversion in real time in a batch emulsion polymerization process. The business value of this monitoring... 

The peak of double bonds disappears while intensity of the carbonyl group peak is independent of reaction time. Using this method, the conversion of double bonds vs. time was calculated to monitor polymerization of multiacrylate at two different temperatures (Figure 11.2). 

Fig. 14. Polymerization of methyl methacrylate (MMA) with the enolatealuminum porphyrin (2, R=Me)-triphenylboron [( 5445)36] system (TPP)AlMe (1, X=Me) as initiator ([MMA]q/[1 (X=Me)]o=200, [1 (X=Me)]o=16.2 mM), C6H5 as solvent, rt, (C6H5)3B added ([( 5445)36]q/[2 (R=Me)]o=1.0) after irradiation for 3h (9.3% conversion). GP profiles (T44F as eluent) of the polymerization mixture at 100% conversion (4 h), monitored with differential refractometer and UV (269 nm) detectors...

The first polymerizations reported by Kops and Schuerch147 were those of l,4-anhydro-2,3,6-tri-0-methyl-/3-D-galactopyranose and 1,4-anhydro-2,3-di-0-methyl-a -L-arabinopyranose. The latter compound was slightly contaminated with l,4-anhydro-2,3-di-0-methyl-a-D-xy-lopyranose, but the course of the polymerization could nevertheless be monitored reasonably accurately. For the most part, the polymerizations were conducted at 10% concentration (g/mL) in dichloro-methane, or aromatic hydrocarbons, with 1-5 mol% of phosphorus pentafluoride, or boron trifluoride etherate. At low temperature (—78 to —97°), the d.p. of both polymers produced was —90 at increasing temperatures of polymerization, termination processes became more severe, and the d.p. lower. Usually, the reaction times were long (perhaps unnecessarily so), and the conversions were 50 to 90%. The specific rotations of the D-galactans prepared at —28 and —90° differ by only —10° ( — 85 to — 95°), but those of the L-arabinans varied from + 6... 

It is important to monitor the time that the proanthocyanidin is allowed to react with the phloroglucinol solution. The products formed are not stable under acidic conditions, and it is therefore critically important that the reaction not exceed 20 min. Of particular concern are the flavan-3-ol monomers, which degrade more rapidly than the phloroglucinol adducts (Kennedy and Jones, 2001). Excessive degradation of the flavan-3-ol monomers will result in reduced amount of terminal subunits. This in turn will reduce the conversion yield and increase the average degree of polymerization calculated. 

The use of a precision digital density meter as supplied by Mettler Instruments (Anton Paar, Ag.) appeared attractive. Few references on using density measurements to follow polymerization or other reactions appear in the literature. Poehlein and Dougherty (2) mentioned, without elaboration, the occasional use of y-ray density meters to measure conversion for control purposes in continuous emulsion polymerization. Braun and Disselhoff (3) utilized an instrument by Anton Paar, Ag. but only in a very limited fashion. More recently Rentsch and Schultz(4) also utilized an instrument by Anton Paar, Ag. for the continuous density measurement of the cationic polymerization of 1,3,6,9-tetraoxacycloundecane. Ray(5) has used a newer model Paar digital density meter to monitor emulsion polymerization in a continuous stirred tank reactor train. Trathnigg(6, 7) quite recently considered the solution polymerization of styrene in tetrahydrofuran and discusses the effect of mixing on the reliability of the conversion data calculated. Two other references by Russian authors(8,9) are known citing kinetic measurements by the density method but their procedures do not fulfill the above stated requirements. 

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. 

This work has shown that by monitoring conversion curves by a computer, emulsifier metering can be varied to produce a desired particle size distribution of smalls in a seeded PVC emulsion polymerization. 

Contrary to viscosity measurements GPC provides number average molar mass data (Mn). For a few studies on the polymerization of BD with Nd-carboxylate-based catalyst systems GPC was systematically applied for the monitoring of Mn as a function of monomer conversion. In these studies three catalyst systems were used (1) NdV/DIBAH/EASC [178], (2) NdV/TIBA/EASC [179] and (3) NdO/TIBA/DEAC [188]. In the first two studies linear Mn-conversion plots were obtained at various molar ratios Al/ Ndv- In these studies, however, molar mass data at low monomer conversions (<20%) are lacking and positive intercepts on the Mn-axis were found. For the ternary catalyst system NdO/TIBA/DEAC used in the third of these studies the concentrations of Nd and TIBA were varied. A linear increase of Mw and Mn on monomer conversion was found. Deviations from linearity were also observed for low monomer conversions. 

In summary, it has to be mentioned that in many studies intrinsic viscosity, inherent viscosity and dilute solution viscosity (DSV) were used in order to monitor the increase of molar mass on monomer conversion. Unfortunately, only a few studies use GPC rather than viscosity measurements. For a few Nd-carboxylate-based catalyst systems linear dependencies of Mn on monomer conversion were established and proof in favor of requirement No. 2 linear increase of Mn with monomer conversion (no irreversible chain transfer) was provided. A more detailed analysis of the data, however, reveals deviations from linearity particularly at low monomer conversions (< 20%). These deviations are particularly pronounced for polymerizations with induction periods. Also the extrapolation of the straight lines to zero monomer conversion reveals intercepts on the Mn-axis. 

The oxidation of propylene oxide on porous polycrystalline Ag films supported on stabilized zirconia was studied in a CSTR at temperatures between 240 and 400°C and atmospheric total pressure. The technique of solid electrolyte potentiometry (SEP) was used to monitor the chemical potential of oxygen adsorbed on the catalyst surface. The steady state kinetic and potentiometric results are consistent with a Langmuir-Hinshelwood mechanism. However over a wide range of temperature and gaseous composition both the reaction rate and the surface oxygen activity were found to exhibit self-sustained isothermal oscillations. The limit cycles can be understood assuming that adsorbed propylene oxide undergoes both oxidation to CO2 and H2O as well as conversion to an adsorbed polymeric residue. A dynamic model based on the above assumption explains qualitatively the experimental observations. 

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