Online Monitoring Of Polymerization Reactions Research Articles

Continuous Monitoring and Characterization of Copolymerization Reactions of Acrylate Monomers with Indistinguishable Ultraviolet Spectra Using Infrared Spectroscopy

AbstractApplications of Automatic Continuous Online Monitoring of Polymerization reactions (ACOMP) have steadily expanded over the past 20 years. An ultraviolet (UV) detector, often combined with a differential refractive index detector (RID), is the usual method to track monomer conversion, but the method can fail when comonomers have similar UV spectra. Here, Fourier‐transform infrared spectroscopy (FTIR) is coupled to ACOMP to separate comonomer conversion during solution copolymerization of tert‐butyl acrylate (tBA) and n‐butyl acrylate (nBA). The UV spectra of these comonomers are too similar for separation. The FTIR region between 1100 and 1410 cm−1, however, shows a strong difference between tBA and nBA peaks—1403 and 1192 cm−1, respectively. Reaction rates determined from FTIR data show that tBA's is higher than nBA's. The terpolymerization of tBA, nBA, and methyl methacrylate (MMA) is also monitored, using MMA's 1325 cm−1 peak. Reaction rates are significantly lower than in the nBA/tBA copolymerization. The incorporation of FTIR into ACOMP opens the path for further expansion to a wide variety of copolymerization reactions, including systems of three or more comonomers. It can be a key new detector in the next steps for development of fully automatic feedback control of composition and molecular weight.

Rapid Online Analysis of Photopolymerization Kinetics and Molecular Weight Using Diffusion NMR.

Online, high-throughput molecular weight analysis of polymerizations is rare, with most studies relying on tedious sampling techniques and batchwise postanalysis. The ability to track both monomer conversion and molecular weight evolution in real time could underpin precision polymer development and facilitate study of rapid polymerization reactions. Here, we use a single time-resolved diffusion nuclear magnetic resonance (NMR) experiment to simultaneously study the kinetics and molecular weight evolution during a photopolymerization, with in situ irradiation inside the NMR instrument. As a model system, we used a photoinduced electron transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization. The data allow diffusion coefficients and intensities to be calculated every 14 s from which the polymer size and monomer conversion can be extracted. Key to this approach is (1) the use of shuffled gradient amplitudes in the diffusion NMR experiment to access reactions of any rate, (2) the addition of a relaxation agent to increase achievable time resolution and, (3) a sliding correction that accounts for viscosity changes during polymerization. Diffusion NMR offers a uniquely simple, translatable handle for online monitoring of polymerization reactions.

Kinetic analysis of continuous reaction data for RAFT and free radical copolymerization with acrylic and styrenic monomers

Free radical (FR) and Reversible Addition Fragmentation Chain Transfer (RAFT) copolymerization rates for styrenic monomers in bulk and aqueous reactions were accelerated by acrylic comonomers, whose own rates were both delayed and decreased. The effect was found with styrenics as low as 0.6% by mole, and is directly observable from Automatic Continuous Online Monitoring of Polymerization reactions (ACOMP) data. Rapid radical transfer from acrylic to styrenic comonomer, through lower energy radical resonance stabilization in styrenics, was the suspected cause. FR and RAFT rates were equal, suggesting that FR termination controlled radical concentrations and rates. Despite strongly changing concentrations of initiator and comonomer radicals during reactions, the Quasi-Steady-State Approximation was demonstrated to hold. While rates were identical, molecular weights showed the characteristic increase with conversion for RAFT and decrease for FR. A new, generalized method for reactivity ratio determination was found and acceleration/deceleration behavior was consistently interpreted within a penultimate model.

Coupling of NMR to ACOMP for Terpolymerization Monitoring and Control

AbstractMonitoring and control of free radical copolymerization using automatic continuous online monitoring of polymerization reactions (ACOMP) with UV detection has been recently achieved. It is difficult, however, to achieve spectral separation of similar monomers with UV alone. Here, nuclear magnetic resonance (NMR) is coupled to ACOMP for the first time and terpolymerization reactions involving acrylamide (Am), sodium acrylate (Ac), and styrene sulfonate (SS) are monitored, and a first attempt at active composition control is made. The NMR resolves the concentrations of Am and Ac, while the UV resolves SS and the sum of Am and Ac. NMR analysis is performed in water, using signal suppression, to eliminate the need for deuterated solvents. From this, instantaneous fractions of each comonomer in the terpolymer are continuously known, along with weight average molecular weight and intrinsic viscosity, IV. Am and Ac have similar reactivities, whereas the reactivity ratio of SS is much larger. Hence, there is high composition drift in batch polymerization and SS is rapidly consumed, leaving a final copolymer of Am and SS to form. Maintaining constant terpolymer composition is taken as a first step toward active control.

A geometric observer design for a semi-batch free-radical polymerization system

In this work, a geometric observer (GO) is formulated and tested using experimental data from the Automatic Continuous Online Monitoring of Polymerization reactions (ACOMP) system for a semi-batch free-radical polymerization reactor that synthetizes polyacrylamide. The available measurements include the weight-average molecular weight, the concentration of monomer, and the volume of internal contents. Different combinations between innovated states and measurements offer a number of possible structures of the GO. The computation of the minimum singular values and condition numbers permit to select the most promising structures. To overcome inadequacies observed in full-order architectures, low order GOs with passive structures are explored. The best observer is selected, compared with a standard estimation strategy and tested under various experimental operating conditions. The observer performance is evaluated qualitatively and quantitatively as a tradeoff between dynamic property estimation and signal processing.

Framework design for weight-average molecular weight control in semi-batch polymerization

A framework that embraces a state-of-the-art sensor, multi-objective dynamic optimization, nonlinear state estimation and control, is designed and implemented to achieve target weight-average molecular weight trajectories. The Automatic Continuous Online Monitoring of Polymerization reactions (ACOMP) is combined for the first time with a nonlinear state observer for full polymer characterization and signal processing. A hybrid variation of the discrete-time extended Kalman filter (h-DEKF) is formulated based on an auto-tuning procedure that uses a stochastic global optimization technique. A number of optimal policies are generated and experimentally tested. Results are provided through investigations into the free-radical aqueous polymerization of acrylamide using potassium persulfate as initiator.

Online Optimal Feedback Control of Polymerization Reactors: Application to Polymerization of Acrylamide–Water–Potassium Persulfate (KPS) System

This paper deals with the design and implementation of optimal real-time control strategies for controlling polymerization reactors in free radical polymerization processes. A multiobjective optimization problem is first formulated to determine optimal trajectories for a range of target products. A tailor-made control module is then formulated and implemented in python for controlling the weight-average molar mass (Mw) to robustly achieve the desired polymer molar mass distribution (MMD). Multiple control case studies are discussed and applied to force the system along the optimal targets. Performance of the controller is evaluated in each case using Automatic Continuous Online Monitoring of Polymerization reactions (ACOMP). Results are provided through investigations into the free radical polymerization of the acrylamide in water using potassium persulfate (KPS) as an initiator.

Polysaccharide depolymerization from TEMPO-catalysis: Effect of TEMPO concentration

Polysaccharide TEMPO-oxidation was monitored using automatic continuous online monitoring of polymerization reactions (ACOMP). The products of oxidation, obtained at different pHs (9, 7 and 5) and different concentrations of catalyst TEMPO, were evaluated by Automatic Continuous Mixing (ACM) and Size Exclusion Chromatography (SEC). The degree of oxidation was higher at pH 9 and polysaccharide degradation was observed under different pH conditions, but was much higher without catalyst TEMPO. The rate constant (k) was dependent on reaction pH and TEMPO concentration. The amount of −COOH per g of polysaccharide, at pH 9, in the presence and absence of TEMPO was different, 0.215 and 0.395mmolg−1, respectively. This suggested a secondary and non-selective polysaccharide oxidation occurring at a lower rate in the absence of catalyst. TEMPO protects the polysaccharide from degradation caused by secondary oxidant species, acting as a catalyst and “sacrificial molecule” at higher concentrations.

Simultaneous Monitoring of the Effects of Multiple Ionic Strengths on Properties of Copolymeric Polyelectrolytes during Their Synthesis

A new Automatic Continuous Online Monitoring of Polymerization reactions (ACOMP) system has been developed with multiple light scattering and viscosity detection stages in serial flow, where solution conditions are different at each stage. Solution conditions can include ionic strength (IS), pH, surfactants, concentration, and other factors. This allows behavior of a polymer under simultaneous, varying solution conditions to be monitored at each instant of its synthesis. The system can potentially be used for realtime formulation, where a solution formulation is built up additively in successive stages. It can also monitor the effect of solution conditions on stimuli responsive polymers, as their responsiveness changes during synthesis. In this first work, the new ACOMP system monitored light scattering and reduced viscosity properties of copolymeric polyelectrolytes under various IS during synthesis. Aqueous copolymerization of acrylamide (Am) and styrene sulfonate (SS) was used. Polyelectrolytes in solution expand as IS decreases, leading to increased intrinsic viscosity (η) and suppression of light scattering intensity due to electrostatically enhanced second and third virial coefficients, A2 and A3. At a fixed IS, the same effects occur if polyelectrolyte linear charge density (ξ) increases. This work presents polyelectrolyte response to a series of IS and changing ξ during chemical synthesis.

Automatic Synthesis of Multimodal Polymers

An automatic molecular weight controller is used in semi‐batch reactions to produce final polymers that contain distinct subpopulations with different molecular weight distributions (MWD). While blending polymers with different MWD is frequently used to make multimodal polymer products, a method is introduced here allowing the multimodal product to be made in successive, automatically controlled stages in the same reactor. The method is demonstrated using free radical polymerization of acrylamide to produce widely separated multimodal MWD. Automatic Continuous Online Monitoring of Polymerization reactions with a Control Interface (ACOMP/CI) was used. Weight average molecular weight (Mw) in the ACOMP/CI was continuously measured with multi‐angle light scattering and ultra‐violet absorption. The controller used two principles: both polymerization rate and instantaneous molecular weight are proportional to monomer concentration. By controlling monomer flow rate into the reactor, the controller produced a targeted amount of polymer with a desired first Mw and then automatically introduced chain transfer agent (CTA) into the reactor to produce a second population of polymers with much lower Mw. Subsequently, reactions were performed to produce trimodal populations. This approach is kindred to multi‐stage synthesis of polymers where distinctly different processes are carried out in succeeding stages to produce polymers with highly specific properties. image

Automatic Control of Polymer Molecular Weight during Synthesis

The continuous record of monomer and polymer concentrations, Cm and Cp, and cumulative weight-average mass, Mw, furnished by automatic continuous online monitoring of polymerization reactions (ACOMP) has been harnessed to provide feedback to control reactor monomer flow in order to follow a target trajectory Mw,t(t) during linear chain growth free radical polymerization. This was achieved without a detailed kinetic model. Two proportionality parameters to pilot the controller, α and p, result from (i) reaction rate = αCm and (ii) Mw,inst = pCm, where Mw,inst is instantaneous Mw. Using Ansatz values for α and p, the controller periodically recomputes these, based on the ACOMP data stream, in order to follow Mw,t(t). A histogram of concentration vs Mw,inst estimates the molecular weight distribution width. Invoking an instantaneous distribution provides polydispersities. Results are compared to GPC analysis on end products. The concept of “isomorphic reaction pair” is introduced: two reactions that follow t...

Online, continuous monitoring of the sensitivity of the LCST of NIPAM-Am copolymers to discrete and broad composition distributions

The lower critical solution temperature (LCST) of NIPAM/Acrylamide copolymers was monitored during synthesis using second generation ACOMP (automatic continuous online monitoring of polymerization reactions) for a series of discrete starting compositions. A full sweep of NIPAM/Am compositions was subsequently obtained in a semi-batch feed of NIPAM into Am, allowing LCST behavior to be assessed over all compositions of the two copolymers. A separate Simultaneous Multiple Sample Light Scattering (SMSLS) instrument was used to assess compositional polydispersity via the broadening of the LCST temperature transition as compositional heterogeneity of samples increases. This was achieved by combining LCST data as a function of composition and added salt into a master equation. Extended period SMSLS monitoring of the aggregates formed when the LCST is reached showed they are reversible on the short time scale of hours, but are irreversible for homopolymer NIPAM with salt and for copolymers on the scale of days.

Simultaneous multiple sample light scattering detection of LCST during copolymer synthesis

This work has two objectives in the investigation of polymer solution lower critical solution temperature (LCST): First, to develop a new instrument to monitor LCST onset during copolymer synthesis by coupling several thermostatted light scattering flow cells to the output stream of an ACOMP system (Automatic Continuous Online Monitoring of Polymerization reactions). Second, to use this to investigate effects on LCST when N-isopropylacrylamide (NIPAM) is copolymerized with a charged comonomer, styrene sulfonate (SS). This comonomer pair has widely separated reactivity ratios. SS is rapidly consumed, yielding composition drift toward NIPAM rich polymer. High content SS chains inhibited LCST, and SS was dramatically effective at suppressing LCST down to copolymers of 5% molar SS in 10 mM NaCl aqueous solvent. LCST for higher content SS chains was elevated and required significant additional ionic strength to occur. It was determined that the suppression of the LCST by SS is chiefly an intramolecular effect.

Polymerization Online Monitoring

AbstractOnline monitoring of polymerization reactions is not only important due to the high exothermic nature of most polymerization systems; the data gained also provides information on product composition and quality and control possibilities thereof. Many inline and online methods are still in development and are better suited for application on the laboratory or pilot‐plant scale. Industrial polymerization plant environments pose additional technical and financial challenges and constraints for the use of such systems. Available methods and current developments are reviewed with regard to their practicability and usefulness under these aspects.

Online monitoring of the copolymerization of 2-(dimethylamino)ethyl acrylate with styrene by RAFT. Deviations from reaction control

Kinetics of copolymerization reactions of 2-(dimethylamino)ethyl acrylate (DMAEA) and styrene by reversible addition fragmentation transfer (RAFT) in N,N-dimethylformamide (DMF) are reported. Novel approaches in the online monitoring of the synthesis of the amphiphilic copolymers by Automatic Continuous Online Monitoring of Polymerization reactions (ACOMP) are presented. Automatic withdrawal of separate reactor streams and their subsequent dilution throughout the reaction with organic and aqueous solvents, respectively, allowed different features to be captured. Thus, light scattering data combined with spectroscopic and viscometric measurements in DMF, together with conductivity measurements in aqueous medium provided in real time comonomer conversion, copolymer mass, reduced viscosity, and composition. Continuous data gathered allowed observing and quantifying the DMAEA-linked macroRAFT control agent decomposition, with significant effects on the reaction kinetics. Deviations from controlled behavior were investigated during (co)polymerization reactions under different conditions.Multi-detector Size Exclusion Chromatography (SEC) and NMR were used to bring additional information on the system investigated.

Fundamental Measurements in Online Polymerization Reaction Monitoring and Control with a Focus on ACOMP

AbstractFundamental measurements in online polymerization reaction monitoring and control seek to avoid empirical and inferential models in data interpretation. One such approach making use of multiple detectors is automatic continuous online monitoring of polymerization reactions (ACOMP), wherein a continuous reactor stream is automatically, continuously diluted and conditioned to where measurements reflect intrinsic particle properties and not the interactions that can dominate measurements in concentrated media. Examples where dilute regime measurements are needed include static and dynamic light scattering, and reduced viscosity. This review focuses on ACOMP to illustrate a number of reaction contexts where fundamental measurements are used to gain a comprehensive picture of reaction characteristics.magnified image

New micro viscosity sensor—A novel analytical tool for online monitoring of polymerization reactions in a micro reaction plant

The authors developed within a project funded by the German government a micro reaction plant which allows to produce polymers out of up to 8 monomers by using the Suzuki coupling method. Micro reactors can have a positive influence on the quality of the polymer product. Especially the mixing quality of the monomers, the catalyst solution, and end capper solutions have a great influence on the molecular weight distribution. With micro reactors it is possible to have a very narrow distribution which is not achievable in typical batch processes. To monitor the product quality a new sensor was developed to measure online the product viscosity and with this the molecular weight of the polymer.

Online Monitoring of Molecular Weight and Other Characteristics during Semibatch Emulsion Polymerization under Monomer Starved and Flooded Conditions

Reaction kinetics in semibatch emulsion polymerization of methyl methacrylate, were monitored by automatic continuous online monitoring of polymerization reactions (ACOMP). Semibatch reagent feed to the reactor at different flow rates allowed the transition between the monomer starved and flooded regimes and the approach to and maintenance of near steady state conditions to be monitored and quantified, and yielded relationships between flow regimes and monomer conversion kinetics, polymer weight-average molecular weight Mw, and intrinsic viscosity (η)w. Polymer molecular weight distributions togetherwithlatexparticlesizeandnumberplayanimportantroleindefiningthemacroscopiccharacteristics and performance of polymeric materials. The change in the monomer feed rates during reaction allowed simultaneous influence on and verification of the evolution of these parameters during synthesis. When flow rate changes to the reactor were made it was verified that the characteristic rate of achieving the new pseudosteady state was close to the first order reaction rate. Multidetector size exclusion chromatography was used to cross-check results and measure full distributions of endproducts. Latex polymer size was measuredwithdynamiclightscattering.AnimportantadvanceforthisfieldisthatpolymerMwisdetermined continuously during the reaction and in an absolute fashion, based on multiangle light scattering detection available.

Predictive Control of Average Composition and Molecular Weight Distributions in Semibatch Free Radical Copolymerization Reactions

Control of copolymer composition and molecular weight distributions is important, since they partially govern material performance. This work uses a simple but robust approach to approximate predictive control of average composition and molecular weight during free radical copolymerization, based on detailed kinetics obtained by ACOMP (automatic continuous online monitoring of polymerization reactions). Using these kinetics and elementary computations as a guide, the trends in the composition and molecular weight drifts during synthesis can be approximately controlled by predetermined, constant reagent flows into the reactor (“semibatch” operation). Two comonomer systems were chosen: one comonomer pair with very different reactivity ratios, having a tendency to high composition drift, and the other one with much closer reactivity ratios resulting in relatively constant composition during batch copolymerization. Each system had high downward molecular weight drift during synthesis, typical of free radical polymerization. Appropriate feed polices were found to control the composition and molecular weight drift tendencies at will; i.e., it was possible to keep composition constant in the first case and to increase or decrease composition in the latter, while desired trends for the average molecular weight were also obtained. The approach allows practical, approximate composition and molecular weight control in free radical copolymerization with simple reactors and pumps and without computationally intensive requirements. The results can also be useful as an Ansatz for reaction trajectory prediction and, together with the online monitoring signals, can be used for feedback controlled corrections to the reagent flows and other reaction conditions.

Predictive control and verification of conversion kinetics and polymer molecular weight in semi-batch free radical homopolymer reactions

Polymer molar mass distributions critically affect macroscopic characteristics and performance of polymeric materials. While multi-detector methods coupled to size exclusion chromatography (SEC) are widely used to measure endproduct mass distributions, less progress has been made in simultaneously controlling and verifying the evolution of these distributions during synthesis. This work focuses on quantitative predictions and online verification of conversion kinetics and of molecular weight during free radical homopolymerization of acrylamide, where reagents were fed into the reactor during the reaction. The central task is to establish and experimentally test a formalism combining free radical polymerization kinetics with time dependent processes related to flows of material into and out of the reactor. Monomer feed experiments were performed that alternately hold molecular weight constant and ramp the weight up, in contrast to batch reactions, where molecular weight decreases. Three types of initiator feed ‘tapers’ were also used to produce predictable conversion kinetics and mass distributions: (i) constant initiator feed, (ii) linearly stepped feed to produce Gaussian conversion kinetics, and (iii) booster shots to produce multi-modal masses. Automatic Continuous Online Monitoring of Polymerization reactions (ACOMP) was used to follow the conversion and evolution of the average mass distribution, and multi-detector SEC was used to cross-check results and measure full distributions of endproducts. In general, there was good agreement between the predictions and results. In future work this approach can be used as an Ansatz for reaction trajectory prediction, and the online monitoring signals exploited to make feedback controlled corrections to the reagent flows and other reaction conditions.