Eduardo Vivaldo-Lima

DETAILED MODELING, SIMULATION, AND PARAMETER ESTIMATION OF NITROXIDE MEDIATED LIVING FREE RADICAL POLYMERIZATION OF STYRENE *

A kinetic model based on a detailed reaction mechanism for the nitroxide-mediated radical polymerization (NMRP) of styrene is presented. The reaction mechanism includes the following reactions: chemical initiation, reversible nitroxyl ether decomposition, monomer dimerization, thermal initiation, propagation, reversible monomeric and polymeric alkoxyamine formation (production of dormant species), alkoxyamine decomposition, rate enhancement, transfer to monomer and dimer, as well as conventional termination. By simple manipulation of the ODEs initial conditions and tuning of the model by turning on/off the appropriate kinetic steps via their corresponding kinetic rate constants, the model presented here is capable of representing two technologically important variations of nitroxyl mediated polymerization techniques: 1) use of traditional radical initiator together with a nitroxide-type stable radical; and 2) use of a nitroxyl ether or alkoxyamine compound as controller. Model predictions are validated against experimental data provided by Ciba Specialty Chemicals, Inc. Parametric sensitivity analyses and non-linear estimation procedures are used to estimate the unknown kinetic rate constants. The overall agreement between model predictions and experimental data is generally good. The qualitative simulations and the detailed mechanistic discusssions presented here provide deeper insight into some of the aspects of NMRP processes not well understood to date. *An earlier version of this paper was presented at the Microsymposium on Polymer Reaction Engineering of the 7th Pacific Polymer Conference, Dec. 3–7, 2001, Oaxaca, México.

Controlled Free‐Radical Copolymerization Kinetics of Styrene and Divinylbenzene by Bimolecular NMRP using TEMPO and Dibenzoyl Peroxide

An experimental study on the kinetics of nitroxide‐mediated free radical copolymerization (NMRP) of styrene (STY) and divinylbenzene (DVB) is presented. The experiments were carried out in bulk from a mixture of monomers, stable free radical controller (2,2,6,6‐Tetramethyl‐1‐piperidinyloxy, TEMPO), and initiator (dibenzoyl peroxide, BPO), at 120°C, without using a TEMPO‐capped prepolymer in the initial mixture. The system studied is a case of bimolecular NMRP, as opposed to the monomolecular NMRP of styrene and other crosslinker previously addressed in the literature by others. The results on total monomer conversion (polymerization rate), molecular weight development, gel fraction, and swelling index are compared against a conventional reference system (a STY/DVB copolymer, also synthesized for this study). No significant auto‐acceleration effect was observed in the early and intermediate conversion ranges of the TEMPO‐controlled copolymerization of STY/DVB, and the gelation point was significantly delayed. †On research leave from UNAM.

Another Perspective on the Nitroxide Mediated Radical Polymerization (NMRP) of Styrene Using 2,2,6,6‐Tetramethyl‐1‐piperidinyloxy (TEMPO) and Dibenzoyl Peroxide (BPO)

Polymerization conditions for the bimolecular NMRP of styrene using TEMPO and BPO were revisited and expanded with the objective of creating a more complete and reliable source of experimental data for parameter estimation and model validation purposes. Three different experimental techniques were assessed for the NMRP of styrene. The reliability of results produced in vials with inert nitrogen atmosphere was evaluated, taking as reference the more reliable technique using sealed ampoules with inert atmosphere. Polymerization rate data obtained in vials could be considered reliable if monomer loss was taken into account, but the reliability of molecular weight data at high conversions may be questionable. Polymerizations at 120 and 130°C and with TEMPO to BPO, molar ratios of 0.9 to 1.5 were carried out. Comparison of the experimental data collected against predictions obtained with a detailed kinetic model previously reported in the literature suggest that either the present understanding of the reaction system is incomplete, or some of the kinetic rate constants reported in the literature are not accurate, or both. Guidelines on how to address and design future experimental and modeling studies are offered.

Assessing the importance of diffusion-controlled effects on polymerization rate and molecular weight development in nitroxide-mediated radical polymerization of styrene

A previously derived kinetic model for the nitroxide‐mediated radical polymerization (NMRP) of styrene has been modified by considering diffusion‐controlled (DC) effects on the bimolecular radical termination, monomer propagation, dormant polymer activation, and polymer radical deactivation reactions. Free‐volume theory was used to incorporate the DC‐effects into the model. It was found that DC‐termination enhances the living behavior of the system, whereas DC‐propagation, DC‐activation and DC‐deactivation worsen it. Although the inclusion of overall DC‐effects into the kinetic model improved the performance of the model by slightly reducing the deviations obtained from experimental data of polymerization rate and molecular weight in the bimolecular NMRP of styrene with 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) and dibenzoyl peroxide (BPO), it does not seem to justify adding the extra four free‐volume parameters. In the case of the semi‐batch addition of azo‐bis‐iso‐butyronitrile (AIBN) (several single shots at definite time intervals) in the NMRP of styrene, recently reported in the literature, it was found that DC effects are more significant, but it was observed that there was a strong dependence of polymerization rate on the frequency of addition of the shots of initiator (a maximum on polymerization rate being observed at a given frequency of addition of the shots), which could not be adequately explained in terms of DC‐effects.

RAFT Copolymerization of Styrene/Divinylbenzene in Supercritical Carbon Dioxide

An experimental study on the kinetics of the reversible addition–fragmentation chain transfer (RAFT) dispersion copolymerization with crosslinking of styrene and divinylbenzene in supercritical carbon dioxide (scCO2) is presented. This is the first time that such a controlled polymer network synthesis is carried out in scCO2. S-Thiobenzoyl thioglycolic acid (TBTGA) and dibenzoyl peroxide were used as RAFT agent and initiator, respectively. The polymerizations were carried out in a high pressure cell with lateral sapphire windows at 80°C. The effect of RAFT agent concentration, including the case without RAFT controller, on polymerization rate, molecular weight development, gel fraction, swelling index, and particle morphology was analysed.

Effect of Microwave Activation on Polymerization Rate and Molecular Weight Development in Emulsion Polymerization of Methyl Methacrylate

Polymerization rate and molecular weight development experimental results for the emulsion polymerization of methyl methacrylate (MMA), in the presence of potassium persulphate (PPS) as initiator, and activated with a 50 W microwave source, are reported. The polymerization kinetics of the microwave activation experiment (MA) was compared against a traditional conductive heating (CH) polymerization reaction. The number average molecular weights, Mn, of the polymer samples obtained with microwave activation were significantly higher than those obtained from conductive heating. These high values of Mn were obtained from the beginning of the polymerization reaction. Polydispersity index (PDI) values in the range of 1.18 to 1.83 were obtained in the microwave irradiated samples. These values are lower than those produced by conventional emulsion polymerization of styrene and other vinyl monomers, and resemble the PDI values obtained in controlled‐radical polymerization processes. Polymer particles of submicron size (60 to 100 nm) were obtained.

Modeling of the Microwave Initiated Emulsion Polymerization of Styrene

The emulsion polymerization of styrene, activated by microwave irradiation and conductive heating, was modeled using the Predici® simulation package of CiT. Microwave activated initiation was modeled as adding a second conventional free‐radical chemical initiator, whose concentration is given by the intensity of microwave irradiation, and its “decomposition” kinetic rate constant is related to the ratio of monomer concentration to the rate of absorbed radiation. Most of the kinetic rate constants and model parameters used in the model were taken from the literature, in order to avoid unnecessary parameter estimation procedures. Model predictions of conversion, number and weight average molecular weights, for microwave and thermally activated systems, agree well with the experimental data reported in the literature, including experimental data previously reported by our own group.

DEVELOPMENT OF A KINETIC MODEL FOR INIFERTER CONTROLLED/“LIVING” FREE-RADICAL POLYMERIZATION CONSIDERING DIFFUSION-CONTROLLED EFFECTS *

A kinetic model incorporating the effects of diffusion-controlled reactions on INIFERTER (initiator-transfer agent-terminator) thermal free-radical polymerization has been developed. Molecular weight development is done using the method of moments. Diffusion-controlled effects are modeled using free-volume theory. The reactions considered to be diffusion-controlled are: monomer propagation, bimolecular radical termination, dormant termination, chain transfer to monomer, and chain transfer to iniferter. Radical generation by thermal self-initiation is also included in the model. Model predictions indicate that diffusion-controlled propagation reduces the “living” behavior of the system, whereas a diffusion-controlled termination enhances its livingness. The livingness of the system is also enhanced by diffusion-controlled dormant termination. When diffusion-controlled termination is important, our simulations show that the overall effect of the diffusion controlled phenomena in iniferter polymerization is to enhance the livingness of the system. Experimental data from the literature for styrene, and methyl methacrylate homopolymerizations have been used to validate the kinetic model. The agreement is reasonably good. *An earlier version of this paper was presented at the Microsymposium on Polymer Reaction Engineering of the 7th Pacific Polymer Conference, Dec. 3–7, 2001, Oaxaca, México.

RAFT Copolymerization with Crosslinking of Methyl Methacrylate and Ethylene Glycol Dimethacrylate in Supercritical Carbon Dioxide

An experimental study on the kinetics of the reversible addition-fragmentation chain transfer (RAFT) copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) in supercritical carbon dioxide (scCO2) at 65°C and 172 bar is presented. Azobisisobutyronitrile (AIBN) was used as initiator, S-Thiobenzoyl thioglycolic acid as RAFT agent, and Krytox 257 FSL (Dupont) as stabilizer. A 38 mL, high pressure view cell, equipped with one frontal and two lateral sapphire windows, was used as the reacting vessel. The effect of RAFT agent concentration on polymerization rate, molecular weight development, gel fraction, swelling index and particle morphology was analyzed.

Characterization of Blue Agave Bagasse (BAB) as Raw Material for Bioethanol Production Processes by Gravimetric, Thermal, Chromatographic, X-ray Diffraction, Microscopy, and Laser Light Scattering Techniques

A detailed characterization of the main types of blue agave bagasse (BAB) obtained from the four largest tequila factories in the State of Jalisco (Mexico) is presented here. After milling/sieving the agave bagasses, two particle size fractions were identified, one rich in fibers and the other consisting of dust/fine particles. Both fractions were analyzed to determine the content of cellulose, hemicellulose, lignin, organic-soluble compounds, absorbed remaining sugars, minerals, and organic matter. After detailed analyses of both fractions by wet, thermal (thermo-gravimetric analysis (TGA)/differential thermo-gravimetric analysis (DTA)), and other methods (high-performance liquid chromatography (HPLC), microscopy, particle size by laser diffraction light scattering, and crystallinity by X-ray diffraction), a moderate-to-intensive method was devised for further processing the fibrous fraction, which had a high crystalline cellulose content, as well as for its subsequent enzymatic saccharification under well-defined moderate conditions. Alternative processing options were also devised for the dust/fine particle fraction, which has a moderate crystalline cellulose that is rich in adsorbed sugars and that has a high mineral matter content.