Molecular Weight Distribution Of Products Research Articles

Influence of Radical Distribution in a Polymer Particle during Emulsion Polymerization on Product Molecular Weight Distribution

Influence of Radical Distribution in a Polymer Particle during Emulsion Polymerization on Product Molecular Weight Distribution

Olefin oligomerization by main group Ga3+ and Zn2+ single site catalysts on SiO2

In heterogeneous catalysis, olefin oligomerization is typically performed on immobilized transition metal ions, such as Ni2+ and Cr3+. Here we report that silica-supported, single site catalysts containing immobilized, main group Zn2+ and Ga3+ ion sites catalyze ethylene and propylene oligomerization to an equilibrium distribution of linear olefins with rates similar to that of Ni2+. The molecular weight distribution of products formed on Zn2+ is similar to Ni2+, while Ga3+ forms higher molecular weight olefins. In situ spectroscopic and computational studies suggest that oligomerization unexpectedly occurs by the Cossee-Arlman mechanism via metal hydride and metal alkyl intermediates formed during olefin insertion and β-hydride elimination elementary steps. Initiation of the catalytic cycle is proposed to occur by heterolytic C-H dissociation of ethylene, which occurs at about 250 °C where oligomerization is catalytically relevant. This work illuminates new chemistry for main group metal catalysts with potential for development of new oligomerization processes.

Degradation of chitosan with self-resonating cavitation

For the degradation of chitosan, a novel physical method of self-resonating cavitation with strong cavitation effects was investigated in this paper. The effects of initial concentration, pH, temperature, inlet pressure and cavitation time on the degradation efficiency of chitosan were evaluated. It was found that the degradation efficiency was positively correlated with temperature and cavitation time, but was negatively correlated with the solution concentration. The degradation efficiency was maximized at pH of 4.4 and inlet pressure of 0.4 MPa. Under the experimental conditions, the intrinsic viscosity of chitosan solution was reduced by 92.2%, which was twice as high as the degradation efficiency where a Venturi tube cavitator was used. The viscosity-average molecular weights of initial and degraded chitosan were 651 and 104 kD, respectively. The deacetylation degree of chitosan slightly decreased from 89.34% to 88.05%. Structures and polydispersity of initial and degraded chitosan were measured by Fourier-transform infrared spectroscopy (FT-IR), nuclear magnetic resonance hydrogen spectroscopy (1H NMR), X-ray diffraction (XRD) and gel permeation chromatography (GPC). The results showed that the degradation process did not change the natural structure of chitosan. XRD peaks of the original chitosan were observed at 2θ of 9.59° and 20.00°, and the one at 2θ of 20.00° was obviously weakened after the degradation process, which indicated that the crystallinity of chitosan decreased significantly after the degradation. The polydispersity index of chitosan samples decreased from 3.17 to 2.75, indicating that the molecular-weight distribution of products after the degradation was more concentrated. The results proved that self-resonating cavitation prompted the degradation of chitosan and could reduce the polydispersity of the products for the production of oligochitosan with homogeneous molecular weights.

Study on mechanism of chitosan degradation with hydrodynamic cavitation.

Hydrodynamic cavitation is an effective method for chitosan degradation, of which the mechanism directly determines the molecular weight distribution of degradation products. In this study, based on the Monte Carlo simulation and experimental results, the mechanism of chitosan degradation with hydrodynamic cavitation and molecular weight distribution of products were analyzed. The results showed that the algorithm established in the simulation could effectively analyze degradation mechanism and the factors that influenced degradation mechanism and molecular weight distribution of products. The degradation with hydrodynamic cavitation was caused by chemical and mechanical effects, of which the former dominated the degradation process. The outlet and inlet angles and throat length of the cavitator had major and minor influences on the degradation pattern, respectively. The chemical effect led to random cuts resulting in wide distribution of the products, while the mechanical effect led to central cuts resulting in narrow distribution of the products. With more central cuts, the slide-shaped molecular weight distribution curve of degradation products was gradually transferred into a bell-shaped curve. These results provide instructions for researches on the molecular weight distribution of chitosan products degraded with hydrodynamic cavitation.

Selected Physicochemical and Pharmaceutical Properties of Poly-ε-caprolactone and Poly(d,l-lactide-co-ε-caprolactone) Conjugates of Lamivudine Synthesized via Ring-Opening Polymerization.

The modification of drug fate after administration may be achieved by the covalent coupling of active pharmaceutical ingredients with macromolecules. To prolong or delay the release, slowly degrading polymers such as polyesters may be applied for conjugation. The detachment of a covalently conjugated drug from the polymeric matrix relies mostly on the material degradation profile and barely on the weak interaction between the drug and macromolecules. In the present study, lamivudine was conjugated via ring-opening polymerization with poly-ε-caprolactone and poly(d,l-lactide-co-ε-caprolactone). The influence of the reaction parameters on the course of the polymerization and physicochemical properties of obtained conjugates were investigated. Subsequently, selected reaction products were formulated into submicron particles, and drug release profiles in physiological-like conditions were investigated. The course of the reaction was monitored via gel permeation chromatography. The structure and physicochemical properties of products were evaluated via spectroscopic, calorimetric, and diffractometric methods. The profile of the drug release from particles prepared by the slow evaporation of conjugate solution from o/w emulsion was monitored with high-performance liquid chromatography. Both an elevated reaction temperature and higher catalyst concentration increased the polymerization rate and simultaneously promoted the side reactions, resulting in a broad molecular weight distribution of products in the range from 1.30 to 2.15. The physicochemical properties of conjugates obtained in different conditions varied and had a direct influence on the drug release. The release curve of lamivudine from particles based on low molecular weight conjugates achieved a plateau between 18.9 and 22.2 μg per mg of conjugate within a month. Drug detachment from particles composed of high molecular weight conjugates exhibited a distinct delay period preceded by a drug burst release at a maximal level of 13.3 μg per mg of conjugate. Conjugate chemical composition and the degree of crystallinity were also found to influence the release.

The chain scission extent of polystyrene in different shear flow fields

A mechanochemical reactor providing strong shear‐force has been developed in order to graft the polar groups onto the polymers. The extent of polymers' chain scission is a key factor in grafting process. In order to evaluate the scission extent, the shearing experiments were carried out for GPPS152P, which were produced by SECCO Petrochemical Company, Shanghai, China, at different revolving speeds, and temperatures. The decrease in molecular weight of GPPS152P was evaluated through gel permeation chromatography method and was estimated to be 13.8%. Additionally, the molecular weight distribution (MWD) of this sample became narrower. The chain scission theory and the shear rate, simulated through POLYFLOW software, were simultaneously employed to conduct quantitative analysis of molten PS' chain scission process. According to this method, the decrease in molecular weight was estimated to be 12.88%. Therefore, this method can be taken as a guideline for estimating molecular weight and MWD of products in manufacturing industry. POLYM. ENG. SCI., 58:913–919, 2018. © 2017 Society of Plastics Engineers

Microwave radiation in the synthesis of urethane prepolymers

This paper describes the use of microwave radiation in the synthesis of urethane prepolymers in bulk (without solvent). The prepolymers were synthesized using 1,6-hexamethylene diisocyanate (HDI) and poly (ε-caprolactone) diol (PCL) at a molar ratio of 4:1. The reaction was carried out without a catalyst in the reactor with a conventional heating system (oil bath) or in the microwave reactor (MW Reactor NOVA 09) at temperatures of 60, 80 and 100°C. The progress of the reaction was monitored based on the degree of HDI and PCL conversion, as assessed by acidimetric titration of non-bonded isocyanate groups (0–120min). The chemical structure of the resulting products was determined via Fourier Transform Infrared Spectroscopy. Size exclusion chromatography (GPC) was applied to determine the molecular weight distribution of products at different time points of prepolymer synthesis. The obtained results indicate that the use of microwave radiation as a heat source during prepolymer synthesis accelerates the reaction in the temperature range from 60 to 80°C. This finding allows a significant reduction in energy required for heating the reaction mixture. The acceleration of prepolymer synthesis by the application of microwave radiation as a heat source also allows for the elimination of catalysts from the system. The aforementioned catalysts are necessary to perform the prepolymer synthesis with the use of aliphatic diisocyanates and polyols, particularly at temperatures below 100°C. In the case of prepolymers that are further used in the synthesis of biomedical polyurethanes, the catalyst elimination is especially advantageous as the presence of residual catalyst may significantly influence the final material’s biocompatibility and disturb host cell viability.

The Modeling of Residue Solvent Deasphalting Product Molecular Weight Distribution with Continuous Thermodynamics

A product molecular weight distribution calculating model of residue solvent deasphalting has been built based on continuous thermodynamics and combined with simplified perturbed hard chain theory equation of state (SPHCT EOS). Deasphalted oil (DAO) and deoiling asphalt (DOA) molecular weight distribution as well as DAO yield (w) of butane solvent deasphalting have been obtained with the model under different temperature, pressure, and solvent ratios for a specific vacuum residue. Results indicated that the model was feasible and could be used as a basis to predict product properties and optimize operating conditions as well as as a foundation to build other refinery process models with continuous thermodynamics.

Modeling Nd‐Catalyzed Butadiene Rubber Production

AbstractSummary: A simulation tool was developed for the industrial solution polymerization of 1,3‐butadiene with a Nd‐based homogeneous Ziegler‐Natta catalyst system. Insight into underlying reaction mechanisms was gained from laboratory experiments. Besides the chain growth reaction, the following steps were identified: catalyst formation, deactivation reactions, and molecular weight control reactions. A kinetic model based on this reaction scheme was developed to quantitatively describe butadiene conversion and product molecular weight distribution. By including process characteristics, the laboratory (batch) model was transferred to the industrial production process. A correlation function relates product molecular weight to the relevant product property Mooney viscosity. This polymerization model was successfully applied, e.g. to optimize product grade transitions and to maintain high product quality by predicting the influence of process changes.

Synthesis of isomaltooligosaccharides and oligodextrans in a recycle membrane bioreactor by the combined use of dextransucrase and dextranase

A recycle ultrafiltration membrane reactor was used to develop a continuous synthesis process for the production of isomaltooligosaccharides (IMO) from sucrose, using the enzymes dextransucrase and dextranase. A variety of membranes were tested and the parameters affecting reactor stability, productivity, and product molecular weight distribution were investigated. Enzyme inactivation in the reactor was reduced with the use of a non-ionic surfactant but its use had severe adverse effects on the membrane pore size and porosity. During continuous isomaltooligosaccharide synthesis, dextransucrase inactivation was shown to occur as a result of the dextranase activity and it was dependent mainly on the substrate availability in the reactor and the hydrolytic activity of dextranase. Substrate and dextranase concentrations (50-200 mg/mL(-1) and 10-30 U/mL(-1), respectively) affected permeate fluxes, reactor productivity, and product average molecular weight. The oligodextrans and isomaltooligosaccharides formed had molecular weights lower than in batch synthesis reactions but they largely consisted of oligosaccharides with a degree of polymerization (DP) greater than 5, depending on the synthesis conditions. No significant rejection of the sugars formed was shown by the membranes and permeate flux was dependent on tangential flow velocity.

Molecular Weight Distribution Design with Living Polymerization Reactions

It has been previously shown that it is feasible to control the molecular weight distribution (MWD) of polymers produced from living polymerization processes in flow reactors through the control of reactant feeds. Here, attention is given to the problem of establishing an inverse process model as a step toward a fully automatic control strategy for the synthesis of polymers with predefined MWD. Particular attention is given to the prediction of instantaneous reactor feed conditions for a specified product MWD. The reactor is modeled as an ideal continuous stirred tank reactor with constant monomer concentration in the reactor. The way in which a real laboratory-scale polymerization system can be developed from this approach is outlined.

Thermal pyrolysis of polypropylene: effect of reflux-condenser on the molecular weight distribution of products

When polypropylene is heated in an inert atmosphere and the volatile products are condensed, the molecular weight distribution (MWD) of the product obtained is very wide. In addition, this product is very waxy, has poor flow properties, and is unusable as a liquid fuel. To improve the product quality, ZSM-5 and other catalysts are usually used in the recycling process. However, these catalysts are costly. On the other hand, if the higher boiling fractions of the volatiles evolved due to degradation are not allowed to escape, then the MWD of the final product can be improved. This paper compares the effects of degradation with and without a reflux on the MWD of the product. Initial MWD of the polypropylene determined by GPC was used for all the simulations. In solving the governing population balance equation for the polymer degradation process, the rate constant for degradation has been fitted to the total weight loss of the polypropylene at a given temperature. The problems arising from the use of moment method with an assumed gamma-distribution are also discussed. A fixed-pivot discretization technique has been adopted for solving the governing equations.

Grafting on polysilanes using atom transfer radical polymerisation

AbstractAtom transfer radical polymerisation (ATRP) was used in an attempt to prepare a graft polymer based on poly(methylphenylsilane) (PMPS). A partially bromomethylated PMPS was subjected to ATRP conditions in order to grow poly(methyl methacrylate) (PMMA) grafts onto that backbone. Following the grafting reaction, a substantial increase in molecular weight was observed by size exclusion chromatography, analysis using refractive index detection. However, a bimodal product molecular weight distribution was observed using a UV detector set at wavelengths at which only the PMPS backbone can be detected. The lower molecular weight fraction was identified as homopolymeric PMPS although it appears to include some material with short grafts, while the higher molecular weight fraction corresponds to PMPS with long grafts. These observations are interpreted as the formation of a substantial amount of PMMA that is not grafted onto PMPS through a radical process initiated independently of the ATRP. However, since MMA is barely susceptible to thermal polymerisation, this is not easily rationalised.© 2002 Society of Chemical Industry

Investigations on the laccase-catalyzed polymerization of lignin model compounds using size-exclusion HPLC

The reaction of laccase from Trametes hirsuta with monomeric and dimeric lignin model compounds was studied with special attention to the formation of polymeric reaction products. Different HPLC techniques were evaluated for the analysis of the reaction products. Methods requiring a solvent change from aqueous to organic phase were found not to be adequate for the analysis of product mixtures since evaporation over nitrogen led to the polymerization of reactive monomers and extraction with dichloromethane was not quantitative in all cases. Therefore, aqueous phase size-exclusion chromatography (SEC) at high pH was used to characterize reaction product mixtures. The product molecular weight distribution (MWD) after reaction with laccase was observed to be highly dependent on the type of model compound whereas the enzyme concentration had only a limited impact.

LIVING POLYMERISATION REACTORS: MOLECULAR WEIGHT DISTRIBUTION CONTROL USING INVERSE NEURAL NETWORK MODELS

In principle, it is possible to exercise control over the molecular weight distribution (MWD) of the polymers produced from living polymerisation processes in flow reactors through the control of reactant feeds in a predetermined fashion. Some of the factors that influence the extent to which control can be achieved with feed perturbations to a single stage continuous flow stirred tank (CSTR) reactor have been reported previously. Here, attention is given to the problem of establishing inverse process models as a first step towards a fully automatic control strategy for the synthesis of polymers with pre-ordained MWD in a real process. Particular attention is given to the development of a neural network model for predicting the instantaneous reactor feed conditions for a specified product MWD and characterising the MWD for the purpose of dimension reduction using principal component analysis. Data collected from a simulated ideal reactor process are used in the study. The way in which this approach will u...

Investigations of restricted mass transport effects on hydrocarbon pyrolysis mechanisms through silica immobilization

In the pyrolysis of polymers and other macromolecular systems including coal, kerogen, lignin, and cellulose, the importance of mass transport in affecting pyrolysis kinetics, tar and char yields, and product molecular weight distribution is becoming increasingly appreciated. However, detailed molecular level information on the effects of restricted mass transport on the reaction mechanisms for organic constituents in these complex organic materials is understandably more limited. Well-designed, experimental studies of the pyrolysis of model compounds and polymers are capable of providing such insights. In this paper, research employing silica-immobilized compounds is reviewed and selected examples from other types of model systems that explore mass transport effects are presented.

A population balance approach for the description of particle size distribution in suspension polymerization reactors

There exist modelling and experimental methodologies to meet productivity (conversion and batch time) and product molecular weight distribution (MWD) quality specifications in suspension polymerization reactors. However, no counterpart exists to address the issue of particle size distribution (PSD), which is of fundamental importance in determining suspension stability and product quality attributes. In industrial practice, PSD considerations are guided by mean-size correlations from the Weber number theory. The setting up of an industrial reactor usually demands considerable scaling and testing efforts. In this work, we obtain a model to describe the evolution of the PSD in a suspension polymerization reactor. First, relevant phenomena are identified and modelled individually: suspension MWD, suspension viscoelasticity, interfacial tension, injection—dissipation of mechanical energy and drop breakage—coalescence mechanism. A central point is the derivation of analytic expressions for breakage and coalescence rate distributions. A drop population balance leads to an integrodifferential equation whose numerical solution yields the evolution of the PSD. Breakage—coalescence parameters are estimated from the reported experimental data. Modelling identifies the role of individual phenomena and their interplay.

Studies on functional diene oligomers with an amino end group, 3. Synthesis of oligoisoprenes containing a secondary amino end group

AbstractLithium monoalkylamide, in the presence of monoalkylamine, was found to initiate the polymerization of isoprene to form oligoisoprenes. The molecular weight distribution of products formed in the lithium isobutylamide initiated isoprene oligomerization strongly depends on the initial mole ratio [Isoprene]0/[Amine]0/[Amide]0; with an initial mole ratio of 60/1/8, oligomers with a number‐average molecular weight of about 800 were obtained. The presence of N,N,N′,N′‐tetramethylethylenediamine in the reaction system was found to be effective for the selective formation of secondary amino‐ended oligoisoprenes.

Optimization of batch polymerization processes—narrowing the MWD. I. Model simulation

AbstractSeveral control strategies for the free‐radical polymerization of methyl methacrylate are analyzed with a detailed constitutive model incorporating the gel and glass effects. Optimal temperature history, photoinitiation intensity variation, and programmed monomer and solvent additions, employed alone and in combination, represent cases simulated by this model. Solvent addition is selected for further experimental studies, due to some attractive features. The product molecular weight distribution is narrowed, while the molecular weight averages are maintained high. Model predictions of the solvent injection history exhibit a strong sensitivity to the constitutive equations for describing the gel and glass effects.

Optimization of batch polymerization processes—narrowing the MWD. II. Experimental study

AbstractProgrammed solvent injection was previously proposed and analyzed in the first part of this two‐paper series as a viable alternative for controlling the average molecular weight and narrowing the polydispersity of products from a free‐radical polymerization process with an inherent tendency to exhibit strong gel and glass effects. Here, methyl methacrylate is chosen for experimental verification of this idea. The product molecular weight distribution is indeed narrowed.