Polymer Molecular Weight Distribution Research Articles

Synthesis and characterization of the new pyridine‐containing poly(azomethine‐urethane)s: The effect of methyl substituent as electron‐donating group on some physical properties

In this study, a series of pyridine‐ring based poly(azomethine‐urethane)s were synthesized to investigate the effect of electron‐donating group on some physical properties such as thermal stability, optical, and electrochemical properties. For this reason, firstly the Schiff bases were synthesized using amino‐methyl pyridines (2‐amino‐3‐methyl pyridine, 2‐amino‐4‐methyl pyridine, and 2‐amino‐6‐methyl pyridine) with 2,4‐dihydroxy benzaldehyde by the condensation reaction. Then, these Schiff bases were converted to poly(azomethine‐urethane) derivatives via step‐polymerization reaction. Secondly, structural characterizations were carried out by FTIR, NMR, and UV–Vis. Cyclic voltammetry was used to determine the electrochemical oxidation‐reduction characteristics. Optical properties were investigated by UV–vis and fluorescence analyses. Thermal properties were clarified by TG‐DTA and DSC techniques. The molecular weight distributions of polymer were determined by size exclusion chromatography. POLYM. ENG. SCI., 54:1664–1674, 2014. © 2013 Society of Plastics Engineers

Rheological measurement of molecular weight distribution of polymers

Rheological measurement of molecular weight distribution of polymers

Solid state self-assembly of the single-walled carbon nanotubes and poly(γ-benzyl-l -glutamate)s with different conformations

A series of pyrenyl-terminated poly(γ-benzyl-l-glutamate)s (py-PBLGs) with controlled polymer molecular weight (MW = 2.3–14.8 kg mol−1) and molecular weight distribution (PDI = 1.17–1.55) have been prepared from 1-pyrenemethylamine hydrochloride-mediated ring-opening polymerization (ROP) of γ-benzyl-l-glutamic acid based N-carboxyanhydride (BLG-NCA). FTIR analysis revealed that the py-PBLG9 was conformationally heterogeneous with 35.0% α-helix, 55.6% β-sheet, and 9.4% random coil conformations in the solid state, whereas the py-PBLG66 adopts 100% α-helix conformation. Py-PBLGs promote the dispersion of SWCNTs in organic solvents and in the PBLG solid through π–π interaction, as evidenced by the Raman spectroscopic studies. WAXD analysis revealed that the SWCNTs significantly affect the ordering of the py-PBLG self-assembly: the long range hexagonal packing of py-PBLG66 rods is notably enhanced by the addition of SWCNTs, whereas the lamellar packing of py-PGLG9 β-sheets is weakened. In the hexagonal lattice, the SWCNTs are intercalated parallel to the py-PBLG66 rods, in contrast to the normal orientation of the SWCNTs with respect to the extended py-PBLG9 chains in the β-sheets. The relative packing structure also affects the intermolecular interaction among the PBLGs: SWCNTs promote the interaction among the py-PBLG9 chains packed in a lamellar structure and weaken the intermolecular interaction among the py-PBLG66 columnar hexagonal array. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4489–4497

Kinetics and Mechanisms of Radical-Based Branching/Cross-Linking Reactions in Preformed Polymers Induced by Benzophenone and Bis-Benzophenone Photoinitiators

The general mechanism for photo-cross-linking of preformed polymers is studied by comparing the efficacy of monofunctional benzophenone (BP) and bifunctional bis-benzophenone (BP-BP) photoinitiators for inducing radical chain branching reactions in glassy polystyrene (PS) and rubbery poly(n-butyl acrylate) (PnBA). Upon UV irradiation, macroradicals form and initiate a variety of cross-linking and scission reactions. The kinetics and mechanisms of these macroradical reactions were monitored by gel permeation chromatography (GPC) measurements of changes in the polymer molecular weight distributions. Molecular weight increases are associated with chain branching while decreases in molecular weight are indicative of chain scission. We study the early stages of radical recombination where branching is manifest as the formation of three- and four-arm star polymers that are soluble and can be detected/differentiated by GPC. Branching is observed even in glassy PS; however, the reactions are much faster in rubbery PnBA, consistent with the expected influence of main chain mobility. At equal chromophore equivalents, BP-BP was found to be more efficient than BP for producing macroradicals, primarily due to a lower degree of self-quenching in BP-BP. When added to glassy PS, the higher efficiency of BP-BP did not translate into more chain branching, except at high additive concentration where the probability of BP-BP chain bridging reactions becomes significant, but instead led to a higher degree of main chain scission. The latter result was attributed to the larger distance between chromophores for BP-BP than for BP at equal benzophenone equivalents. In marked contrast, almost no main chain scission was found for either additive in PnBA, and BP-BP proved more effective for promoting chain branching. The susceptibility to main chain scission is found to be dependent upon the location of radical formation by hydrogen abstraction from the polymer. In PS, radicals can form on the chain backbone, and radical scission reactions lead to fragmentation of the polymer chain. In PnBA, radicals form preferentially on the pendant side chains, and radical scission reactions do not lead to main chain breakage. A simple probability-based model was found to capture the salient features of radiation-induced branching in preformed polymers.

Heterogeneous Ethylene and Alpha‐Olefin Copolymerization Using Zirconocene Aluminohydride Complexes

AbstractIn this work we studied the copolymerization of ethylene and different alpha‐olefins (propylene, 1‐butene, 1‐hexene, and 1‐octene) using aluminohydride zirconocene systems supported on modified silica. The copolymerization reactions were carried out in hexanes, at different Al/Zr ratios, using two different comonomer concentrations. The effect of comonomer type on catalyst activity and polymer molecular weight and chemical composition distributions (MWD and CCD) was compared. Average comonomer incorporation was determined by 13C NMR and the CCD by crystallization elution fractionation (CEF). Most of the copolymerizations showed very high catalytic activity (up to 4000 Kg PE/mol Zr), and the molecular weight was higher for the copolymers containing 1‐octene, demonstrating that aluminohydride zirconocene systems can copolymerize ethylene and alpha‐olefins effectively.

Three-dimensional analysis of membrane formation via thermally induced phase separation by dissipative particle dynamics simulation

The effects of polymer molecular weight (polymer chain length), polymer molecular weight distribution (polymer chain length distribution) and diluent structure on the membrane formation of polymer–diluent system via the thermally induced phase separation (TIPS) method were analyzed on the basis of a dissipative particle dynamics (DPD) simulation platform. The simulation platform was extended into three-dimensions (3-D) to make sure that the results were more comprehensive. The simulation results proved that both the polymer chain length and polymer chain length distribution had significant influences on the phase separation process. As the polymer chain length increased, phase separation rate first increased when the polymer chain was relatively short and then decreased when the polymer chain length was larger. In addition, as the chain length distribution got wider, the phase separation rate decreased gradually at the middle and last stages of the phase separation process. It was also demonstrated that the interaction between polymer and diluent could be regulated by changing the interaction of the polymer and diluent local part by the local modification on the diluent molecular structure, which was beneficial to the membrane preparation. The information obtained from this study is consistent with the experimental results in literatures, proving that the simulation method could be widely applied in dynamics analysis of the membrane formation process.

Reverse Atom Transfer Radical Polymerization of N-Vinylpyrrolidone in Bulk Catalyzed by AIBN)/FeCl3/ PPh3

The reverse atom transfer radical polymerization (RATRP) of N-vinylpyrrolidone (NVP) using azobisisobutyronitrile (AIBN)/FeCl3/triphenylphosphine(PPh3) as the initiating system, was successfully carried out in bulk at 80°C. Plots of In ([M]0/[M]) vs time and molecular weight evolution vs monomer conversion presented a linear dependence and the polymerization was proved to accord with the first-order kinetics. After 10 hours’ reaction, the monomer conversion was up to 84%. Gel permeation chromatography (GPC) was used in testing the molecular weight of polymer and molecular weight distribution, the results showed that polymer molecular weight distribution was as low as 1.018 (Mn=3288 g/mol). Moreover, the resultant polymer was characterized by 1H-NMR, 13C-NMR spectroscopy and Pyrolysis GC-MS, and the results showed that the polymerization mechanism is consistent with RATRP.

Revisiting molecular weight distribution of polystyrenes using adsorption high-performance thin-layer chromatography

A repeatable separation of polystyrenes according to MW, from 1920 to 520000 u.m.a, has been obtained under adsorption conditions using a method that comprises the use of Lichrospher HPTLC plates, and a controlled, isocratic elution with a 78:22 (v/v) mixture of cyclohexane (Cy)-tetrahydrofuran (THF). Likewise, UV-densitometric quantification of polystyrenes in mixtures can be achieved, by an intra-plate or an inter-plate procedure, using the corresponding polystyrene calibration curve. In the case or overlapped, unresolved peaks, an average curve of the corresponding polystyrenes can be used. Migration of polystyrenes strongly varies with slight variations in the relative proportion of Cy and THF in the mobile phase. This allows different ranges of MW to be separated as a function of mobile phase composition. Other factors influencing repeatability have been identified. Some reasons have been advanced to explain the current lack of activity in the research on polymer characterization by HPTLC. Old literature results concerning polystyrene separation have also been discussed in the light of modern HPTLC instrumentation. Developed method provides similar information on Molecular Weight Distribution (MWD) to that obtained using Gel Permeation Chromatography (GPC). Advantages and limitations of HPTLC for obtaining polymer MWD have also been discussed.

Synthesis and evaluation of two new FI‐Ti catalysts for living polymerization of ethylene

AbstractTwo new FI complexes, bis[N‐(3‐allylsalicylidene)‐pentafluoroanilinato]titanium(IV) dichloride (AFI) and bis[N‐(3‐propylsalicylidene)‐pentafluoroanilinato]titanium(IV) dichloride (PFI) were designed and synthesized as catalysts for living polymerization of ethylene. The two complexes were characterized by elemental analysis, spectroscopy and X‐ray single diffraction. The catalysts were evaluated in ethylene polymerization under atmospheric pressure. It was found that both catalysts exhibited high activity and good livingness. The effects of temperature and dMAO/Ti molar ratio on the polymerization behavior of AFI were studied in detail. Elevating temperature increased self‐immobilization of the AFI catalyst, which broadened the polymer molecular weight distribution. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Exceptionally high lactide polymerization activity of zirconium complexes with bridged diketiminate ligands

A cyclohexanediyl-bridged, bis(N-xylyl) diketiminate ligand, (±)-C6H10(nacnac(Xyl)H)2, LH2 (Xyl = 2,6-dimethylphenyl), was obtained from the reaction of [(2,6-dimethylphenyl)amino]-pent-3-en-2-one first with Meerwein's salt, then with (±)-cyclohexanediamine. The reaction of the ligand with Zr(NMe2)4 yielded LZr(NMe2)2. Protonation of the remaining diamide ligands with EtOH or [H2NMe2]Cl yielded LZr(OEt)2 and LZrCl2, respectively. The latter complex was also obtained by the reaction of LH2 first with nBuLi and then with ZrCl4(THF)2. The dichloride complex yielded LZr(OEt)2 and LZrMe2 upon reaction with NaOEt or MeLi/AlMe3, respectively. X-ray diffraction studies showed a trans-configuration of the ancillary ligands in LZrCl2 and LZrMe2, and a cis-configuration in LZr(NMe2)2 and LZr(OEt)2. LZr(OEt)2 was tested as a catalyst for the polymerization of rac-lactide. Kinetic investigations yielded a rate law first order in catalyst and monomer and a rate constant k = 14(1) L mol(-1) s(-1), the latter being orders of magnitude higher than typical activities for group 4 complexes in lactide polymerization. Analyses of the obtained polymer revealed an atactic polymer and broad polymer molecular weight distributions with sizeable fractions of cyclic oligomers. The influence of contaminants on the polymerization activity was examined: while lactic acid deactivates the catalyst, addition of up to 1 equiv. of water or para-toluenesulfonic acid revitalized catalysts not showing maximum activity.

Membrane‐dispersion reactor in homogeneous liquid process

AbstractFor homogeneous liquid processes, mixing at molecular scale may influence selectivity, yield and quality of final products. In a membrane‐dispersion reactor, microporous membranes are employed as dispersion media for controlled feeding of one solution into another one to intensify micromixing. The reactor has been widely used in the preparation of nanoparticles, preparation of nanocapsules and liposomes, synthesis of polymers, parallel and consecutive reactions to improve nanoparticles quality, molecular weight distribution of polymer, or selectivity of complex reactions. This paper reviewed the progress of the membrane‐dispersion reactor in homogeneous liquid processing including features, applications, advantages and limits. © 2012 Society of Chemical Industry.

Synthesis, characterization and thermal degradation of dual temperature‐ and pH‐sensitive RAFT‐made copolymers of N,N‐(dimethylamino)ethyl methacrylate and methyl methacrylate

AbstractPoly{[(N,N‐(dimethylamino)ethyl methacrylate]‐co‐(methyl methacrylate)} copolymers of various compositions were synthesized by reversible addition‐fragmentation chain transfer (RAFT) polymerization at 70 °C in N,N‐dimethylformamide. The polymer molecular weights and molecular weight distributions were obtained from size exclusion chromatography, and they indicated the controlled nature of the RAFT polymerizations; the polydispersity indices are in the range 1.1–1.3. The reactivity ratios of N,N‐(dimethylamino)ethyl methacrylate (DMAEMA) and methyl methacrylate (MMA) (rDMAEMA = 0.925 and rMMA = 0.854) were computed by the extended Kelen–Tüdös method at high conversions, using compositions obtained from 1H NMR. The pH‐ and temperature‐sensitive behaviour were studied in aqueous solution to confirm dual responsiveness of these copolymers. The thermal properties of the copolymers with various compositions were investigated by differential scanning calorimetry and thermogravimetric analysis. The kinetics of thermal degradation were determined by Friedmann and Chang techniques to evaluate various parameters such as the activation energy, the order and the frequency factor. © 2012 Society of Chemical Industry

Gray-box modeling and control of polymer molecular weight distribution using orthogonal polynomial neural networks

A method of modeling and control on polymer molecular weight distribution (MWD) is presented in this paper. An orthogonal polynomial feedforward neural network (OPFNN) and a recurrent neural network (RNN) are combined to model the shape of MWD. In this combined neural networks, the weights of OPFNN are equivalent to moments of MWD through a linear transformation, when the polynomial used as the basis function of OPFNN satisfies some requirements. So the weights are given practical feature, and terms the neural network model a gray-box model. Then the requirements of polynomial are deduced. After modeling, an optimal control scheme is discussed on tracking the desired MWD during the polymerization process. The modeling error is added into the performance function to improve the control effect. The modeling and control method is tested on styrene polymerization reacted in CSTR, and simulation results proved the effectiveness of the method.

Polymerization of Ethylene over rac-Et(1-indenyl)2ZrCl2/MAO Catalyst Supported on Pseudo-Inverse Opal Silica Particles

The polymerization of ethylene with rac-Et(1-indenyl)2ZrCl2/MAO catalyst supported on ultraporous and spherical pseudo-inverse opal silica (PIOS) particles with unique morphology is reported. The novel silica particles with an inverse opal-like structure provide wide-open pore and surface structures favorable for the catalyst deposition and monomer access with minimal intraparticle diffusion resistance. The metallocene catalyst supported on the PIOS support exhibited very high initial catalyst activity and longer catalyst lifetime compared with conventional silica-supported catalysts. The experimental data that show the effects of support geometry on the catalytic activity, catalyst particle fragmentation, polymer particle morphology, and polymer molecular weight distribution in ethylene polymerization are presented.

Producing bimodal molecular weight distribution polymers through facile one-pot/one-step RAFT polymerization

Well-defined bimodal molecular weight distribution (MWD) polystyrene and polystyrene-b-poly(acrylonitrile) were successfully synthesized using a pair of mono/difunctional trithiocarbonate RAFT agents 1 and 2 via one-pot RAFT polymerization. The kinetics of RAFT polymerization for styrene in bulk with a molar ratio of [St]0:[AIBN]0:[1]0:[2]0 = 1200:1:2.5:2.5 was studied at 75°C. The results indicated that the system showed excellent controllability and “living” characteristics to both higher and lower molecular weight fractions, providing an efficient and facile way to producing bimodal MWD (co)polymers with both controlled molecular weight (MW) and MWD in molecular level, and the plausible mechanism was discussed in this work. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Continuum lumping modelling for step growth polymerisation mechanism

Polymerisation occurs by chain addition or step-growth mechanisms. The yield and the concentration distribution of monomers, dimers, trimers and higher order polymers can depend on the molecular weight or some other variable which can be unequivocally linked to any given compound. Continuum lumping is a methodology widely used to describe the kinetic and thermodynamic behaviour of complex reactive mixtures of diverse components all undergoing similar types of reactions (e.g., cracking, pyrolysis, oligomerisation). The methodology is particularly convenient when the number of species involved is very large and the species can be characterised by properties which are measured in a continuous fashion, as for instance in a chromatogram. By applying the lumping methodology, a large reactive system can be reduced to a simpler and more tractable one. A model, based on the continuum lumping approach, is proposed to predict the molecular weight distribution of polymers during polymerisation. The main aim of this work is to assess the suitability of the application of the continuum lumping approach to polymerisation. The results obtained from a continuous model are assessed, at this stage, only qualitatively; nevertheless, by analysing the weight distribution and the average of such distribution, conclusions can be reached to assess the predictive capability of the lumping methodology. The model is tested for two selected case studies and the product distribution is obtained at various times.

Effect of Ga- and BCl3-modified silica-supported [t-BuNSiMe2(2,7-t-Bu2Flu)]TiMe2/MAO catalyst on ethylene/1-hexene copolymerization

In this present study, the copolymerization of ethylene and 1-hexene was conducted over the [t-BuNSiMe2(2,7-t-Bu2Flu)]TiMe2 (CGC)/MAO catalyst immobilized on different supports. The effects of Ga and the Lewis acid BCl3 modification of the silica support on the copolymerization behavior were investigated based on catalytic activity and polymer properties. It was found that the silica support modified with BCl3 exhibited the highest activity. However, both Ga and BCl3 modifiers are capable of enhancing the catalytic activity, probably attributed to stronger interaction between the MAO and support together with the acidic sites exerted by the modification which could assist MAO to activate the catalyst during polymerization. Besides, a role of BCl3 as a spacer to keep apart the catalyst on the silica surface was proposed as another probable reason for activity increased. This led to the more homogeneous-like behavior with less effect of support and also caused the higher comonomer incorporation content. Moreover, the results revealed that narrow polymer molecular weight distribution can be achieved by the supported CGC catalyst, especially for acidic modified supports. On the other hand, there was no noticeable effect with regards to the melting temperature and copolymer microstructure of the Ga and BCl3 modification. Therefore, based on the study it may be regarded that the efficient supported CGC catalyst can be accomplished through the acidic modification namely Ga and BCl3.

Effect of molecular weight of amine end-modified poly(β-amino ester)s on gene delivery efficiency and toxicity

Amine end-modified poly(β-amino ester)s (PBAEs) have generated interest as efficient, biodegradable polymeric carriers for plasmid DNA (pDNA). For cationic, non-degradable polymers, such as polyethylenimine (PEI), the polymer molecular weight (MW) and molecular weight distribution (MWD) significantly affect transfection activity and cytotoxicity. The effect of MW on DNA transfection activity for PBAEs has been less well studied. We applied two strategies to obtain amine end-modified PBAEs varying in MW. In one approach, we synthesized four amine end-modified PBAEs with each at 15 different monomer molar ratios, and observed that polymers of intermediate length mediated optimal DNA transfection in HeLa cells. Biophysical characterization of these feed ratio variants suggested that optimal performance was related to higher DNA complexation efficiency and smaller nanoparticle size, but not to nanoparticle charge. In a second approach, we used preparative size exclusion chromatography (SEC) to obtain well-defined, monodisperse polymer fractions. We observed that the transfection activities of size-fractionated PBAEs generally increased with MW, a trend that was weakly associated with an increase in DNA binding efficiency. Furthermore, this approach allowed for the isolation of polymer fractions with greater transfection potency than the starting material. For researchers working with gene delivery polymers synthesized by step-growth polymerization, our data highlight the potentially broad utility of preparative SEC to isolate monodisperse polymers with improved properties. Overall, these results help to elucidate the influence of polymer MWD on nucleic acid delivery and provide insight toward the rational design of next-generation materials for gene therapy.

Reduction-sensitive core-cross-linked mPEG–poly(ester-carbonate) micelles for glutathione-triggered intracellular drug release

A functionalized six-membered cyclic carbonate monomer containing a protected thiol group, 2-(2,4-dinitrophenylthio)ethyl-5-methyl-2-oxo-1,3-dioxane-5-carboxylate (MTC), was prepared. Ring-opening polymerization (ROP) of MTC and L-lactide (LA) initiated by monohydroxyl poly(ethylene glycol) (mPEG) was successfully performed with controlled polymer molecular weight and molecular weight distribution. The structures of the obtained monomer and copolymers were confirmed by NMR and FTIR. After deprotection under mild conditions, the amphiphilic block copolymer was self-assembled into micelles in aqueous solution. By oxidation of the free thiol groups in the PMTC segments, the micelles with the cross-linked core were formed, and the enhanced stability against disruptive conditions was demonstrated by dynamic light scattering (DLS) measurement in the presence of DMF. Doxorubicin (DOX) was loaded into the micelles as a model drug. The in vitro DOX release behaviors indicated that cross-linking of the micelle cores resulted in a slow drug release and this process was greatly accelerated by adding glutathione (GSH) solution with comparable concentrations to intracellular levels in tumor cells. The cross-linked micelles were demonstrated to be efficiently internalized into the cells. Faster intracellular DOX release was observed by confocal laser scanning microscopy (CLSM) in the GSH pretreated MCF-7 cells than in the unpretreated MCF-7 cells. In vitro MTT assay revealed that the cross-linked micelles were biocompatible with L929 cells and MCF-7 cells. As expected, DOX-loaded cross-linked micelles showed higher cellular proliferation inhibition towards glutathione pretreated MCF-7 cells than that towards the unpretreated ones at various GSH concentrations. These results indicated that the bioreducible core-cross-linked micelles can be used as drug carriers for intelligent drug delivery.

Photo-cross-linked mPEG-poly(γ-cinnamyl-l-glutamate) micelles as stable drug carriers

A functionalized L-glutamate N-carboxyanhydride monomer containing a cinnamyl moiety, γ-cinnamyl-L-glutamate N-carboxyanhydride (CLG-NCA), was prepared. Ring-opening polymerization (ROP) of CLG-NCA initiated by n-hexylamine and amino-terminated poly(ethylene glycol) (mPEG-NH2) was successfully performed with controlled polymer molecular weight and molecular weight distribution. The structures of the obtained monomer and polymers were confirmed by 1H NMR, 13C NMR and FTIR. The block copolymer mPEG-b-PCLG could self-assemble into micelles in aqueous solution which consist of a hydrophobic PCLG core and a hydrophilic mPEG shell. Under UV-irradiation at 254 nm, the photo-cross-linking process of the cinnamyl-carrying block copolymer micelles was tracked by UV-Vis spectra, 1H NMR, dynamic light scattering (DLS) and transmission electron microscopy (TEM). In vitro MTT assay demonstrated that the micelles were biocompatible to L929 cells. Paclitaxel was loaded into the micelles as a model drug. The in vitro paclitaxel release behaviors indicated that cross-linking of the micelle cores results in a slow drug release in comparison with the non-cross-linked micelles. These results indicated that the photo cross-linked mPEG-b-PCLG micelles can be used as drug carriers for intelligent drug delivery.