EFFECT OF POLYMERIZATION CONDITIONS ON THE MOLECULAR WEIGHT OF POLYPHENYLSILSISQUIOXANE

Samples of polyphenylsilsisquioxane (PPSQ)using CaF_2 or MgF_2 as the main catalysts hadbeen prepared under different polymerization conditions. The results were treated on anorthogonal design L_9 (3~4). All weight-average molecular weights (M_W ) of PPSQ had beenmeasured by gel permeation chromatography (GPC). Effects of polymerization conditionsincluding reaction temperature, composition of the dual catalysts (CaF_2 or MgF_2 and DCC),mixture of solvents and reaction time on M_W of PPSQ have been discussed. The quantity of thecatalysts is the most important factor that affects M_W of PPSQ. Reaction temperature is thesecond important factor. Appropriate polymerization condition has been established to get PPSQwith high M_W.

Effect of Molecular Weight on Thermoelectric Performance of P3HT Analogues with 2-Propoxyethyl Side Chains

By replacing hexyl chains in poly(3-hexylthiophene)(P3HT)with 2-propoxyethyls,four poly(3-(2-propoxyethyl)thiophene)(P3POET)homopolymers with comparable polydispersity indexes(PDI)and regioregularities were prepared herein in addition with step increment of about 7 kDa on numberaverage molecular weight(M_(n))from around 11 to 32 kDa(accordingly denoted as P11k,P18k,P25k,and P32k).When doped in film by FeCl_(3)at the optimized conditions,the maximum power factor(PF_(max))increases greatly from 4.3μW·m^(-1)·K^(-2)for P11k to 8.8μW·m^(-1)·K^(-2)for P18k,and further to 9.7μW·m^(-1)·K^(-2)for P25k,followed by a slight decrease to 9.2μW·m^(-1)·K^(-2)for P32k.The close Seebeck coefficients(S)at PF_(max)are observed in these doped polymer films due to their consistent frontier orbital energy levels and Fermi levels.The main contribution to this PF_(max)evolution thus comes from the corresponding conductivities(σ).Theσvariation of the doped films can be rationally correlated with their microstructure evolution.

Enhancement of Adhesive Strength of Ultrahigh Molecular Weight Polyethylene Fibers Prepared by Polar Polymer Implantation

A new technique was proposed to enhance the adhesive strength of ultrahigh molecular weight polyethylene （UHMWPE） fibers. Polar polymer was implanted into UHMWE gel fibers during extracting process and can then be trapped on the surface of the fibers after subsequent ultra-drawing. The physical and chemical changes in the fiber structure were examined with scanning electron microscopy （SEM） and Fourier transform infrared （FTIR） spectroscopy. The mechanical and interfacial adhesion properties of UHMWPE fibers were investigated with tensile testing. The results showed that there were polar groups on the smface of pretreated UHMWPE fiber. The interracial shear strength of UHMWPE fibers with epoxy resin was greatly improved without sacrificing the excellent mechanical p^perties of fibers. After pretreated with ethylene/vinyl acetate copolymer （EVA）, the shear strength of the interface between fiber and epoxy resin increased from 1.06 to 2.49 MPa, while the integrated mechanical properties d the pretreated UHMWPE fibers were still optimal.

A HIGHLY ACTIVE NEODYMIUM CHLORIDE ISOPROPANOL COMPLEX/MODIFIED METHYLALUMINOXANE CATALYST FOR PREPARING POLYISOPRENE WITH HIGH CIS-1,4 STEREOSPECIFICITY AND NARROW MOLECULAR WEIGHT DISTRIBUTION

Neodymium chloride isopropanol complex （NdCl3.3＇prOH） activated by modified methylaluminoxane （MMAO） was examined in isoprene polymerization in hexane, with regards to Nd compounds, aluminum （Al） compounds, [Al]/[Nd] ratio, polymerization temperature and time. NdCl3＇3iprOH exhibited high activity producing polymers feasting high cis-l,4 stereospecificity （〉 96%）, very high molecular weight （Mn 〉 1.0 × 10^6） and fairly narrow molecular weight distribution （MWD, Mw/Mn 〈 2.0） simultaneously. In comparison, neodymium isopropoxide also showed high activity providing polymers with narrow MWD （Mw/Mn = 2.07）, but somewhat low cis-1,4 content （ca. 92%）, while neodymium chloride had no activity under present polymerization conditions. The Al compounds affected the polymer yield in the order of Al（i-Bu）3 〉 MMAO 〉 Al（i-Bu）2H. MMAO as cocatalyst afforded polyisoprene with high Mn over 1.0 × 10^6, whereas as stronger chain transfer agent than MMAO, AI（i-Bu）3 and AI（i-Bu）EH yielded polymers with low Mn （1.0 × 10^5-8.0 × 10^5）. NdCl3·3＇PrOH/MMAO catalyst showed a fairly good catalytic activity even at relatively low [Al]/[Nd] ratio of 30, and the produced polymer remained high cis-1,4 content of 95.8% along with high Mn over 1.0× 10^6 even at elevated temperatures up to 70℃. The polymerization rate is of the first order with respect to the concentration of isoprene. The mechanism of active species formation was discussed preliminarily.

The Ring-opening Polymerization of D, L-Lactide Initiated with Stannous Octoate

The ring-opening polymerization of D, L-lactide in the melt was systematically investigated by using stannous octoate as the initiator. The molecular weight of poly (D, L-lactide) was characterized with M,. Mn and Mw respectively. The results indicated that five variables, namely purity of monomer, initiator to monomer ratio, vacuum level, polymerization temperature and polymerization time had different influences on the molecular weight and molecular weight distribution of poly(D. L-lactide).

ON THE KINETICS AND MECHANISM OF THE COPOLYMERIZATION OF ACRYLONITRILE WITH STYRENE IN THE PRESENCE OF COPPER CHLORIDE

The copolymerization of styrene (St) and acrylonitrile (AN) complexed with CuCl_2 monomer by a free radicalmechanism was performed using benzoyl peroxide as an initiator at 65℃ under N_2 atmosphere for 150 min. The rate ofpolymerization (R_p) was found to increase linearly with the concentration (in mol/L) of CuCl_2, AN and St through scalingrelations. The activation energy of the copolymerization process in the presence and absence of CuCl_2 was found to be46.5 kJ/mol and 102 kJ/mol, respectively. The viscosity average molecular weigh of the copolymer and the k_p^2/k_t ratio weredctermired to further assess the accelerating effect of CuCl_2 on the copolymerization process. The copolymerization processin the presence of CuCl_2 has a radical complex mechanism.

Nitroxide-Mediated Photo-Controlled/Living Radical Polymerization of Methacrylic Acid

The photo-controlled/living radical polymerization of methacrylic acid (MAA) was performed at room temperature by irradiation with a high-pressure mercury lamp using azo initiators and 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl as the mediator in the presence of (4-tert-butylphenyl)diphenylsulfonium triflate (tBuS) as the accelerator. Whereas the bulk polymerization yielded polymers with a bimodal molecular weight distribution in both the absence and presence of tBuS, the solution polymerization in methanol produced unimodal polymers with the molecular weight distribution of 2.0 - 2.3 in the presence of tBuS. The molecular weight distribution of the resulting poly (MAA) decreased with an in- crease in tBuS. The dilution of the monomer concentration also reduced the molecular weight distribution. The use of the initiator with a low 10-h half-life temperature also effectively controlled the molecular weight. The livingness of the polymerization was confirmed by obtaining linear increases in the first-order conversion versus time, the molecular weight versus the conversion, and the molecular weight versus the reciprocal of the initiator concentration.

KINETICS OF SUSPENDED EMULSION POLYMERIZATION OF METHYL METHACRYLATE

The kinetics of suspended emulsion polymerization of methyl methacrylate (MMA), in which water acted as the dispersed phase and the mixture of NMA and cyclohexane as the continuous phase, was investigated. It showed that the initial polymerization rate (R-p0) and steady-state polymerization rate (R-p) were proportional to the mass ratio between water and oil phase, and increased as the polymerization temperature, the potassium persulphate concentration ([I]) and the Tween20 emulsifier concentration ([S]) increased. The relationships between the polymerization rate and [I] and [S] were obtained as follows: R-p0 proportional to [I](0.73)[S](0.32) and R-p proportional to [I](0.71)[S](0.23). The above exponents were close to those obtained from normal MMA emulsion polymerization. It also showed that the average molecular weight of the resulting poly(methyl methacrylate) decreased as the polymerization temperature, [I] and [S] increased. Thus, MMA suspended emulsion polymerization could be considered as a combination of many miniature emulsion polymerizations proceeding in water drops and obeyed the classical kinetics of MMA emulsion polymerization.

TERMINATION AND TRANSFER OF THE CHAIN RADICALS IN THE POLYMERIZATION OF ACRYLONITRILE INITIATED BY VANADIUM(V)-THIOUREA REDOX SYSTEM

The dependence of the molecular weights on the concentration of reactants in the polymerization of acrylonitrile initiated by vanadium (V)-thiourea redox system has been investigated. It was found that the molecular weights of the polymer change nonlinearly with increasing concentrations of nitric acid and thiourea. Probably, the composition of the complexes exert a great influence on the chain initiation and termination. The reaction of 'complextermination' gives rise to the decrease of the molecular weights markedly while the concentrations of thiourea and vanadium (V)in the range from one to three molar ratios.

Synthesis and Antibacterial Action of Poly(DMAEMA-BC) with Various Molecular Weights

Antibacterial polymers of dimethylaminoethyl methacrylate benzyl ammonium chloride（DMAEMA-BC） with various molecular weights（Mws） were prepared under controlling radical polymerization conditions. The Mws of these polymers were determined by means of static multiangle laser light scattering and viscosity method. A Mark-Houwink equation was established to be [η]=0.154M0.764 for the Mw evaluation of poly（DMAEMA-BC）s. The effects of the Mws of these poly（DMAEMA-BC）s on their antibacterial activities against E. coli and S. aureus were investigated by various methods including viable cell counting, electrical conductivity titration, intracellular constituent leakage tests and electron microscopy. Our results indicate that the antibacterial efficiency of DMAEMA-BC was significantly enhanced after the monomers were polymerized into a polymer and increased obviously with the Mws,as a result of the increase of charge density. Further investigation of the molecular basis underlying the anti-bacterial role of these polymers revealed that poly（DMAEMA-BC） promoted the release of potassium ion from the membrane of bacterial cells and the release increased significantly with the Mws of the polymers used.

Numerical simulation of effects of operating conditions on the molecular weight of polypropylene using a response surface method

A L463^5 Box-Behnken design was used for developing a model to predict and optimize the molecular weight （Mw ） of polypropylene （PP） ; a second-order polynomial regression equation was derived to predict responses. The significance of variables and their interactions were tested by means of the ANOVA with 95% confidence limits; the standardized effects were investigated by Pareto chart, the optimum values of the selected variables were obtained by analyzing the response surface contour plots. The optimized Mw value of 1. 217 × 10^5 g/mol was very close to the industrial value （ （ 1.22 ±0. 004） ×10^6 g/tool） at the optimum values.

INFLUENCE OF MOLECULAR WEIGHT OF CHITIN AND ITS THREE DERIVATIVES ON CRITICAL CONCENTRATION OF LYOTROPIC LIQUID CRYSTALLINE PHASE TRANSITION

The critical concentration of lyotropic liquid crystalline phase transition for chitin derivatives was determined using a polarization microscope. The influence of molecular weight on critical concentration of liquid crystalline solution for chitin, chitosan, cyanoethyl chitosan and propionyl chitin successively increases as the chain rigidity decreases. Therefore it can be used as an indicator of the chain rigidity.

Molecular Weight Distribution of Polystyrene Produced in a Starved Feed Reactor

A starved feed reactor (SFR) is a semi-batch polymerization reactor where initiator and monomer are fed slowly into a fixed amount of solvent. The polymerization is carried out isothermally at elevated temperatures. The added initiator decomposes instantaneously and the added monomer polymerizes immediately. The molecular weight (MW) and molecular weight distribution (MWD) of the product polymer can be effectively controlled by the feed ratio of monomer to initiator. This paper presents a study on the MWD of styrene polymerization in a SFR. The MWD model parameters are regressed with experimental data. Although the solids fraction in the SFR is high (higher than 50%), viscosity is not too high and the 'gel effect' is weak due to the low molecular weight of the products. It is found that the termination rate constant is a power function of molecular weight, radicals terminate via 100% combination, the thermal initiation can be neglected even at high reaction temperature studied. And calculated results indicate that in the SFR, the validity of the long chain assumption becomes doubted. It appears that other alterative assumption should be found for an improved model.

Rheological behavior of hydrolyzed polyacrylamide solution flowing through a molecular weight adjusting device with porous medium

The separate-layer injection in different interlayers and the injection of the same-molecular-weight polymer so- lution in a layer are necessary in the polymer flooding process because of heterogeneous multilayer sandstone reservoirs in EOR projects. To alleviate the matching problems between the layer permeability and the injected polymer molecular weight, a molecular weight adjusting device with porous medium was designed on the basis of mechanical degradation principle. In terms of four variables （polymer concentration, pore diameter, length of shear component and flow rate ）, the theological behavior of hydrolyzed polyacrylamide （HPAM） solu- tion flowing through the device was investigated in detail. The change of these variables is able to control the shear rate of HPAM solutions through ceramic foam, and achieve the desired degree of shear degradation and the final theological parameters-viscosity loss, viscoelasticity and pressure drop. Therefore, a linear relationship between viscosity loss and shearing rate was established so as to obtain the targeted viscosity easily. Field tests in the Daqing Oil Field showed that the polymer molecular weight could drop 20% to 50%. In a word, the results could guide the industrial application of the novel device and the further study of polymer degradation flowing through the porous medium.

Green Synthesis and Physical and Chemical Characterization of Chitosans with a High Degree of Deacetylation, Produced by a Binary Enzyme System

Chitosan is a biopolymer obtained from chitin, where the N-acetylglucosamine monomer is in its deacetylated form; this polymer is useful for a wide variety of industrial applications. The properties and uses of chitosan depend on its physical and chemical characteristics, which result from the treatments used for its production. In this study, we report the preparation and characterization ofchitosan oligosaccharides by a green synthesis from crystalline shrimp chitin, using a sequential enzyme treatment by chitinase and chitin deacetylase. Chitinases were purified from grapes and used to rupture the crystalline shrimp chitin structure, modifying the crystallinity index from 57.6% to 15.9%. The resultant polymers were deacetylated using a recombinant chitin deacetylase from Saccharomyces cerevisiae, which was cloned and expressed in Pichia pastoris. The chitosans produced showed an estimated DA （degree of acetylation） of approximately 20%, and the molecular weights ranged from -7,600 to -3,700 after treatment in pH 3.0 and pH 6.0 for 10 min and 40 min, respectively. Physical and chemical characterization of the products indicated that enzyme fragmentation of chitin probably makes the acetamide groups more accessible to deacetylation, forming homogeneous polymers that are free of hazardous sub-products, have defined low molecular weights, and are highly deacetylated.

Metallocene Catalyzed Homopolymerization of Propylene with a <i>C</i>₂Symmetric Isospecific Zirconecene/MMAO Catalysts under Different Polymerization Conditions

In this work, homopolymerization of propylene for synthesis of different molecular weights polypropylene was done using an isospecific metallocene catalyst by changing the polymerization conditions such as Al/Zr (Aluminium/Zirconium) ratio, temperature, pressure and polymerization time. The effects of the polymerization conditions on the molecular weight and other polymer properties have been discussed according to the results obtained. The results indicate that the activity of polymerization increased with increasing the Al/Zr mole ratio and reached a maximum value at a ratio of 2000. In relation to the mole ratio, the molecular weight also followed the same trend. The activity was also seen to increase with increase in temperature. The molecular weight increased with increasing the polymerization time at both 0&#176;C and 25&#176;C, but reduced with increase in time at 40&#176;C. The increase of pressure also increased activity but lowered the molecular weight, indicating β-H elimination to the monomer.

SYNTHESIS OF HIGH MOLECULAR WEIGHT POLYISOBUTYLENE VIA CATIONIC POLYMERIZATION AT ELEVATED TEMPERATURES

A novel simple but effective initiating system of H2O/AlCl3/veratrole （VE） has been developed to synthesize high molecular weight polyisobutylene （PIB） at elevated temperatures via cationic polymerization of isobutylene （IB） in solvent mixture of hexane/methylene dichloride （n-Hex/CH2Cl2 = 2/1, V/V）. VE played very important roles in decreasing cationicity of the growing chain ends, suppressing side reactions of chain transfer and termination during polymerization, leading to production of high molecular weight PIBs. PIBs with high yields, having very high weight-average molecular weight （Mw） of 1117000 and 370000 g/tool could be synthesized with H2O/AICl3/VE initiating system at VE concentration of 5.4 mmol/L at -80 and -60 ℃ respectively. Molecular weight of PIB increased remarkably with increasing VE concentration. The reaction order with respect to VE concentration was determined to be -3.52 via FTIR spectroscopy in combination with a diamond tipped attenuated total reflectance （ATR） immersion probe. The negative reaction order of VE was consistent with its retarding effect on IB polymerization by interacting with the propagating species. Molecular weight of PIB decreased with increasing polymerization temperature. The activation energy for polymerization degree （Eop） could be determined to be around -23 kJ/mol when VE concentration was 5.4 mmol/L or 6.4 mmol/L.

Preparation of Ultralow Molecular Weight Poly(vinyl chloride) with Submicrometer Particles via Precipitation Polymerization

Poly(vinyl chloride), with ultralow molecular weight, produced by free radical polymerization either at high temperature or in the presence of chain transfer agents, is widely used as special resins and polymer process additives. This paper reports a new process, called self-stabilized precipitation polymerization, in which the polymerization of vinyl chloride monomer (VCM) is conducted in hydrocarbon diluents without addition of any suspending agent or emulsifier. The merits of this novel strategy include:(1) PVC resins with ultra-low number-average molecular weight (Mn) from 4000 to 15000, which is much lower than Mn of those prepared by conventional suspension and emulsion polymerizations,(2) sub-micrometer PVC particles with near spherical morphology, and (3) the very simple post-polymerization separation process. Under mild stirring, polymerization proceeds stably and smoothly. The influences of main process factors, such as solvents, initiator and monomer concentrations, polymerization time, and temperature on both particle morphology and Mn of the polymer products are investigated systematically. The molar ratio of-CH2-CHCl-/-CH=CH-CH2CHCl, a good indicator of structural defects, is about 1000/0.1 which means the low molecular weights do not result from chain transfer to the monomers. Then the mechanism of this polymerization is proposed. In summary, this novel polymerization technology provides a straightforward method for preparing PVC particulate products with low Mn.

Aqueous RAFT Polymerization of Acrylamide： A Convenient Method for Polyacrylamide with Narrow Molecular Weight Distribution

Controlled and homogeneous flee-radical polymerization of acrylamide （AM） in aqueous phase was realized by using S,S＇-bis（α, α＇-dimethyl-α＂-acetic acid）-trithiocarbonate as a reversible addition-fragmentation transfer （RAFT） agent. Linear increases in molecular weight with conversion and narrow molecular weight distribution were observed for polyacrylamide （PAM） throughout the polymerization. By this method, PAMs with controlled molecular weight （up to 1.0 ~ 106） and narrow molecular weight distribution （Mw/Mn 〈 1.2） were prepared. This study provides an effective method for synthesis of PAMs with narrow molecular weight distribution under environmentally friendly conditions.

Anionic polymerization initiated by lithium amides for preparing high molecular weight polyacrylonitrile

Lithium amides have been proved to be effective anionic initiators for the anionic polymerization of acrylonitrile to get high molecular weight polyacrylonitrile in this study. Polyacrylonitrile with weightaverage molecular weight ranging from 1.02 × 10~6 g/mol to 1.23 ×10~6 g/mol (M_w/M_n= 1.9-2.2) could be prepared utilizing lithium amides derived from diisopropylamine, diethylamine, hexamethyldisilazane,dicyclohexylamine, and 2,2,6,6-tetramethylpiperidine as initiators. The polymerization of acrylonitrile proceeded in a homogeneous manner in N,N-di methyl for mamide and insignificant contribution of side reactions was confirmed.