Molecular weight distribution of PVC blends from resins with different K values

The molecular weight distribution (MWD) of blends of commercial suspension grade PVC resins with different molecular weights (MW) or K values is calculated. If the K‐value difference is not too high, the polydispersity of the blend is only slightly increased as compared to those of the components. It is not possible to get bimodal MWDs by blending. Additionally, a model calculation of mean molecular weights has been performed for temperature‐programmed polymerization. On the basis of the most realistic model, which takes into account the increasing polymerization reaction rate with increasing temperature and increasing conversion, it can be predicted that a linear temperature program will give a product corresponding to an isothermal resin polymerized at a temperature near the high end of the temperature program and showing only a slightly increased polydispersity.     

Molecular weight distribution of commercial PVC

The molecular weight distribution (MWD) of commercial suspension grade poly(vinyl chloride) (PVC) resins with K values from 50 to 93 and mass grade PVC resins with K values from 58 to 68 has been determined by size exclusion chromatography (SEC), using literature Mark‐Houwink coefficients. The MWD is characterized by the number average molecular weight (M_n), the weight average molecular weight (M_w) and the polydispersity (M_w/M_n). Our results for M_w are consistent with recently published data, but we find different results for M_n and consequently for M_w/M_n. The polydispersity of PVC increases with increasing K value. This effect can be explained by two mechanisms. The first mechanism is a reduced terminating reaction rate between two growing polymer chains (disproportionation) at higher molecular weight owing to the reduced mobility of the polymer chains. The second mechanism is long‐chain branching of molecules with high molecular weight which lets the molecules grow at two ends. For two examples graphs of the measured MWD are compared with the theoretically expected MWD.     

Molecular weight distributions of tetrafluoroethylene-hexafluoropropylene copolymers

A new methodology has been developed to determine molecular weight distributions (MWD) from dynamic rheometry data. The present work extends and modifies previous results and illustrates that the MWDs of tetrafluoroethylenehexafluoropropylene copolymers (FEP) are broader than previously thought. The major change in methodology involves the relationship between a reduced form of the elastic shear modulus as a function of frequency, G′(ω), and the cumulative weight fraction as a function of molecular weight. The new method uses a square root versus the former linear relationship. Spectroscopic analysis of the end group concentration was used to determine the number-average molecular weight. These data were used to reference the MWDs determined by dynamic rheometry and indicate that the molecular weights of these polymers are 20% lower than previously found.     

Prediction of molecular weight distribution of cellulose by using the rheological method

In this article, a rheological method that can predict the molecular weight distribution (MWD) of polymer was introduced. Using this method, the MWDs of four cellulose samples were compared from rheological data of the cellulose / N‐methyl morpholine N‐oxide (NMMO) / H₂O solutions. The MWDs of cellulose also were determined by gel permeation chromatography (GPC) calibrated with narrow distribution polystyrene standards, using 0.5% lithium chloride (LiCl) in N,N‐dimethylacetamide (DMAc) as the eluent. Comparison of the results from rheology and GPC showed that the MW and MWD of cellulose could be roughly inferred from their rheological data. Although the differential MWD obtained from the rheological method was bell shaped and can not reflect the fine characteristics of cellulose as GPC, it may be feasible to compare the MWDs of cellulose by using the rheological method. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 598–603, 2004     

Living ring-opening polymerization of ɛ-caprolactone with combinations of tert-butyllithium and bilky aluminium phenoxides

Summary The polymerization of ɛ-caprolactone (ɛ-CL) with a combination of tert-butyllithium (t-BuLi) and bis(2,6-di-t-butylphenoxy)methylaluminum [MeAl(ODBP)₂] in toluene at 0°C proceeded in a living manner to give polymers with narrow molecular weight distributions (MWD) within a few minutes, while the polymerization with t-BuLi alone gave a polymer with much broader MWD. The yield of the polymer did not reach 100 % at the Al/Li ratio of 5, because the excess MeAl(ODBP)₂ coordinates with ɛ-CL to protect it from the attack by the propagating species. The polymerization with t-BuLi/EtAl(ODBP)₂ gave polymers in quantitative yields regardless of Al/Li ratio, and also narrower MWD even for higher molecular weight polymers. Received: 1 August 2000/Accepted: 11 August 2000     

Propene polymerization with MgCl2-supported TiCl4/dioctylphthalate catalyst. III. Effects of polymerization conditions on molecular weights and molecular weight distribution

In propene polymerization over the MgCl₂-supported TiCl₄/dioctylphthalate (DOP) catalyst, the weight- and number-average molecular weights and the molecular weight distribution (MWD) of polypropene products and of the isotactic and atactic polymer portions were studied. The average molecular weights and MWD were found to be independent of time. The isotactic polymer had higher molecular weight and broader distribution than the atactic portion by almost an order of magnitude. An increase in temperature and cocatalyst/catalyst ratio resulted in lowering molecular weight due to increasing transfer reaction. Alkyl aluminum was used as a cocatalyst, and the molecular weight did not vary significantly with different alkyl groups. Of the three external bases studied, 2,2,6,6-tetramethyl piperidine (TMPIP), dimethoxydiphenyl silane (DMDPS), and t-butylmethyl ether (TBME), the addition of a small amount of one of the first two bases caused a substantial increase in both molecular weight and polydispersity of the isotactic polymer. Those increases leveled off quickly with increasing amounts of the external base. On the other hand, both average molecular weights and polydispersity of the atactic polymer decreased with a net increase in the molecular weight of the whole polymer. TBME, however, has no significant effect on either molecular weight or MWD. These effects are discussed in the context of the roles of the external base in propene polymerization. © 1995 John Wiley & Sons, Inc.     

Synthesis of emulsion styrene butadiene rubber by reversible addition–fragmentation chain transfer polymerization and its properties

Emulsion polymerization is generally used to synthesize styrene butadiene rubber (SBR), and the molecular weight of this rubber can be easily increased. However, the broad molecular weight distribution (MWD) of SBR increases energy loss and adversely affects the dynamic viscoelastic properties. To overcome this disadvantage, reversible addition–fragmentation chain transfer (RAFT) polymerization, which is a type of living polymerization, is applied to emulsion polymerization for preparing RAFT emulsion SBR (ESBR). The molecular weight and microstructure of RAFT ESBR are compared to those of commercially available ESBR 1502 by gel permeation chromatography and proton nuclear magnetic resonance spectroscopy. The aforementioned two polymers are used to prepare unfilled ESBR compounds, which are compared in terms of key physical properties (abrasion resistance, mechanical properties, and dynamic viscoelastic properties). It is confirmed that various physical properties of RAFT ESBR are improved due to its narrow MWD. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47069.     

可逆加成-断裂链转移聚合制备聚氯乙烯-b-聚乙二醇-b-聚氯乙烯共聚物

合成了含黄原酸酯端基的聚乙二醇（X-PEG-X）大分子链转移剂,并以其为可逆加成-断裂链转移试剂调控氯乙烯（VC）溶液和悬浮聚合,合成聚氯乙烯-b-聚乙二醇-b-聚氯乙烯（PVC-b-PEG-b-PVC）三嵌段共聚物。X-PEG-X调控VC溶液聚合得到的共聚物的分子量随聚合时间增加而增大,分子量分布指数小于1.65。X-PEG-X具有水/油两相分配和可显著降低水/油界面张力的特性,以X-PEG-X为链转移剂和分散剂,通过自稳定悬浮聚合也可合成PVC-b-PEG-b-PVC共聚物,共聚物颗粒无皮膜结构,分子量随聚合时间增加而增大;由于VC悬浮聚合具有聚合物富相和单体富相两相聚合特性,共聚物分子量分布指数略大于溶液聚合共聚物。通过乙酸乙烯酯（VAc）扩链反应进一步证实了PVC-b-PEG-b-PVC的“活性”,并合成PVAc-b-PVC-b-PEG-b-PVC-b-PVAc共聚物。水接触角测试表明PVC-b-PEG-b-PVC的亲水性优于PVC。     

Copolymerization of ethylene with α-olefins over supported titanium–magnesium catalysts. II. Comonomer as a chain transfer agent

The data on the effect of comonomer (propylene and 1-hexene) on molecular weight (M_w), molecular weight distribution (MWD), and content of terminal double bonds were obtained for ethylene/α-olefin copolymers produced over a supported titanium–magnesium catalyst (TMC) upon polymerization in the absence of hydrogen. The experimental data on the effect of comonomer concentration on M_w of polymers were used to calculate the ratios between the effective rate constants of chain transfer with monomer and the propagation rate constant. It was shown that the effective rate constant of chain transfer with monomers increases in the row of monomers: ethylene < 1-hexene < propylene. Meanwhile, the data on the effect of copolymers on content of terminal double bonds of various types demonstrate that different reactions of chain transfer with comonomer may simultaneously occur during copolymerization. It results in simultaneous formation of terminal vinylidene and trans-vinylene bonds. Therefore, the calculated rate constants of chain transfer with comonomer are complex values, which include the rate constants of chain transfer with comonomer occurring via different mechanisms. The data on MWD, short chain branching (SCB) and terminal double bonds content of different types were obtained by molecular weight fractionation of copolymers followed by the analysis of narrow fractions. The analysis of the data on MWDs of SCB and terminal double bonds shows that active sites of the TMC are considerably heterogeneous with respect to the rates of different chain transfer reactions with monomers. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Molecular weight distribution of polystyrene and of butadiene–styrene copolymers prepared in emulsion systems

Polystyrene and butadiene–styrene copolymers (SBR) were prepared in emulsion systems with a homologous series of commercial mercaptan modifiers. The molecular weight distribution (MWD) of the sets of polymers changed in a consistant manner when the regulating index of the mercaptan was relatively low. However the shape of the MWD curves appeared distorted in comparsion to theoretical curves when the modifier depleted rapidly and when divinylbenzene was present in the system. The divergence from the theoretical curve is attributed to a higher degree of branching in the high molecular weight fractions. Differences in MWD of SBR made with n- and tert- dodecyl mercaptans was marked. Notable differences were also found for SBR 1500 samples from the industry at random, but only slight differences were seen in a set of SBR 1503 samples. This study shows how the MWD of polymers prepared in emulsions can be varied simply by use of modifiers with different regulating indexes.     

Synthesis and characterization of water-based poly(vinyl acetate-co-butyl acrylate) latexes containing oligomeric protective colloid

Poly(vinyl acetate-co-butyl acrylate) latexes having oligomeric N-methylol acrylamide were prepared by semi-continuous emulsion polymerization. The effects of new protective colloid and comonomer ratios on the physicochemical and colloidal properties of latexes were investigated. The changes in homopolymer and copolymer latexes were determined by measuring viscosity, particle size, molecular weight (MW), molecular weight distribution (MWD), and surface tension. [`(M)]<sub>n</sub> \bar{M}_{n} values of copolymer latexes were found to be lower than the MWs of the poly(vinyl acetate) and poly(butyl acrylate) homopolymers. In general, [`(M)]_n \bar{M}_{n} and [`(M)]_\textw \bar{M}_{\text{w}} values of copolymer latexes changed irregularly with increasing BuA ratio in the copolymer composition.     

Emulsion polymerization kinetics and its reactor design. II. The MWD behavior in isothermal batch operation

The emulsion polymerization of styrene in an isothermal batch operation can be divided into three stages, and the corresponding kinetic rate equations are obtained from the experimental conversion-reaction time curve. The course of polymerization between the beginning of the reaction and any subsequent time is successfully computed for the reaction rate, the MWD, and the average degree of polymerization using the intergrals of these rate equations, which could readily explain the behavior of the emulsion polymerization. A comparison is made between the theoretical results and the experimental data. Both the theoretical treatment and the experimental data predict that the MWD for a typical emulsion polymerization is characterized by the rapid decrease in M _w/M _w ratio at the entrance of the zero-order stage and a gradual increase in the first-order stage regardless of an autoacceleration effect. An autoacceleration effect in the first-order stage is, however, evidence for a typical emulsion polymerization.     

Mechanochemical tuning of molecular weight distribution of styrene homopolymers as postpolymerization modification in solvent-free solid-state

Mechanochemical degradation by planetary ball milling (PM) is used for postpolymerization modification of styrene homopolymers (PS). A complete factorial design was chosen to study the effect of radical scavengers, milling time, initial molecular weight, and revolution radius (R_p), on the shape of molecular weight distributions (MWDs) of PS. Size-exclusion chromatography analysis shows the feasibility of fine-tuning MWD of PS at up to 40% conversion. Distributions ranged from unimodal to bimodal in a PM with R_p = 150 mm at different stage of milling, whereas in a PM with R_p of 60.8 mm the adjustment of unimodal distributions is achieved. Initial polydispersity is more important to develop bimodal distributions when compared with initial molecular weight. Fourier transform infrared and X-ray electron spectrometry analysis show some suppression of PS degradation and complete oxidation inhibition of macromolecular radicals with the incorporation of radical scavengers, which we considered as additional aids when adjusting the MWDs.     

A segment probability method for calculating the molecular weight distributions of linear polycondensates in a continuous reactor

The molecular weight distribution (MWD) of polymers is a target of the optimization and control of industrial polymerization processes, as it dictates the processability and properties of polymers. A method, named as segment probability method, is developed to calculate the MWD of polycondensates produced by monomers of types A₂ and B₂ in a continuous reactor. It considers a growing chain as being composed of A and B segments in the middle of the chain and two terminal segments at the chain ends. It calculates the propagation probabilities of these different types of segments upon taking into account both the polycondensation and side reaction kinetics as well as the residence time distribution of the continuous reactor. The method is validated by poly(butylene terephthalate) (PBT) obtained from an industrial polymerization process composed of a continuous esterification reactor. The MWDs of the PBT calculated by this method are in agreement with those measured by size exclusion chromatography with mean square errors less than 10%.     

Numerical inversion techniques in the recovery of molecular weight distribution expressed in different transformed domains. Experimental validation

In this work we analyze the feasibility of using numerical inversion techniques for recovering MWD of actual polymers from a transformed domain, specifically the one defined by probability generating functions. We start from known experimental MWDs, transform them, and then apply two different numerical techniques to recover the MWD. We analyze the influence of noise in the calculated probability generating functions on the quality of the recovered molecular weight distributions. We also study how the range of molecular weight selected for the inversion procedure affects the results. We compare the recovered distributions obtained by both methods and suggest a criterion for establishing the reliability of a given solution. We find that this general strategy is appropriate for the recovery of MWDs whether they are monomodal, multimodal, wide or narrow. This provides a tool for the treatment of actual polymerization systems for which there is no analytical solution for the mass balance equations.     

Molecular control of polystyrene in the reverse iodine transfer polymerization (RITP) – Suspension process

The reverse iodine transfer polymerization (RITP)-suspension polymerization of styrene was carried out in the presence of molecular iodine (I₂) using AIBN as an initiator and polyvinylalcohol (PVA) as a stabilizer under argon atmospheres at 70 °C for 10 h in the absence of light. The effect of iodine and styrene contents on the molecular characteristics was investigated. The result was summarized into three categories; (1) the two different polymerizations, the emulsion and suspension, simultaneously occurred in the RITP-suspension polymerization. (2) The emulsion fraction decreased, whereas the suspension fraction increased with the I₂ content from 0 to 0.25 mmol, resulting in the decreased molecular weight of the resultant polymer from 320,000 to 59,000 g/mol, respectively. This was arisen from the higher reaction rate of the RITP-suspension polymerization due to the better solubility of AIBN in the monomer droplet in the suspension phase than in the micelle in the emulsion phase upon the increased I₂ content. (3) As the styrene content increased up to 40 wt% under fixed I₂ content, the suspension fraction dominated, resulting in the reduced molecular weight from 59,000 to 38,000 g/mol, respectively. This was rationalized that the temperature increment upon the relatively small amount of solvent, which means due to the large amount of monomer, induced the rapid polymerization in the monomer droplets where the suspension polymerization was favorable. Thus, the RITP-suspension process dramatically decreased the molecular weight of PS not only by the presence of I₂, but also by the dominated suspension fraction.     

宽分子量分布高聚合度PVC的合成和性能

针对高聚合度聚氯乙烯加工性能较差的缺点，采用２段聚合温度法合成了宽分子量分布的ＰＶＣ树脂，并对树脂的热稳定性、加工性能和力学性能进行了研究。     

Rheological behaviors and molecular weight distribution characteristics of bimodal high‐density polyethylene

A series of bimodal high density polyethylene (PE) with different molecular weight distributions (MWDs) were prepared by melt blending, and the fitting multipeaks on Gaussian were used to analyze the MWD curves, and the ratio of the areas under unimodal curves was as a tool to characterize the MWD; the phase behaviors and rheological behaviors were studied by dynamic rheological. The results showed that homogeneous bimodal high density PEs could be successfully prepared via melt blending, and the bimodal characteristic could be adjusted as expected. For samples with the MWD peak positions unvaried, the storage modulus, complex viscosity, and zero‐shear viscosity decreased rapidly with the value of A_L/U increasing. Especially in the low frequency region, the loss modulus surpassed the storage modulus (G″ > G′) when A_L/U > 10.17 and the dynamic cross‐point Gx appeared and increased with increasing A_L/U, with an increasing extent much larger than that due to the width of MWD. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synthesis of a cis-rich,living poly[(o-methylphenyl)acetylene] by use of the MoOCl4-n-Bu4Sn-EtOH catalyst

Summary Polymerization of (o-methylphenyl)acetylene (o-MePA) by MoOCl₄-n-Bu₄Sn-EtOH catalyst in toluene at 0°C provided a cis-rich living polymer; cis 77%, M _w/M _n=1.21. The polymerization at -30°C gave results similar to those for 0°C, whereas the polymer obtained at 30°C exhibited a broader molecular weight distribution (MWD) and a lower cis content. Among several organotin compounds, only n-Bu₄Sn was effective for the living polymerization of o-MePA. The bulkier the alkyl group of alcohols as the third catalyst component, the broader the MWD of the polymer, while the geometrical structure was not affected by the alcohols.     

Scale‐Free Power‐Law Distribution of Emulsion‐Polymerized Branched Polymers: Power Exponent of the Molecular Weight Distribution

Summary: The molecular weight distribution (MWD), formed in emulsion polymerization that involves the polymer transfer reaction during Interval II, may approach the power‐law distribution as polymerization proceeds. The power exponent, α, of the weight fraction distribution W(M) = M^?α conforms to the relationship, α = 1/P_b, where P_b is the probability that the chain end is connected to a backbone chain. The MWD of emulsion‐polymerized polyethylene reported in literature agrees reasonably well with the relationship, W(M) = M^?α with α = 1/P_b. This simple relationship could be used to estimate the P_b value from the MWD data, possibly leading to determining the polymer transfer constant under well‐designed experimental conditions. Because α > 1, the number‐average MW always approaches a finite value, but the weight‐ and higher order‐averages of MWD may continue to increase as the particle grows without limit depending on the magnitude of P_b. The power‐law distributions are self‐similar, possessing the nature of fractals and lacking a characteristic scale. The i‐th moment of the MWD for the present reaction system continues to increase without limit during Interval II for P_b ≥ 1/i.

Molecular weight distribution of the emulsion‐polymerized polyethylene.