Synthesis of high molecular weight and narrow molecular weight distribution poly(acrylonitrile) via RAFT polymerization

Well‐defined polyacrylonitrile (PAN) of high viscosity‐average molecular weight (M_η = 405,100 g/mol) was successfully synthesized using reversible addition‐fragmentation chain transfer polymerization. The polymerization exhibits controlled characters: molecular weights of the resultant PANs increasing approximately linearly with monomer conversion and keeping narrow molecular weight distributions. The addition of 0.01 equiv (relative to monomer acrylonitrile) of Lewis acid AlCl₃ in the polymerization system afforded the obtained PAN with an improved isotacticity (by 8%). In addition, the influence of molecular weights and molecular weight distributions of PANs on the morphology of the electrospun fibers was investigated. The results showed that, under the same conditions of electrospinning, average diameter (247–1094 nm) of fibers increased with molecular weights of PANs, and it was much easier to get “uniform” diameter fibers while using PANs with narrow molecular weight distributions as the precursor of electrospinning. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

High molecular weight and low dispersity polyacrylonitrile by low temperature RAFT polymerization

High molecular weight polyacrylonitrile (PAN) with low dispersity has been successfully synthesized utilizing reversible addition‐fragmentation chain transfer (RAFT) polymerization. A comprehensive study was performed to understand the influence of reaction temperature, RAFT agent structure, and [M]₀:[CTA]₀[I]₀ on the polymerization kinetics, molecular weight, and dispersity. Enhanced control is attributed to reduction of side reactions by conducting the polymerization at lower temperature, and optimizing the radical exchange between active and dormant states via appropriate selection of RAFT agent and initiator. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 553–562     

Preparation of high cis‐1,4 polyisoprene with narrow molecular weight distribution via coordinative chain transfer polymerization

High cis‐1,4 polyisoprene with narrow molecular weight distribution has been prepared via coordinative chain transfer polymerization (CCTP) using a homogeneous rare earth catalyst composed of neodymium versatate (Nd(vers)₃), dimethyldichlorosilane (Me₂SiCl₂), and diisobutylaluminum hydride (Al(i‐Bu)₂H) which has strong chain transfer affinity is used as both cocatalyst and chain transfer agent (CTA). Differentiating from the typical chain shuttling polymerization where dual‐catalysts/CSA system has been used, one catalyst/CTA system is used in this work, and the growing chain swapping between the identical active sites leads to the formation of high cis‐1,4 polyisoprene with narrowly distributed molecular weight. Sequential polymerization proves that irreversible chain termination reactions are negligible. Much smaller molecular weight of polymer obtained than that of stoichiometrically calculated illuminates that, differentiating from the typical living polymerization, several polymer chains can be produced by one neodymium atom. The effectiveness of Al(i‐Bu)₂H as a CTA is further testified by much broad molecular weight distribution of polymer when triisobutylaluminum (Al(i‐Bu)₃), a much weaker chain transfer agent, is used as cocatalyst instead of Al(i‐Bu)₂H. Finally, CCTP polymerization mechanism is validated by continuously decreased M_w/M_n value of polymer when increasing concentration of Al(i‐Bu)₂H extra added in the Nd(ver)₃/Me₂SiCl₂/Al(i‐Bu)₃ catalyst system. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Butyl acrylate RAFT polymerization in miniemulsion

Few successes about butyl acrylate (BA) RAFT miniemulsion homopolymerization were reported, even though styrene, methyl methacrylate, and vinyl acetate had been successfully applied in reversible addition fragmentation transfer (RAFT) miniemulsion polymerization. In this article, four types of RAFT agent with various designed R and Z groups [benzyl dithioisobutyrate (BDIB), 1-phenylethyl phenyldithioacetate (PEPDTA), cumyl dithioisobutyrate (CDIB), benzyl dithiobenzoate] were used to mediate BA miniemulsion polymerization using the conditions (5 wt % hexadance and sodium dodecyl sulfate) effective for styrene and methyl methacrylate systems. All four types of the RAFT agents effectively control over the bulk polymerization. In contrast, only BDIB resulted in a rather narrow molecular weight distribution in the miniemulsion polymerization. A pronounced inhibition and rate retardation were observed in both bulk and miniemulsion polymerizations mediated by CDIB and benzyl dithiobenzoate. When compared with the bulk polymerization, a much longer inhibition period (over eight times) was observed in the CDIB-mediated miniemulsion polymerization. It was concluded that only the RAFT agent with the primary R group and Z group with less stabilizing ability to the intermediate radicals is effective to mediate BA miniemulsion polymerization in terms of achieving a narrow molecular weight distribution and short inhibition period. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2304–2315, 2007     

Simulation model for the molecular weight distribution in emulsion polymerization

A Monte Carlo simulation model for the kinetics of emulsion polymerization is proposed. In the present model, the formation of each polymer molecule is simulated by the use of only a couple of probability functions; therefore, the calculation can be handled well even on personal computers. It is straightforward to account for virtually any kinetic event, such as the desorption of oligomeric radicals and chain length dependence of kinetic parameters, and as a consequence very detailed information such as the full distributions of the dead polymer molecular weights and the macroradicals among various polymer particles can be obtained. When bimolecular terminations are the dominant chain stoppage mechanism, the instantaneous molecular weight distribution (produced in a very small time interval) becomes broader than that for homogeneous polymerizations due to a higher possibility that short and long polymer radicals react with each other if bimolecular reactions are fast enough. The increase in the polydispersity of the MWD is fairly large, especially when bimolecular termination by disproportionation is significant; however, the gel permeation chromatography (GPC) may not be a suitable analytical technique to detect such broadening since oligomeric peaks may not be observed in the elution curve. The present simulation method provides greater insight into the complicated phenomena of emulsion polymerizations. © 1995 John Wiley & Sons, Inc.     

The molecular weight distribution for chain polymerization plus side reaction

The kinetics of chain polymerization is investigated for the case of a complicating side reaction. In addition to the polymerization reaction, A_i + M → A_i+1, there is a reversible side reaction, A_i + Q ↔ B_i. Initiation is assumed to be instantaneous. The monomer concentration M, and the concentration of the reacting species Q, are assumed to be constant. The reaction kinetics are solved exactly, yielding the distribution of living and dormant polymer, as well as the molecular weight distribution, as explicit functions of the reaction rate constants and the time. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1711–1725, 1997     

Thermosensitive diblock copolymers with designed molecular weight distribution: Synthesis by continuous living cationic polymerization and micellization behavior

The synthesis of diblock copolymers with designed molecular weight distributions (MWDs) was successfully demonstrated in a continuous living cationic polymerization system using simple equipment. The control of MWDs was achieved by gradually feeding a polymerization reaction mixture into a terminating agent. As thermosensitive diblock copolymers, poly(vinyl ethers) containing a thermosensitive segment with oxyethylene side chains and a hydrophilic segment were prepared. The polymerization was carried out in a gas‐tight microsyringe, and the polymerization mixture was added continuously into methanol during the second‐stage polymerization. The self‐association behavior of the resulting diblock copolymers was evaluated by dynamic light scattering in water. MWD‐designed polymers with thermosensitive segments that varied continuously in length and hydrophilic segments of nearly uniform lengths formed micelles with a broad size distribution. Conversely, polymers with nearly uniform thermosensitive segments and hydrophilic segments of different lengths formed micelles with a narrow size distribution, as observed with conventional narrow MWD diblock copolymers. Thus, the MWD of the thermosensitive segment proved a decisive factor in achieving fine control of self‐association. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2212–2221, 2008     

Gas-phase polymerization of propylene: Reaction kinetics and molecular weight distribution

Gas-phase polymerizations have been executed at different temperatures, pressures, and hydrogen concentrations using Me₂Si[Ind]₂ZrCl₂ / methylaluminoxane / SiO₂(Pennsylvania Quarts) as a catalyst. The reaction rate curves have been described by a kinetic model, which takes into account the initially increasing polymerization rate. The monomer concentration in the polymer has been calculated with the Flory–Huggins equation. The kinetic parameters have been determined by fitting the reaction rate curves with the model. At high temperatures, pressures, and hydrogen concentrations a runaway on particle scale may occur leading to reduced polymer yields. The molecular weight and molecular weight distribution of the polymer samples could be described by a “two-site model.” At constant temperature the chain-transfer probability of sites 1 and 2 depends only on the hydrogen concentration divided by the monomer concentration. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 500–513, 2001     

Modeling molecular weight distribution in emulsion polymerization reactions with transfer to polymer

A mathematical model was developed for the computation of the dynamic evolution of molecular weight distributions (MWDs) during nonlinear emulsion polymerization reactions. To allow the direct computation of the whole MWD, an adaptive orthogonal collocation technique was applied. The model was validated with experimental methyl methacrylate/butylacrylate (BuA) semicontinuous and vinyl acrylate (VA)/Veova10 continuous emulsion polymerization results. Both systems considered introduce significant chain‐transfer reactions to polymer chains as a result of the presence of BuA and VA, respectively. The model developed was able to represent quite properly the kinetics and MWD of polymer samples during emulsion polymerizations. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3513–3528, 2001     

An experimental investigation on the evolution of the molecular weight distribution in styrene emulsion polymerization

Styrene ab initio emulsion polymerizations were conducted at 70°C in an automated reaction calorimeter. Two polymerizations were performed, one above and the other below the critical micelle concentration (CMC) of the surfactant, thus ensuring differing polymerization kinetics between the two: the system below the CMC gave large particles that were expected to follow pseudobulk kinetics, while that above the CMC gave small particles that were expected to follow zero-one kinetics. The evolutions of the molecular weight distributions (MWDs) were characterized by removing samples periodically during the course of the reactions and analyzing with gel permeation chromatography. Interpretation of the data used average molecular weights, the GPC MWDs, and the number MWDs, as functions of conversion. It was found that all of the number MWDs (plotted as ln (number of polymer chains) vs. molecular weight of polymer chains) were concave-up at low molecular weights and become nearly linear at molecular weights (≥3−4 × 10⁶); this linearity is expected from theory. The slope of the high molecular weight region was consistent with theory for the dominant mode for chain stoppage: termination and transfer for the pseudobulk system and (predominantly) chain transfer to monomer for the zero-one system. The most likely explanation for the concavity of the number MWDs is a heterogeneity of radicals: some surface anchored with sulfate end groups and others (with hydrogen end groups arising from transfer to monomer and/or reentry) being more mobile. Thus, two types of termination are proposed: slow reaction-diffusion for the less mobile surface anchored chains, and rapid short-long (center of mass) termination for the more mobile hydrogen-terminated chains. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 989–1006, 1997     

Bimodal molecular weight distribution formed in the emulsion polymerization of ethylene

It is known that the molecular weight distribution (MWD) formed in an emulsion polymerization of ethylene can be bimodal. However, the origin of the bimodality has not been elucidated. In this article, a Monte Carlo simulation is conducted, mostly with parameters reported in the literature. The simulated MWDs are bimodal because of the limited volume effect; that is, the high molecular weight profiles are distorted by the small particle size, which is comparable to the size of the largest branched polymer molecule in a particle. The simulated MWDs agree reasonably well with the experimentally obtained MWDs. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3426–3433, 2002     

Modeling molecular weight distribution and effect of termination in controlled radical polymerization: A novel and transformative approach

A theory for polymer molecular weight distribution (MWD) in atom transfer radical polymerization (ATRP) has been derived by using analogy to a series of continuous stirred tank reactors (CSTRs). This approach relates one activation cycle in ATRP to one reactor in CSTRs. The derived MWD expression includes effect of radical termination and allows detailed investigation on the factors that determine the polymer distribution, namely the level of “control” and “livingness.” The level of control means the average number of activation cycles experienced by individual chains. The degree of livingness is quantified by the fraction of terminated chains. It was found that the effect of livingness and control on MWD is complex. Large number of activation cycles does not guarantee a narrow distribution. There exists an optimum value of activation cycles to achieve the lowest polydispersity by balancing the control (achieved by increasing activation cycles) with the loss of livingness (also caused by increasing activation cycles). The distribution for ATRP is in‐between Poisson and Flory distributions, determined by the level of control and livingness. In general, having high degree of livingness and maintaining control are both necessary conditions to obtain narrow MWD. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 639–651     

Synthesis of poly(n‐butyl acrylate) containing multiblocks with a narrow molecular weight distribution using cyclic trithiocarbonates

The polymerization of n‐butyl acrylate in the presence of two cyclic trithiocarbonates (CTTCs) and the synthesis of multiblock poly(n‐butyl acrylate) have been investigated. The CTTCs not only can be stepwise incorporated into the polymer chain via reversible addition–fragmentation chain transfer (RAFT) but also can be polymerized into polytrithiocarbonate, which functions as a macro‐RAFT agent in turn. Through two kinds of mechanisms, multiblock poly(n‐butyl acrylate) containing narrow‐polydispersity blocks can be prepared. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6600–6606, 2006     

Modeling the molecular weight distribution of block copolymer formation in a reversible addition–fragmentation chain transfer mediated living radical polymerization

Block copolymers have become an integral part of the preparation of complex architectures through self‐assembly. The use of reversible addition–fragmentation chain transfer (RAFT) allows blocks ranging from functional to nonfunctional polymers to be made with predictable molecular weight distributions. This article models block formation by varying many of the kinetic parameters. The simulations provide insight into the overall polydispersities (PDIs) that will be obtained when the chain‐transfer constants in the main equilibrium steps are varied from 100 to 0.5. When the first dormant block [polymer–S? C(Z)?S] has a PDI of 1 and the second propagating radical has a low reactivity to the RAFT moiety, the overall PDI will be greater than 1 and dependent on the weight fraction of each block. When the first block has a PDI of 2 and the second propagating radical has a low reactivity to the RAFT moiety, the PDI will decrease to around 1.5 because of random coupling of two broad distributions. It is also shown how we can in principle use only one RAFT agent to obtain block copolymers with any desired molecular weight distribution. We can accomplish this by maintaining the monomer concentration at a constant level in the reactor over the course of the reaction. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5643–5651, 2005     

超高分子量聚丙烯酰胺的表征

采用环己烷及乳化剂ＥＡ合成了分子量（■≥１２００万）的速溶型、粉状聚丙烯酰胺（ＰＡＡ），采用扫描电镜及透射电镜观察了其微细结构，对其速溶性作出了解释。给出了该产品的综合性能数据，与进口样品进行了对比。     

Molecular weight distribution in nonlinear emulsion polymerization

A modelistic study of the molecular weight distribution (MWD) formed in emulsion polymerization that involves chain transfer to polymer is conducted, by focusing our attention to the effect of very small reaction volume on the formed MWD. In emulsion polymerization, a polymer radical that causes polymer transfer reaction must choose the partner only within the same particle, which makes the expected size of the polymer molecule to be chosen smaller compared with the corresponding polymerization system that involves an infinitely large number of polymeric species. The usual assumption for homogeneous polymerization that the rate of chain transfer to a particular polymer molecule is proportional to its chain length cannot be used, except when branching frequency is low and particle size is large enough. This fact invalidates the direct use of models developed for homogeneous nonlinear polymerizations to emulsion polymerizations. Model equations that could be used to assess the significance of the limited space effects on the MWD under a given polymerization condition are also proposed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1515–1532, 1997     

Frontal polymerization as a new method for developing drug controlled release systems (DCRS) based on polyacrylamide

Stimuli-responsive polymers are macromolecular materials that undergo changes in response to small external stimuli in the environmental conditions. Among stimuli-responsive hydrogels are several polyacrylamides. Frontal polymerization is a fast, easy and inexpensive polymerization technique used for the synthesis of macromolecules.Aim of this work was the evaluation of the Frontal polymerization technique as new method for the preparation of controlled release dosage forms in which drug loading and polymer preparation occur together, as well as the possibility of obtaining more dosage units by a unique preparation. Hydrogels based on polyacrylamide containing diclofenac sodium salt were prepared using the Frontal polymerization and compared with similar systems obtained by the classic batch method. Polymers characterized by three different degree of cross-linking were prepared. The stability of the drug during the sample preparation was evaluated by IR analysis. The obtained samples were characterized in terms of drug content, morphology, in vitro drug release and swelling properties. Samples were studied also divided into disks. The results show that hydrogels based on polyacrylamide can be prepared by Frontal polymerization; these samples show similar properties to those obtained by batch polymerization. The drug is stable in the polymerization reaction conditions. Samples characterized by the lowest degree of cross-linking show drug loading values always higher than samples with the highest one regardless of the preparation method employed. The swelling ratio decreases as the degree of cross-linking increases. Loaded samples swell more than drug free ones. From a single preparation of hydrogel, three disks showing same drug content and in vitro release behaviour can be obtained and thus they can be used as three single dosage units.