The role of chain transfer agents in the emulsion polymerization of styrene

The effect of chain transfer agents on the nucleation and growth of polymer particles in the emulsion polymerization of styrene were examined extensively. The chain transfer agents used are carbon tetrachloride, carbon tetrabromide, and four primary mercaptans (C₂, n-C₄, n-C₇, and n-C₁₂). It is shown that with an increase in the amount of chain transfer agents charged the rate of polymerization per particle decreases progressively. The number of polymer particles formed, on the other hand, increases initially then decreases. These effects can be enhanced by using a chain transfer agent with higher values of chain transfer constant and solubility in water. It is also demonstrated that with increasing radical desorption from the particles, aided by chain transfer agents, the emulsifier dependence exponent for the number of polymer particles formed increases from 0.6 to 1.0 and the initiator dependence exponent decreases from 0.4 to 0. The effect of chain transfer agents on the nucleation and growth of polymer particles in the emulsion polymerization of styrene can be explained in terms of desorption of chain-transfered radicals from the polymer particles.     

Long-chain branching in free-radical polymerization due to chain transfer to polymer

The Monte Carlo sampling technique is used to investigate the branched structure formation during free-radical polymerization that involves chain transfer to polymer. This method accounts for the history of the generated branched structure and can provide virtually any structural information, because one can observe each polymer molecule directly. In this paper, we investigate the whole molecular weight distribution (MWD) for both pre- and postgelation periods, the MWDs for polymer molecules containing 0, 1, 2, 3, … branch points, the branching density of polymer molecules as functions of both size and the number of branch points, the spatial distribution of the branched chains at the theta state, etc. Contrary to the term ‘long-chain’ branching, many branch chains are relatively small, and the branched structures formed are significantly different from those usually depicted to introduce ‘branched polymers’ in many introductory textbooks. The radii of gyration at the theta state can be approximated by the Zimm-Stockmayer equation for random branching, in spite of various violations against the assumptions used in deriving the equation © 1995 John Wiley & Sons, Inc.     

Chain transfer in anionic polymerization. Determination of chain-transfer constants by using carbon-14-labeled chain transfer agents

A method is described in which ¹⁴C-labeled chain-transfer agents are employed to measure chain-transfer constants in anionic polymerization as low as 10^?6. Each chain-transfer step incorporates one molecule of the chain-transfer agent into the polymer so that measurement of the activity and conversion allows evaluation of the chain-transfer constant. This method is independent of the initiator concentration and efficiency, making the technique especially useful when problems with the initiator are encountered. The experimental procedure is described in detail for the case of chain transfer to toluene in the n-butyllithium-initiated polymerization of styrene, where C_RH was found to be 5 × 10^?6. A mathematical treatment is given showing the relationship between the degree of polymerization (DP _n) and chain transfer.     

Miniemulsion polymerization of styrene with chain transfer agent as cosurfactant

Miniemulsion polymerization of styrene with the chain transfer agent n-dodecyl mercaptan (DDM) used as cosurfactant was studied. Droplet size and shelf life for unpolymerized miniemulsions were measured and compared with those of equivalent macroemulsions. The miniemulsion monomer droplets with dodecyl mercaptan as cosurfactant were very stable. Shelf lives were from 17 h to 3 months. The kinetics of miniemulsion polymerization were studied. Unlike other miniemulsion systems where the cosurfactant does not act as a chain transfer agent, the polymerization rate falls with cosurfactant level because the chain transfer agent enhances radical desorption from the particles. The polymerization rate in all the miniemulsions was lower than that of the corresponding macroemulsions. Polymerized particles were larger than in the corresponding macroemulsions, but molecular weights were lower. Results indicate that DDM can serve as an effective cosurfactant as well as a chain transfer agent. The fact that the molecular weights are lower in the miniemulsion reactions indicates predominant droplet nucleation. © 1997 John Wiley & Sons, Inc.     

Borane chain transfer reaction in olefin polymerization using trialkylboranes as chain transfer agents

This article discusses a new borane chain transfer reaction in olefin polymerization that uses trialkylboranes as a chain transfer agent and thus can be realized in conventional single site polymerization processes under mild conditions. Commercially available triethylborane (TEB) and synthesized methyl‐B‐9‐borabicyclononane (Me‐B‐9‐BBN) were engaged in metallocene/MAO [depleted of trimethylaluminum (TMA)]‐catalyzed ethylene (Cp₂ZrCl₂ and rac‐Me₂Si(2‐Me‐4‐Ph)₂ZrCl₂ as a catalyst) and styrene (Cp*Ti(OMe)₃ as catalyst) polymerizations. The two trialkylboranes were found—in most cases—able to initiate an effective chain transfer reaction, which resulted in hydroxyl (OH)‐terminated PE and s‐PS polymers after an oxidative workup process, suggesting the formation of the B‐polymer bond at the polymer chain end. However, chain transfer efficiencies were influenced substantially by the steric hindrances of both the substituent on the trialkylborane and that on the catalyst ligand. TEB was more effective than TMA in ethylene polymerization with Cp₂ZrCl₂/MAO, whereas it became less effective when the catalyst changed to rac‐Me₂Si(2‐Me‐4‐Ph)₂ZrCl₂. Both TEB and Me‐B‐9‐BBN caused an efficient chain transfer in the Cp₂ZrCl₂/MAO‐catalyzed ethylene polymerization; nevertheless, Me‐B‐9‐BBN failed in vain with rac‐Me₂Si(2‐Me‐4‐Ph)₂ZrCl₂/MAO. In the case of styrene polymerization with Cp*Ti(OMe)₃/MAO, thanks to the large steric openness of the catalyst, TEB exhibited a high efficiency of chain transfer. Overall, trialkylboranes as chain transfer agents perform as well as B? H‐bearing borane derivatives, and are additionally advantaged by a much milder reaction condition, which further boosts their applicability in the preparation of borane‐terminated polyolefins. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3534–3541, 2010     

Controlled radical polymerization of styrene in miniemulsion polymerization using reversible addition fragmentation chain transfer

Miniemulsion polymerizations of styrene in the presence of two reversible addition–fragmentation chain-transfer (RAFT) agents were studied. The rates were significantly retarded by the presence of a RAFT agents S-(thiobenzoyl)thioglycolic acid, 1, or dithiobenzoic acid 1-phenylethyl ester, 2. Control in miniemulsion polymerization is not as good as for bulk polymerizations. The miniemulsions could also be stabilized against Ostwald ripening by a polymer terminated by a dithiobenzoic moiety. In this case, the polymerization was not controlled because of the generation of renucleated particles. To cite this article: I. Uzulina et al., C. R. Chimie 6 (2003).     

In silico allergenicity prediction of several lipid transfer proteins

Non-specific lipid transfer proteins (nsLTPs) are common allergens and they are particularly widespread within the plant kingdom. They have a highly conserved three-dimensional structure that generate a strong cross-reactivity among the members of this family. In the last years several web tools for the prediction of allergenicity of new molecules based on their homology with known allergens have been released, and guidelines to assess potential allergenicity of proteins through bioinformatics have been established. Even if such tools are only partially reliable yet, they can provide important indications when other kinds of molecular characterization are lacking. The potential allergenicity of 28 amino acid sequences of LTPs homologs, either retrieved from the UniProt database or in silico deduced from the corresponding EST coding sequence, was predicted using 7 publicly available web tools. Moreover, their similarity degree to their closest known LTP allergens was calculated, in order to evaluate their potential cross-reactivity. Finally, all sequences were studied for their identity degree with the peach allergen Pru p 3, considering the regions involved in the formation of its known conformational IgE-binding epitope. Most of the analyzed sequences displayed a high probability to be allergenic according to all the software employed. The analyzed LTPs from bell pepper, cassava, mango, mungbean and soybean showed high homology (>70%) with some known allergenic LTPs, suggesting a potential risk of cross-reactivity for sensitized individuals. Other LTPs, like for example those from canola, cassava, mango, mungbean, papaya or persimmon, displayed a high degree of identity with Pru p 3 within the consensus sequence responsible for the formation, at three-dimensional level, of its major conformational epitope. Since recent studies highlighted how in patients mono-sensitized to peach LTP the levels of IgE seem directly proportional to the chance of developing cross-reactivity to LTPs from non-Rosaceae foods, and these chances increase the more similar the protein is to Pru p 3, these proteins should be taken into special account for future studies aimed at evaluating the risk of cross-allergenicity in highly sensitized individuals.     

Merrifield resin-supported chain transfer agents, precursors for RAFT polymerization

The modification of Merrifield resins to form chain transfer agent (CTA) precursors for reversible addition fragmentation chain transfer (RAFT) polymerization is investigated. A series of CTA precursor resins were prepared and characterized by FTIR and elemental analysis (EA). [reaction: see text]     

Regularities of the kinetics of miniemulsion polymerization of styrene in the presence of dithiobenzoates as reversible chain transfer agents

Russian Chemical Bulletin - The results of comparative kinetic study of the miniemulsion controlled radical polymerization of styrene in the presence of reversible chain transfer agents (benzyl...     

The catalytic system tri-n-butyl boron-p-quinone in the free-radical polymerization of styrene

The AIBN-initiated polymerization of styrene is conducted at 60 and 80°C in the presence of tri-n-butyl boron and several p-quinones. The rate of polymerization and the molecular-mass characteristics of the polymers depend on the structure of the used p-quinone and the temperature of the process.     

In situ radical transfer addition polymerization of styrene from silica nanoparticles

The in situ radical transfer addition polymerization of styrene from silica nanoparticles was carried out by the free radical polymerized of styrene in the presence of mercaptopropyl-modified silica nanoparticles as chain-transfer agent. The effects of the amount of the initiator, polymerizing temperature and polymerizing time on the convention of styrene (C) and the percentage of grafting were investigated. Results of elemental analysis, IR, X-ray photoelectron spectrometer and transmission electron microscope demonstrated that the desired polymer chains have been covalently bonded to the surface of the silica nanoparticles. A C of 42.56% and a PG of 38.10% could be achieved with the optimal condition. The polystyrene grafted silica nanoparticles could be separated and used as nanofiller for polymers.     

Effect of polyfunctional chain transfer agents on molecular weight distribution in free-radical polymerization, 1. The ideal case: C equals one

The theoretical approach to free-radical polymerization in the presence of polyfunctional transfer agents can be derived in a simple way if the effects of primary chains and termination reactions are neglected. Under these conditions, and for values of the chain transfer constant C = 1, the analytical expressions for the number- and weight-average degree of polymerization, dispersion index and weight distribution function were derived through the elementary probability theory and generating functions. Numerical examples are reported on the relationship between distribution parameters, functionality and monomer conversion.     

Kinetic model of reversible addition fragmentation chain transfer polymerization of styrene in seeded emulsion

A detailed model describing the kinetics of living polymerization mediated by reversible addition‐fragmentation chain transfer (RAFT) in seeded emulsion polymerization is developed. The model consists of a set of population balance equations of the different radical species in the aqueous phase and in the particle phase (accounting for radical segregation) as well as for the dormant species in the particle phase. The entire population of radicals was divided into several distinguished species, based on their length and their chain end group. The model results are helpful in understanding inhibition and retardation phenomena that are typical for RAFT emulsion polymerizations. While inhibition is due to the radical loss in form of the RAFT leaving group, retardation is mostly caused by a small amount of short dormant chains in the particle phase, leading to a slight increase of radical loss via RAFT exchange with radicals entering a particle. The model results are compared to a series of experiments, using cumyl dithiobenzoate as a RAFT agent in polymerizations of styrene. The agreement between experimental and model results is good and, notably, the only parameters considered adjustable were the RAFT exchange rate coefficients. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6114–6135, 2006     

Effect of polyfunctional chain transfer agents on molecular weight distribution in free-radical polymerization, 2. The case C ≤ 1

In this part of the series, the influence of polyfunctional chain transfer agents with transfer constant C ≤ 1 on the molecular weight distribution was studied. The analytical expressions for the number- and weight-average degree of polymerization, and dispersion index were derived by kinetic and statistical methods. The expression for the molecular weight distribution can only be obtained by statistical methods. Some numerical examples on the dependence of distribution parameters as a function of the functionality of transfer agents f and transfer constants are illustrated. A critical value of the chain transfer constant was found to exist, which permits the synthesis of linear (for f = 2) or branched polymers (f > 2) with DP _w/DP _n approximately equal to 2 during the entire course of the polymerization.     

Reversible addition–fragmentation chain transfer polymerization of styrene under microwave irradiation

Reversible addition–fragmentation chain transfer (RAFT) polymerizations of styrene under microwave irradiation (MI), with or without azobisisobutyronitrile, were successfully carried out in bulk at 72 and 98 °C, respectively. The results showed that the polymerizations had living/controlled features, and there was a significant enhancement of the polymerization rates under MI in comparison with conventional heating (CH) under the same conditions. The polymer structures were characterized with ¹H and ¹³C NMR. The results showed the same structure for both polymers obtained by MI and CH. Successful chain‐extension experimentation further demonstrated the livingness of the RAFT polymerization carried out under MI. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6810‐6816, 2006