Polymerization Of Vinyl Acetate Research Articles

Morpholine‐based RAFT agents for the reversible deactivation radical polymerization of vinyl acetate and N‐vinylimidazole

AbstractReversible addition–fragmentation chain transfer (RAFT) polymerization of less‐activated monomers in a controlled fashion is challenging due to the high reactivity and instability of the propagating radicals. We have designed dithiocarbamate‐based RAFT agents with morpholine as activating ‘Z’ group and benzyl, ethyl(1‐ethanoate)yl, ethyl(2‐propanoate)yl and cyanomethyl as ‘R’ leaving groups and investigated them for the reversible deactivation radical polymerization of vinyl acetate (VAc) and N‐vinylimidazole (N‐VIm). RAFT polymerization of VAc and N‐VIm at 70 °C using azobisisobutyronitrile as a free radical initiator proceeded in a controlled fashion as demonstrated by a linear increase in molar mass with conversion. Interestingly, the polymerization of VAc followed fast kinetics (approx. 60 min) with good to moderate control affording high‐molar‐mass poly(VAc) polymers. Furthermore, the synthesized chain transfer agents were able to polymerize N‐VIm under controlled conditions. The morpholine RAFT agents bearing cyanomethyl and ethyl(2‐propanoate)yl leaving groups showed better control of the polymerization of VAc and N‐VIm compared to the others. © 2020 Society of Chemical Industry

Adsorption process of Co(acac)2 catalyst on the surface of mesoporous silica gel particles: an effective method to make a new supported catalyst for the controlled radical polymerization of vinyl acetate

The supported catalysts, specifically those containing cobalt(II) acetylacetonate (Co(acac)2), are among the important materials for the organometallic-mediated radical polymerization of vinyl acetate (VAc) monomers. In this study, we report an applicable and efficient technique to produce new supported Co(acac)2 catalysts through adsorption of Co(acac)2 on the surface of mesoporous silica gel particles. For this purpose, adsorption process of Co(acac)2 on these particles was studied from different perspectives such as mechanism, kinetics and thermodynamic. Adsorption experiments were conducted under various conditions, including various contact times, initial Co(acac)2 concentrations, and temperatures. At the end, a number of adsorption models with the highest compatibility with the experimental data were implemented. The polymerization of VAc was carried out using a new supported catalyst. This catalyst was highly efficient for either controlling the radical polymerization and synthesis of poly vinyl acetate with controlled molecular weight and molecular weight distribution.

Manufacturing complex Al2O3 ceramic structures using consumer-grade fused deposition modelling printers

Purpose This study aims to fabricate complex ceramic tetrahedron structures, which are challenging to produce by more conventional methods such as injection molding. To achieve this aim, thermoplastic-ceramic composite filaments were developed and printed with unmodified, consumer-grade, fused deposition modelling (FDM) printers instead. Design/methodology/approach Al2O3 ceramic powder was mixed with ethylene vinyl acetate polymer as a binder (50 Vol.- per cent) to form a filament with a constant diameter of 1.75 mm. After the printing and thermal treatment stages, the shrinkage and mechanical properties of cuboid and tetrahedron structures were investigated. Findings The shrinkage of the parts was found to be anisotropic, depending on the orientation of the printing pattern, with an increase of 2.4 per cent in the (vertical) printing direction compared to the (horizontal) printing layer direction. The alignment of the ceramic particle orientations introduced by FDM printing was identified as a potential cause of the anisotropy. This study further demonstrates that using a powder bed during the thermal debinding process yields sintered structures that can withstand twice the compressive force. Originality/value Ceramic FDM had previously been used primarily for simple scaffold structures. In this study, the applicability of ceramic FDM was extended from simple scaffolds to more complex geometries such as hollow tetrahedra. The structures produced in this study contain dense parts printed from multiple contiguous layers, as compared to the open structures usually found in scaffolds. The mechanical properties of the complex ceramic parts made by using this FDM technique were also subjected to investigation.

Thermal analysis: an effective characterization method of polyvinyl acetate films applied in corrosion inhibition field

To prepare a commercial product with economic and technical relevance, polyvinyl acetate (PVAc) was synthesized, under our laboratory conditions, updating an industrial prescription by emulsion polymerization of vinyl acetate using polyvinyl alcohol as emulsifier, Fenton reagent as reaction initiator and acetic acid as aqueous phase pH-regulator. A polymeric film (PSYF) was made and comparatively characterized with a standard PVAc film (PSTF) by scanning electron microscopy with energy-dispersive X-ray spectroscopy, Raman spectroscopy and simultaneous thermogravimetry and derivative thermogravimetry analysis. Similar vibration bands for PSTF and PSYF in the Raman spectra were identified which accurately reproduce the values reported in the literature. Thermal analysis was an efficient method to detect the compositional differences between the two polymeric films, precisely indicating that PSYF contained 92% PVAc and a mixture of 8% containing unreacted monomer, polyvinyl alcohol and humidity. Additionally, the PVAc film inhibitive properties on carbon steel corrosion in acid environments were investigated by potentiodynamic polarization and electrochemical impedance spectroscopy. A high inhibition efficiency of PVAc, ranging between 86.0 and 88.0%, was computed from electrochemical measurements, indicating the polymer adsorption ability on carbon steel surface.

The Effect of Hair Spray Exposure to the Changes in Electrical Properties of Mice Organ

Human activities at this time tend to use substances that are pollutants and have an impact on health. Pollutants can enter the body through the process of respiration and will disrupt the organ function. Substances that are often used in daily life and potentially as pollutants are hairspray. They contain vinyl acetate and methacrylate complex polymers. Cocamide DEA surfactants in hairspray can cause cancer and produce toxins. The use of hairspray continuously will cause organ damage due to the emergence of secondary metabolites and free radicals. The diagnosis of organ damage in the latest research from various scientists is to utilize the electrical characteristics of the organ. The bioelectric characteristic that can be observed is dielectric constant. Based on the impact of organ damage and suspected changes in organ electrical properties due to exposure to pollutants, a study needs to be conducted to observe changes in the resistivity and dielectric constant in organ tissue in mice exposed to hairspray with cocamide DEA surfactant content. The results showed that the more concentration of exposure given causes increasing the level of damage to the organs. The increasing level of damage is very significant in the changing of the resistivity and dielectric constant value of mice organs. The changing of electrical properties of mice organs is possible due to damage to organ cells caused by increasing reactive oxygen species and free radicals due to oxidation reactions resulting from the interaction of cocamide DEA with cell components.

Analytical application of poly(vinyl chloride-co-vinyl acetate) electrode modified with silver for chloride ions determination in real systems

Application possibility of new simple Ag-selective electrode for the determination of chloride ions, was investigated in this paper. The poly(vinyl chloride-co-vinyl acetate) electrode modified with silver (PVCAc/Ag) was prepared by simple immersion a glassy carbon rod coated with poly(vinyl chloride-co- vinyl acetate) polymer, which contained plasticizer and lipophilic additive, but without ionophore, into the silver nitrate solution. This simplicity of construction and extensive pH range through remarkable acidic media (pH 0.7 to 7.0), are the advantages of this modified electrode over many of reported silver selective electrodes based on a polymer matrix containing different ionophores. The response of the electrode was linear with a Nernstian slope of 60.25 mV/decade in the concentration range from 1.0×10-1 to 1.0×10-5 mol/dm3 Ag+ and with a detection limit of 4.25×10-6 mol/dm3. Proposed PVCAc/Ag electrode was applied to the determination of chloride ions in the samples spring waters. The results of chloride ion determination in samples spring waters obtained by proposing electrode and comparative Ag/AgCl electrode were in satisfactory agreement.

Cobalt(II) phenoxy‐imine complexes in radical polymerization of vinyl acetate: The interplay of catalytic chain transfer and controlled/living radical polymerization

Cobalt(II) phenoxy‐imine complexes in radical polymerization of vinyl acetate: The interplay of catalytic chain transfer and controlled/living radical polymerization

L-menthol-based xanthate mediator for RAFT polymerization of vinyl acetate

A xanthate RAFT agent 2-isopropyl-5-methylcyclohexyl 2-(ethoxycarbonothioyl)thio) propanoate has been synthesized via the reactions of L-menthol with 2-bromo-propionyl bromide in THF in the presence of triethylamine leading to the formation of 2-isopropyl-5-methylcyclohexyl 2-bromopropanoate followed by its reaction with potassium-O-ethyl xanthate in ethanol at room temperature and characterized by 1H, 13C NMR and FTIR. RAFT polymerization of vinyl acetate (VAc) using it has resulted in the formation of well-defined polyVAc in controlled manner. This is evidenced by (i) the observation of the pseudo first order kinetics upto ∼90% conversion, (ii) linear increment of molecular weight (unimodal GPC chromatogram) of the polymer keeping its PDI low with the progress of the reaction upto ∼90% conversion, (iii) the linear increment of the molecular weight of the polymers with low PDI on increasing monomer loading, (iv) chain-end analysis of the polymer using 1H NMR confirming the presence of the RAFT agent fragment at the polymer chain-ends, and (v) successful homo- and hetero-chain extension with VAc and NVP, respectively. L-Menthyl-capped polyvinyl acetate has shown higher (i) light transmission properties of its dichloromethane solution at ∼277 nm, (ii) thermal stability, and (iii) glass transition temperature in comparison to its non-L-Menthyl-capped homolog.

Rational Design of Highly Activating Ligands for Cu-Based Atom Transfer Radical Polymerization.

Atom transfer radical polymerization (ATRP) is the most commonly utilized technique in controlled radical polymerization. However, the identification of more active catalysts could further increase its scope, both for polymerization and small-molecule synthesis more generally. To this end, a series of novel ligands were designed on the basis of two strategies: replacing nitrogen-based ligands with their phosphorus equivalents and rigidifying the ligand cap of nitrogen-based ligands so as to enforce short Cu-cap distances. Each ligand was assessed using accurate computational chemistry, which was used to compute the thermodynamics and, in selected cases, kinetics of an ATRP reaction with a model methyl methacrylate propagating radical. In principle, the use of phosphorus ligand caps was found to be a powerful strategy for increasing catalyst activity. Unfortunately, in practice, speciation issues sacrificed much of their advantage. In contrast, cap rigidification increases the activity of nitrogen-based ligands, well beyond existing ATRP ligands such as TPMANMe2. The effectiveness of these ligands was further demonstrated for hard-to-activate initiating systems based on ethylene, vinyl chloride, and vinyl acetate polymerization. Several of these improved ligands are synthetically accessible, with rigid piperidine or quinuclidine analogues of TPMANMe2 possessing improved thermodynamic and kinetic activity by 2 to 3 orders of magnitude.

Cellulose nanocrystal as ecofriendly stabilizer for emulsion polymerization and its application for waterborne adhesive

Soap-free emulsion polymerization of vinyl acetate (AVM) in the presence of cellulose nanocrystals (CNCs) was performed using persulfate/metabisulfite as initiator. The effect of the addition of MPEG comonomer on the locus of CNCs with respect to polymer particles was investigated. It was shown that the presence of MPEG strongly favors the accumulation of CNCs on the polymer particle thus contributing to a stabilization of polymer particle through Pickering effect. The rheological properties of the dispersion as well as the reinforcing effect of CNCs were also meaningfully affected by the presence of MPEG. For purpose of application, the polyvinyl acetate (PVA)/CNC nanocomposite dispersion was used as binder to produce PVA-based waterborne adhesive for wood. This open the way to produce high-value one pot nanocomposite dispersion ready for use, free from any surfactant likely to be used for waterborne adhesive or coating with higher mechanical performance.

Cycloalkyl-substituted salicylaldimine-nickel(II) complexes as mediators in controlled radical polymerization of vinyl acetate

Nickel(II) complexes of Schiff base derived from cycloalkylamines (cycloalkyl = cyclopentyl (cpen), cyclohexyl (chex), and cycloheptyl (chep)) were synthesized: [NiII(Lcpen)2] (1), [NiII(Lchex)2] (2), and [NiII(Lchep)2] (3). The Schiff base-NiII complexes 1-3 were characterized by FTIR, UV–Vis, elemental analysis, and computational methods. Electrodeposited films of complexes 1–3 were obtained by potential cycling CH2Cl2 on platinum electrode, and their electrochemical behavior were characterized by cyclic voltammetry. The polymerization of vinyl acetate (VAc) initiated by azobisisobutyronitrile (AIBN) at 55 °C were conducted according to an organometallic-mediated radical polymerization (OMRP) mechanism. The best conversions were obtained using a ratio of [VAc]/[AIBN]/[Ni] = 542/3.25/1, reaching 50, 69 and 85% in 12 h for 1, 2 and 3, respectively. The kinetic of polymerization mediated by complex 1 exhibited a linear dependence of ln([VAc]0/[VAc]) versus time, supporting a constant radical concentration; while for the complexes 2 and 3, the radical concentration was constant for a short period of time. The increase of molecular weights with the conversion coupled with low polydispersities indicate a certain level of control of the polymerization when using the complexes [NiII(LR)2] as controlling agents.

A New Strategy for the Synthesis of Fluorinated Polyurethane.

An alternating fluorinated copolymer based on chlorotrifluoroethylene (CTFE) and butyl vinyl ether (BVE) was synthesized by RAFT/MADIX living/controlled polymerization in the presence of S-benzyl O-ethyl dithiocarbonate (BEDTC). Then, using the obtained poly(CTFE-alt-BVE) as a macro chain transfer agent (macro-CTA), a block copolymer was prepared by chain extension polymerization of vinyl acetate (VAc). After a basic methanolysis process, the poly(vinyl acetate) (PVAc) block was transferred into poly(vinyl alcohol) (PVA). Finally, a novel fluorinated polyurethane with good surface properties due to the mobility of the flexible fluorinated polymer chains linked to the network was obtained via reaction of the copolymer bearing the blocks of PVA with isophorone diisocyanate (IPDI) as a cross-linking agent.

Dynamic optimization and experimental validation of a pilot scale emulsion polymerization reactor

Dynamic optimization of the emulsion polymerization of vinyl acetate in a pilot scale stirred-tank reactor was studied. The objective function was defined in order to minimize the reaction time. The polymerization reactor was modeled concluding that the effect of the heat removal is decisive to regulate the quality characteristics of the product. The optimization was performed offline using an optimal control dynamic optimization method, selecting as manipulated variables the heat exchange fluid rate and the feeding flow rates of initiator and monomer. The objective function was subjected to several constraints: the heating time until the system reached the activation temperature, final solids content, final product viscosity and final monomer conversion. As a result, it was determined that the polymer could be produced with a maximum time reduction of 29%. The reduction of time affected the polymer final viscosity dropping it, due to an increase in the number of free radicals which intensified the termination reaction rate. Finally, an experimental validation at pilot scale reactor was made showing good results in terms of the polymer end-properties, in accordance to those obtained via dynamic optimization.

Cobalt(II) phenoxy‐imine complexes in radical polymerization of vinyl acetate: The interplay of catalytic chain transfer and controlled/living radical polymerization

ABSTRACTA series of cobalt(II) phenoxy‐imine complexes (CoII(FI)2) have been synthesized to mediate the radical polymerization of vinyl acetate (VAc) and methyl acrylate (MA) to evaluate the influence of chelating atoms and configuration to the control of polymerization. The VAc polymerizations showed the properties of controlled/living radical polymerization (C/LRP) with complexes 1a and 3a, but the catalytic chain transfer (CCT) behaviors with complexes 2a, 1b, 2b, and 3b. The control of VAc polymerization mediated by complex 1a could be improved by decreasing the reaction temperature to approach the molecular weights that not only linearly increased with conversions but also matched the theoretical values and relatively narrow molecular weight distributions. The catalytic chain transfer polymerizations (CCTP) mediated by complexes 2a, 1b, 2b, and 3b were characterized by Mayo plots and the polymer chain end double bonds were observed by 1H NMR spectra. The tendency toward C/LRP or CCTP in VAc polymerization mediated by CoII(FI)2 could be determined by the ligand structure. Cobalt complex coordinated by the ligand with more steric hindered and less electron‐donating substituents favored the controlled/living radical polymerization. In contrast, the efficiency of CCT process could be enhanced by less steric hindered, more electron‐donating ligands. The controlled/living radical polymerization of MA, however, could not be achieved by the mediation of these cobalt(II) phenoxy‐imine complexes. Associated with the results of polymerization mediated by other cobalt complexes, this study implied that the configuration and spin state of cobalt complexes were more critical than the chelating atoms to the control behavior of radical polymerization. © 2019 Wiley Periodicals, Inc. J. Polym. Sci. 2020, 58, 101–113

Synthesis and characterization of well-defined end-chain functional macrophotoinitiators of polystyrene and polyacrylonitrile by RAFT/MADIX polymerization

An alternative approach for the synthesis of well-defined macrophotoinitiators of polystyrene and polyacrylonitrile with an end-chain photofunctional group via reversible addition-fragmentation chain transfer/macromolecular design via the interchange of xanthate (RAFT/MADIX) polymerization has been described. For this purpose, a novel photoreactive chain transfer agent (CTA-PI), namely S-benzyl O-(2-oxo-1,2-diphenylethyl) carbonodithioate, was prepared with an efficient one-pot, two-step synthesis by first reacting benzoin photoinitiator (PI) with CS2 in the presence of NaH and then reacting this intermediate with benzyl bromide in THF. The structure of CTA-PI was determined by FT-IR, 1H NMR, and 13C NMR spectroscopy and confirmed by elemental analysis. Here, we used this xanthate based CTA in conjunction with 4,4′-azobis(4-cyanovaleric acid) (ACVA), as the initiating species, to investigate RAFT/MADIX polymerization of styrene (St), acrylonitrile (AN), methyl methacrylate (MMA), the so-called more activated monomers (MAMs), and vinyl acetate (VAc), one of the less activated monomers (LAMs). While RAFT/MADIX polymerization of styrene and acrylonitrile progressed in a controlled manner producing well-defined macrophotoinitiators with benzoin end-chain functional group, polymerization of methyl methacrylate proceeded uncontrollably. Polymerization of vinyl acetate under the same reaction conditions, on the other hand, did not occur at all. Spectroscopic and GPC measurements indicated that syntheses of polystyrene and polyacrylonitrile macrophotoinitiators bearing a photoreactive benzoin end group, PSt-PI and PAN-PI, respectively, with well-defined structures were achieved. These unique macrophotoinitiators were then used as prepolymers in photoinitiated free radical promoted cationic polymerization of cyclohexene oxide (CHO) and butyl vinyl ether (BVE) monomers to obtain PSt-PCHO, PSt-PBVE, PAN-PCHO, and PAN-PBVE di-block copolymers.

Leonardo's Donna Nuda unveiled

The painting “Donna Nuda” by Leonardo was acquired by Catherine II (the Great) from the R. Walpole collection, Houghton Hall, England, in 1779 for the Hermitage in St. Petersburg. By exploiting the EVA film technology (ethylene vinyl acetate polymer embedded with strong cation and anion exchangers and with C8 and/or C18 resins) we have explored the surface of the painting in order to ascertain the techniques used in its drawing. Five EVA films were affixed on the body and on the landscape for 60 min. Upon elution from the recovered films, the harvested material was analyzed by gas-chromatography/mass spectrometry as well as by liquid chromatography/mass spectrometry. “Tempera grassa” (consisting of linseed oil admixed with egg yolk) was used in the entire painting. The surface was then protected by a layer of conifer resin. It is hypothesized that access to the layer underneath the protective layer was obtained via micro-cracks on the conifer resin itself. Rosemary oil was used as diluent to slow down the drying process and so to perform the glazing technique, thus obtaining the “aerial perspective” in correspondence of the landscape. To our reckoning, this is the first time in which a Leonardo painting is analyzed in depth but also in which his artistic technique is deciphered via modern techniques for exploring Cultural Heritage. The EVA film technology might be used for ascertaining the authenticity of paintings and uncover frauds. Significance statementLeonardo da Vinci was the most famous Italian polymath of the Renaissance and one of the most important innovators of his time. He was the author of several important artworks such as “La Gioconda”, but he also painted the “Donna Nuda” conserved at the Hermitage Museum. Although some attempts permitted the identification of part of the materials used by Leonardo, to date no analytical investigations were able to fully characterize and decipher the recipes. We explored the surface of the “Donna Nuda” painting through a non-invasive approach that uses a functionalized film to adsorb nano-scopic amounts of materials that were then analyzed by mass spectrometry. This method has the potential to revolutionize the approaches used to analyze cultural heritage.

Interplay of Head, Tail, and Mid-Chain Radicals in Bulk Free-Radical and Reversible Degenerative Addition Fragmentation Chain-Transfer Polymerizations of Vinyl Acetate

A detailed kinetic model for isothermal bulk free-radical and degenerative reversible addition–fragmentation chain-transfer (RAFT) polymerizations of vinyl acetate is presented up to monomer conversions of ca. 0.95, considering a distinction between head and tail end-chain radicals and primary and tertiary mid-chain radicals (MCRs). Diffusional limitations are taken into account for conventional initiation and termination, with a reduction of the gel-effect as chain transfer to the monomer lowers the apparent termination reactivities. The tail radicals are essential for the accurate description of backbiting and RAFT deactivation and the primary MCRs for chain transfer to the polymer. The model is based on the application of the method of moments with over 100 macrospecies types and thousands of reactions to obtain the temporal evolution of the monomer conversion, the number and mass average chain length, the number fraction of short- and long-chain branches, the number fraction of unsaturated chains, and the number fraction of chains with head-to-head defects. After a successful model validation of the experimental literature data, the model is applied to understand the complex interplay of the radical types involved and to highlight the control of the branching and unsaturation amounts under RAFT polymerization conditions. Head-to-head defects can however not be avoided for realistic average chain lengths.

Determination Compressive and Bending Strength of Porous Bodies from Plaster Reinforced with Polyacetate of Vinila (PVAc)

Calcium sulfate dehydrate β (CaSO4. 0,5H2O) is a material obtained from the calcination of gypsite (CaSO4.2H2O). This is a low cost ceramic material very used in building industry. The improvement of their applications is of fundamental technological interest. Polymers are an interesting alternative of reinforcement. Polyvinyl acetate (PVAc) is an addition polymer obtained by the polymerization of vinyl acetate. This polymer is well known to achieve good adhesion to cellulosic surfaces. The aim of this work was to evaluate the mechanical properties of porous plaster bodies reinforced with PVAc. Mechanical behavior was characterized by compression and bending strength. Three water / gypsum ratios: 0.5 0.6 and 0.7 were used on this work. An amount of 5% and 10% PVAc promoted increase on the mechanical properties of gypsum. Higher amounts polymer promoted further decrease of the mechanical properties.

Синтез водостойких и биостойких поливинилацетатных дисперсий

The characteristics of the emulsion polymerization of vinyl acetate in the presence of inorganic compounds such as silicon dioxide, titanium dioxide and polyhexamethylene guanidine hydrochloride are studied. It was found that, under the same synthesis conditions, inorganic additives affect the polymerization mechanism of vinyl acetate. The highest conversion of vinyl acetate can be obtained with the introduction of 0.5 wt% silicon dioxide. Polyhexamethylene guanidine hydrochloride also accelerates the reaction rate at the initial moment of time. However, the combined use of silicon dioxide and polyhexamethylene guanidine hydrochloride also reduces the reaction rate in comparison with the syntheses of vinyl acetate, carried out in the presence of each additive separately. The introduction of titanium dioxide, on the contrary, inhibits the reaction. However, when titanium dioxide and polhexamethylene guanidine hydrochloride are used together, the curve of the dependence of vinyl acetate conversion on time is close in process speed to the polymerization process of vinyl acetate without additives. The introduction of silicon dioxide and titanium dioxide makes it possible to increase by 1.5 times the water resistance of the resulting dispersions after short-term keeping glued samples in the water. At the same time, an increase in the water resistance of the samples after long-term contact with water allows only silica. The joint introduction of polyhexamethylene guanidine hydrochloride and silicon dioxide also led to improve the water resistant of adhesive joints, but not as significantly as in the case of the separate introduction of only silica. Polyhexamethylene guanidine hydrochloride increases the fungicidal activity of the resulting dispersions, and its combined use with titanium dioxide made it possible to improve the water resistance of the dispersion during long-term contact with water, in contrast to samples modified only with titanium dioxide. In addition, these samples showed greater fungicidal activity than samples containing silica and polymethylene guanidine hydrochloride.

Experimental and Theoretical Comparison of Addition–Fragmentation Pathways of Diseleno- and Dithiocarbamate RAFT Agents

Theoretical studies suggest that identically substituted dithio- and diselenocarbamate reversible addition–fragmentation chain transfer (RAFT) agents will have similar reactivity but that diselenocarbamate RAFT agents are more likely to undergo intermediate radical termination. These results are supported by an experimental study of polymerizations of vinyl acetate mediated by Se-cyanomethyl N,N-dimethyldiselenocarbamate (1Se) and its sulfur analog, S-cyanomethyl N,N-dimethyldithiocarbamate (1S). While the two polymerizations had similar interchain transfer constants (3.2 for 1S and 3.9 for 1Se), significant retardation was observed for 1Se-mediated polymerizations. External chain transfer constants were also measured for 1S (5.4) and 1Se (45). The low apparent value of the chain transfer constant of 1S is attributed to the selective formation of a single monomer adduct in the early stages of the reaction.