Presence Of Chain Transfer Agent Research Articles

Phenomenon of phase inversion in high impact polystyrene: Physico‐chemical, rheological and morphological study in the presence of chain transfer agent and using different tapered block copolymers as the precursor rubber

AbstractDifferent high impact polystyrenes were synthesized using styrene/butadiene copolymers (SB) with PS/PB composition: 30/70 and 20/80 as the precursor rubber, benzoyl peroxide (BPO) as the initiator, and ter‐dodecyl mercaptane (TDM) as the chain transfer agent. During the polymerization, several samples were taken and analyzed under different techniques to evaluate the phase inversion (PI) phenomenon. The PI was determined through transmission electron microscopy (TEM) and through dynamic oscillatory behavior, where the PI takes places when the relaxation process presents the lower value of activation energy. Finally, the Choi and Schowalter emulsion model was employed to elucidate the PI, and relevant information was revealed about the interfacial tension in the PI. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

A Novel Linear−Hyperbranched Multiblock Polyethylene Produced from Ethylene Monomer Alone via Chain Walking and Chain Shuttling Polymerization

A novel linear−hyperbranched multiblock polyethylene was prepared from ethylene monomer alone via chain walking and chain shuttling polymerization in the presence of chain transfer agent (ZnEt2). In the binary catalyst system, α-diimine nickel(II) bromide complex (CatA) produced chain segments with hyperbranched architecture via chain walking, and ansa-ethylenebis(1-η5-indenyl)zirconium dichloride (CatB) yielded linear chain segments, while ZnEt2 facilitated chain exchanges between two active metal centers. The resultant multiblock polyethylene featured a narrow weight distribution and useful combining properties of linear and branched polyethylene. Further, polymers were expediently prepared in an effective and economical polymerization process from ethylene monomer alone.

Immobilization of RAFT agents on silica nanoparticles utilizing an alternative functional group and subsequent surface‐initiated RAFT polymerization

AbstractSilica–polystyrene core‐shell particles were successfully prepared by surface‐mediated reversible addition fragmentation chain transfer (RAFT) polymerization of styrene monomer from the surfaces of the silica‐supported RAFT agents. Initially, macro‐RAFT agents were synthesized by RAFT polymerization of γ‐methacryloxypropyltrimethoxysilane (MPS) in the presence of chain transfer agents (CTAs). Immobilization of CTAs onto the silica surfaces was then performed by reacting silica with macro‐RAFT agents via a silane coupling. Grafting of polymer onto silica forms core‐shell nanostructures and shows a sharp contrast between silica core and polymer shell in the phase composition. The thickness of grafted‐polymer shell and the diameter of core‐shell particles increase with the increasing ratio of monomer to silica. A control experiment was carried out by conventional free radical emulsion copolymerization of MPS‐grafted silica and styrene under comparable conditions. The resulting data provide further insight into the chemical composition of grafted‐polymers that are grown from the silica surface through RAFT process. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 467–484, 2009

RAFT Polymerization of Acrylamides Containing Proline and Hydroxyproline Moiety: Controlled Synthesis of Water-Soluble and Thermoresponsive Polymers

Two acrylamides containing proline and hydroxyproline moiety, N-acryloyl-l-proline (A-Pro-OH) and N-acryloyl-4-trans-l-proline (A-Hyp-OH), were polymerized by reversible addition-fragmentation chain transfer (RAFT) process to afford well-defined amino acid based polymers. Two chain transfer agents (CTAs), benzyl dithiobenzoate (CTA 1) and benzyl 1-pyrrolecarbodithioate (CTA 2), were compared for the direct polymerization of these monomers without protecting chemistry. With 2,2′-azobis(isobutyronitrile) as an initiator, the dithiocarbamate-type RAFT agent (CTA 2) was efficient for the controlled synthesis of poly(A-Pro-OH)s, which can be regarded as a weak polyelectrolyte. Controlled character of the polymerization of A-Hyp-OH, which has a carboxylic acid and a hydroxyl group in the monomer unit, in the presence of CTA 1 was confirmed by the formation of narrow polydispersity products and the linear relationship between the molecular weight and conversion. Water-soluble poly(A-Hyp-OH)s with number-average ...

Xanthate‐Mediated Controlled Radical Polymerization of N‐Vinylcarbazole

AbstractSummary: Well‐defined poly(N‐vinylcarbazole) [poly(NVC)] was synthesized by macromolecular design via interchange of the xanthates (MADIX)/reversible addition‐fragmentation chain transfer (RAFT) polymerization. The homopolymers with controlled molecular weights ($\overline M _{\rm n}$ = 3 000–48 000) and low polydispersities indices ($\overline M _{\rm w} /\overline M _{\rm n}$ = 1.15–1.20) were obtained by the polymerization of NVC with AIBN in the presence of O‐ethyl‐S‐(1‐phenylethyl) dithiocarbonate as a xanthate‐type chain transfer agent (CTA). Good control of the polymerization was confirmed by the linear first‐order kinetic plot, the molecular weight controlled by the monomer/CTA molar ratio, linear increase in the molecular weight with the conversion, and the ability to extend the chains by the second addition of the monomer.Radical polymerization of NVC in the presence of CTA and plot of number‐average molecular weight (circles) and polydispersity (squares) as a function of conversion.magnified imageRadical polymerization of NVC in the presence of CTA and plot of number‐average molecular weight (circles) and polydispersity (squares) as a function of conversion.

RAFT Polymerization of Acrylamide Derivatives Containing l-Phenylalanine Moiety

We report the controlled synthesis of an amino acid based polymer by reversible addition−fragmentation chain transfer (RAFT) polymerization of N-acryloyl-l-phenylalanine (A-Phe-OH), which has a carboxylic acid group. Three chain transfer agents (CTAs), benzyl dithiobenzoate (CTA 1), benzyl 1-pyrrolecarbodithioate (CTA 2), and O-ethyl-S-(1-phenylethyl) dithiocarbonate (CTA 3), were compared for the direct polymerization of A-Phe-OH without protecting chemistry. With 2,2‘-azobis(isobutyronitrile) as an initiator, the dithiocarbamate-type RAFT agent (CTA 2) is efficient for the preparation of poly(A-Phe-OH) with relatively narrow molecular weight distributions. The effects of several parameters, such as solvent, temperature, CTA to initiator molar ratio, etc., were investigated in order to determine the conditions, leading to optimal control of the direct polymerization. Good control of the polymerization in the presence of CTA 2 in methanol was confirmed by the formation of narrow polydispersity products an...

Mathematical Modeling of Multimonomer (Vinylic, Divinylic, Acidic) Emulsion Copolymerization Systems

A mathematical model for free‐radically initiated semi‐continuous emulsion polymerizations of a multimonomer system containing vinylic, divinylic and acidic monomers in the presence of chain transfer agent (CTA) was deleloped. In addition to the more traditional aspects of emulsion polymerization, the model takes into account both the presence of water‐soluble monomers and the formation of gel due to the polymerization of a divinylic monomer in the presence of CTA. The outputs of the model are the time evolution of monomers conversions, copolymer composition, molecular weight of the sol polymer and gel fraction.

Multimonomer emulsion copolymerization in presence of inhibitors

The comparison of the mathematical model and experimental results of the free radically initiated semi-continuous emulsion polymerization of styrene/butadiene/acrylic acid in the presence of chain transfer agent (CTA) is presented. The mathematical model takes into account the mechanism of gel formation and the effect of acidic monomers. It has also been included the inhibition caused by oxygen and monomer inhibitor. The experimental results (solids content, unreacted amount of monomer and gel content) and those obtained by the model are in rather good agreement.

Synthetic peptide epitope-based polymers: controlling size and determining the efficiency of epitope incorporation.

The assembly of synthetic peptide-based vaccines that incorporate multiple epitopes is a major goal of vaccine development, because such vaccines will potentially allow the immunization of outbred populations against a number of different pathogens. We have shown that free radical-induced polymerization of individual peptide epitopes results in the incorporation of multiple copies of the same or different epitopes into high molecular weight immunogens (O'Brien-Simpson, N.M., Ede, N.J., Brown, L.E., Swan, J. & Jackson, D.C. (1997) Polymerization of unprotected synthetic peptides: a view toward synthetic peptide vaccines. J. Am. Chem. Soc.119, 1183-1188; Jackson, D.C., O'Brien-Simpson, N., Ede, N.J. & Brown, L.E. (1997) Free radical induced polymerization of synthetic peptides into polymeric immunogens. Vaccine 15, 1697-1705). The ability to control the size of these polymers, to determine the physical and chemical properties of the backbone material and also to know the extent to which individual peptide epitopes are incorporated are important manufacturing considerations and form the subject of this study. We show here that the polymerization process is highly efficient with at least 70% of peptides incorporated into the resulting polymer, that acrylamide and acryloylated amino acids can be used as comonomers with peptide epitopes in the polymerization reaction and that the choice of the comonomer can influence the properties of the resulting polymer. We also show that the size of chain growth polymers is restricted in the presence of chain transfer agents, that the resulting polymer size can be predicted and that there is little or no difference in the immunogenicity of polymers that range in apparent molecular size between 18 kDa and 335 kDa. The successful polymerization of peptide epitopes with an acryloyl-amino acid creates the potential for introducing different physical and chemical properties into artificial protein immunogens.

Synthesis of semitelechelic poly[N-(2-hydroxypropyl)methacryl-amide] by radical polymerization in the presence of alkyl mercaptans

The free radical polymerization of N-(2-hydroxylpropyl)methacrylamide (HPMA) in the presence of chain transfer agents was studied. Methyl 3-mercaptopropionate (MMP) and 2-mercaptoethylamine (MEA) were used as chain transfer agents and 2,2′-azobisisobutyronitrile (AIBN) was used as the initiator. Semitelechelic (ST) polymers produced under various reaction conditions were analyzed by Matrix Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) and size exclusion chromatography (SEC). The molecular weight of the ST polymers was regulated by the initial molar ratio of chain transfer agent to HPMA. The free radicals formed by the decomposition of AIBN initiated a fraction of polymer chains; such chains were not terminated in a functional group. The amount of these polymer chains depended on the concentration and efficiency of the chain transfer agent, concentration of AIBN, and conversion. Decreasing the concentration of AIBN and increasing the concentration of the chain transfer agent reduced the formation of nonfunctionalized polymer chains.

Molecular weight distribution formed during free-radical polymerization in the presence of polyfunctional chain transfer agents

Free-radical polymerization of styrene was carried out in the presence of chain transfer agents (CTAs) with functionality, f = 1-4. The size exclusion chromatography (SEC) with an ultraviolet absorption detector (UV) was used to measure the molecular weight distribution (MWD). A Monte Carlo simulation method proposed earlier was used to investigate the experimental results. In this simulation method, one can observe the structure of each polymer molecule directly, and very detailed information can be obtained in a straightforward manner, including the elution curve of SEC. It was found that up to the functionality f = 3, the equal reactivity model that assumes the reactivity of all functional groups in a CTA is equal agrees reasonably well with the experimental results. However, with f = 4, the reactivity of the fourth functional group seems to decrease and the substitution effects may need to be accounted for to fine control the formed branched structure.

Computer Optimization of Molecular Mass Distribution in Low Molecular Mass Acrylic Polymers

A computer model describing the polymerization of acrylic monomers has been developed. The model simulates high conversion polymerization of acrylic monomers in the presence of two types of thiol type chain transfer agents to give low molecular mass acrylates. The model is based on integration of the Mayo equation during the course of the polymerization and keeps into account the relative reagents concentration variations, the changes of the kinetic constant values during polymerization, and the presence of chain transfer agents. The results forecasted by the model have been verified in practice by carrying on a series of polymerization reactions in the conditions above described. A good agreement with the model prediction was obtained.

Molecular Weight Distribution of Living Polymerization Involving Chain-Transfer Agents: Computational Results, Analytical Solutions, and Experimental Investigations Using Ring-Opening Metathesis Polymerization

The number-average degree of polymerization and polydispersity index of living polymerization in the presence of chain-transfer agents are calculated numerically and analytically to determine how the molecular weight of the polymer can be regulated. The solutions reveal that un-steady-state polymerization exists in certain cases. The Mayo plot is nearly linear, but (as opposed to a nonliving system) its slope is not equal to k tr /k p in general (where k i , k p , and k tr are the specific rate constants of initiation, propagation, and chain transfer, respectively), the former differing from the latter by about 0.5-1 order of magnitude when k tr k p = 0.01-1.0. Plots of the slopes of Mayo plot versus k tr k p for different values of k p /k i reveal that only when k tr is equal to or greater than k p by an order of magnitude can the molecular weight be effectively controlled by addition of chain-transfer agent. A sufficient amount of chain-transfer agent is a necessary but not a sufficient condition to ensure monomodal molecular weight distribution. An analytical expression for the number-average degree of polymerization of the dead chains when k tr < k p has been derived and shows excellent agreement with numerical results. An analytical expression relating the slope of the Mayo plot to k tr /k p has also been obtained. These equations hold exactly when the concentration of the catalyst is much less than that of the chain-transfer agent. An experimental investigation of the kinetics of ring-opening metathesis polymerization (ROMP) of norbornene by Mo(=CHCMe 2 Ph)(NAr)(OCMe 3 ) 2 (Ar = 2,6-diisopropylphenyl) (1) in the presence of neohexene suggests that this ROMP system is adequately described by a relatively simple polymerization scheme. As measured from NMR spectroscopy, the specific rate constants of initiation, propagation, and chain transfer at 22° C are 0.57, 17, and 0.00003 M -1 s -1 , respectively.

Optimization of isothermal batchwise bulk free radical polymerization with and without chain transfer agent for minimum reaction time

AbstractIn this work the minimum reaction time was studied for a specific final monomer conversion and number‐average chain length by adjusting the amount of initial initiator concentration in the presence of a fixed amount of chain transfer agent at the best isothermal temperature. A new method for the determination of the best initial initiator concentration ([I]0) and isothermal temperature (T) in the presence of chain transfer agent was developed by the application of a simple optimization algorithm, based on the Lagrangian multiplier, to the basic free radical kinetics. Numerical examples for the polymerization of styrene are presented. It is shown that in the presence of a fixed amount of chain transfer agent the best isothermal policy for minimum time would deviate from dead‐end polymerization. As the concentration of chain transfer agent increases, a stronger deviation from dead‐end polymerization is observed. It was found that for a given pair of desired conversion and number‐average chain length the usage of a chain transfer agent would result in a longer reaction time. The results give insights into operation of batchwise bulk free radical polymerization for minimum reaction time under isothermal conditions.

Modeling of Free-Radical Polymerization Kinetics with Crosslinking for Methyl Methacrylate/Ethylene Glycol Dimethacrylate

A model has been developed to represent polymerization kinetics with crosslinking up to, and past, the gelpoint. Based upon the method of moments, the representation predicts the changes in polymer molecular weight (MW) as branching occurs, and provides a measure of the MW increase caused by the crosslinker. Loss of pendant double bonds by internal cyclization and decrease in branching rate due to diffusion limitations are included. The Ross-Laurence correlation for diffusion-controlled termination has been modified to predict the faster rate acceleration that occurs in lightly crosslinked systems and the delayed Trommsdorff effect observed upon addition of chain transfer agent. The model successfully represents gelpoint and rate data for methyl methacrylate (MMA) polymerization for a wide range of ethylene glycol dimethacrylate (EGDMA) levels, with and without the presence of chain transfer agent.

Kinetics of Emulsifier-Free Emulsion Polymerization of Styrene in the Presence of Chain-Transfer Agents

Abstract The influence of chain-transfer agent on the kinetic behavior of emulsifier-free emulsion polymerization of styrene using potassium persulfate as initiator has been investigated. the presence of a small amount of a chain-transfer agent such as carbon tetrabromide or a mercaptan in the system increased the reaction rate. This rate increase is believed to be caused by a large number of precursor particles which exist in dynamic equilibrium with the mature latex particles in the reacting system. These precursor particles, although colloidally unstable and unable to be detected with an electron microscope, are believed to be an important locus for polymerization. the size of the monomer-swollen polymer particles obtained when CBr4 or ethyl mercaptan (ESH) are used is sensitive to agitation conditions. Large monodisperse polymer particles can be obtained in the presence of CBr4 or ESH under some agitation conditions.

Synthesis of copolymers of N,N‐dimethyl acrylamide and methacrylate esters and their surface tensions

AbstractA systematic study was made on the preparation of N,N‐dimethylacrylamide (NDMA)/ methacrylic ester statistical copolymers. The methacrylic ester comonomers used in the copolymerizations were ethyl, n‐butyl, n‐hexyl, n‐octyl, n‐dodecyl, n‐hexadecyl, and n‐octadecyl methacrylates. It was found that the NDMA copolymers could be prepared in three molecular weight regions: 1.0 × 104, 2.0 × 105, and greater than 1.0 × 106 ([η] in water at 30°C of 0.1, 0.6, and greater than 1.5 dL/g, respectively). The molecular weights of the copolymers were dependent on the solvent employed in the polymerizations and on the presence of a chain transfer agent (t‐dodecyl mercaptan). All copolymer compositional analyses were made by 400 MHz 1H‐NMR spectroscopy. The NDMA/methacrylic ester copolymers decreased the surface tension of water in the order: C4 = C6 &gt; C8 &gt; C12 &gt; C16 &gt; C2 = C18 &gt; P(NDMA), lowest surface tension (35 dyn/cm) to highest surface tension (59 dyn/cm).

Synthesis of copolymers of N,N‐dimethyl acrylamide and methacrylate esters and their surface tensions

AbstractA systematic study of the preparation of N,N‐dimethylacrylamide (NDMA)/methacrylic ester statistical copolymers was conducted. The methacrylic ester comonomers used in the copolymerizations were ethyl, n‐butyl, n‐hexyl, n‐octyl, n‐dodecyl, n‐hexadecyl, and n‐octadecyl methacrylates. It was found that the NDMA copolymers could be prepared in three molecular weight regions 1.0 × 104, 2.0 × 105, and greater than 1.0 × 106 ([η] in water at 30°C of 0.1, 0.6, and greater than 1.5 dL/g, respectively). The molecular weights of the copolymers were dependent on the solvent employed in the polymerizations and on the presence of chain transfer agent (t‐dodecyl mercaptan). All copolymer compositional analyses were made by 400 MHz 1H‐NMR spectroscopy. The NDMA/methacrylic ester copolymers decreased the surface tension of water in the order: C4 = C6 &gt; C8 &gt; C12 &gt; C16 &gt; C2 = C18 &gt; P(NDMA), from lowest surface tension (35 dyn/cm) to highest surface tension (59 dyn/cm).

Minimum end-time policies for batchwise radical chain polymerization: the piecewise initiator addition policy

For the radical chain polymerization in a presence of chain transfer agent, piecewise initiator addition and piecewise reaction temperature are considered as two control actions in N unequal time intervals for minimizing the total reaction time under a given final monomer conversion and the number average chain length. It is found that only one of the two actions are needed to be choosen in the bulk or solution polymerization without chain transfer agent. The initiator addition control is found to be better than the temperature control at the expense of lower polydispersity. The optimal policy of the piecewise initiator addition under the best isothermal temperature is that which gives the highest possible reaction temperature and the least possible total amount of initiator charged just enough for the polymerization to reach the predetermined number average chain length and conversion. With this policy, equal amount of the initiator should be charged into the reactor at every equal time interval. Experimental verifications for solution polymerization of styrene with or without chain transfer agent show that the present method is applicable.

Features of the chain transfer reaction in the polymerization of tetrafluoroethylene in organic media

Features of the formation of low molecular polytetrafluoroethylene have been investigated. Kinetic parameters have been determined for tetrafluoroethylene polymerization in 1,2,2-trifluorotrichloroethane medium and a mechanism for the reaction has been proposed. It is shown that the presence of chain transfer agents is accompanied by increased reactivity of the solvent.