Catalytic Chain Transfer Agent Research Articles

Catalytic Chain Transfer in Crosslinking Photopolymerizations

AbstractPresented here is a detailed account of the development and implementation of macrocyclic cobaloxime complexes as sulfur-free, catalytic chain transfer agents (CTAs) in crosslinking photopolymerizations. Although much of this review is dedicated to understanding the fundamentals of catalytic chain transfer (CCT) in photopolymerizations, its impact on network topology and resultant mechanical properties, future goals of applying this technology to multimaterial 3D printing are also discussed. It is our long-term ambition for catalytic, sulfur-free CTAs to supplant existing consumptive, sulfur-based agents to provide new, unexplored, and not currently possible to fabricate photopolymeric materials with a specific eye towards application in dentistry, additive manufacturing, and responsive materials.1 Introduction2 History of Catalytic Chain Transfer (CCT)3 Understanding Catalyst Purity and Chain Transfer Activity4 Evidencing CCT in a Crosslinking Photopolymerization5 Comparing Cobalt(II)-Catalysts to Other Relevant CTAs6 Performance of Cobalt(II)-Catalysts in Commercial Resins7 Limitations of Approach and Looking Forward

Aluminum Alkyls as Highly Active Catalytic Chain Transfer Agents

AbstractDuring the production of free radical initiated low‐density polyethylene (LDPE), it was discovered that the addition of low levels of alkyl aluminum compounds caused the molecular weight of the LDPE to drop precipitously. Further investigation demonstrated that aluminum‐alkyl compounds are among the most effective chain transfer agents ever utilized. It was also shown that polymer chains, which transfer to Al alkyl species, contain almost exclusively vinyl terminated end groups. A catalytic chain transfer mechanism is proposed in which chain transfer occurs from a growing polymer chain to an aluminum center followed by beta hydride elimination to produce a vinyl terminated polymer chain and a new aluminum hydride bond. This new aluminum hydride bond can then undergo further chain transfer reactions. This is the first time such a catalytic chain transfer mechanism has been reported. As little as 10–20 mol ppm aluminum alkyl species decreased the degree of polymerization by a factor of 2 resulting in chain transfer constant (Cs) values as high as 1000–2000. Density functional theory (DFT) study elucidated the catalytic cycle of triethylaluminum (TEA). It is discovered that, depending on the reaction conditions, TEA can serve as a conventional as well as catalytic chain transfer agent.

Synthesis and Utilization of Low Dispersity Acrylic Macromonomer as Dispersant for Nonaqueous Dispersion Polymerization

The effect of dispersity on the stabilization performance of poly(butyl methacrylate) (P(BMA)) ω-unsaturated macromonomer used as a reactive dispersant in nonaqueous acrylic dispersion (NAD) polymerization is investigated for the first time. Macromonomer with low dispersity (1.3) is synthesized in a one-pot process by the addition of bis(difluoroboryldiphenylglyoximato)cobalt(II) (CoPheBF) as a catalytic chain transfer (CCT) agent to P(BMA) chains mediated by atom transfer radical polymerization (ATRP); addition of the CoPheBF at a BMA monomer conversion of ∼80% leads to good control of chain length and dispersity while achieving a high fraction of terminal double bond (TDB) functionality. The lower dispersity P(BMA) macromonomer provides better stabilization to NAD systems per dispersant chain compared to material of the same number-average molecular weight (∼6000 Da) and TDB functionalization (∼70–75%) but higher dispersity (1.8), as characterized by the smaller average particle sizes and the higher wei...

Recent progress in synthesis of stereoblock copolymers

The synthesis of polymers with tailor-made microstructures and desired properties by controlling the regio- and stereoselectivity of olefin polymerization is an important research area in polymer science. In recent years, achievements were made to synthesis stereoblock copolymer having different tacticity of each segment such as crystalline-amorphous blocks from a single monomer. A stereoblock copolymer consisting of different stereospecific sequences in one polymer chain, shows special physical properties from the original stereoregular polymer which could be applied as thermoplastic polymer. On one hand, it leads to much wider array of polymeric materials from a limited set of commodity monomer feed stocks; on the other hand, it combines the properties of two or more homopolymers and exhibits better performance than the corresponding homopolymer blends. In this paper, research and developments about several kinds of stereoblock copolymers such as polypropylene, polydienes, polyacrylate and polylactide were reviewed and discussed, in particular, the methods to synthesize stereoblock copolymers were also summarized. The successful development of oscillating catalysts for synthesizing isotactic-atactic multi-stereoblock polypropylene opened the door of research on preparing crystalline-amorphous stereoblock polypropylene. Plenty of coordinative catalyst systems and novel polymerization methods were developed for preparing stereoblock polypropylene. The most common way is using living/controlled polymerization system and changing the selectivity simply by varying polymerization temperature, solvent polarity and pressure. Besides, two catalysts with different stereospecificity were mixed together for the propylene polymerization, by successful cross chain transfer reaction, multi-stereoblock polypropylene with thermoplastic elastomeric properties was also obtained. Stereoblock polydienes were also paid much attention for their diverse microstructures which could dramatically influence the properties of resulting materials. Much effort has been attributed to the modification of polydienes microstructure by changing the active species through incorporation of additives such as barium salt, Lewis base (PPh3) and Lewis acids in a living/controlled polymerization system. In addition, chain shuttling polymerization was also applied for diene polymerization to obtained hard-soft multiblock polydiene on the condition of appropriate catalytic systems and chain transfer agent. Progress in stereospecific living polymerization of methyl methacrylate (MMA) made it possible to synthesize stereoblock poly (methyl methacrylate) (PMMA). Stereoblock PMMA should be of interest from the viewpoint of stereocomplex formation which would occur intramolecularly as well as intermolecularly. Preparation of stereoblock PMMA with narrow molecular weight distribution was achieved by living anionic polymerization with incorporation of unique additives in the early time. Isotactic-syndiotactic stereoblock PMMA were synthesized with efficient metallocene and Lewis acid hybrid catalysts capable of switching stereospecificity of the MMA polymerization. Except for the three kinds of stereoblock polymer, other types of stereoblock polymer such as polystyrene, polylactide and polyacrylamide were also introduced. At present, although it is successful in preparing all kinds of stereoblock copolymers, several problems emerged that the feasible catalytic systems are still limited, some polymerization process demanding strict conditions is not easy to control and the obtained stereoblock polymer is often with low stereoregularity. Besides, the study on the properties of stereoblock copolymers also requires more attentions. Therefore, it is necessary for researchers to develop more practical catalytic system and novel polymerization methods for the better control of macromolecular structure, and to deeply and systematically understand the relationship between macromolecular structure and physical properties.

Catalytic chain transfer polymerization for molecular weight control in microemulsion polymerization

Catalytic chain transfer using low-spin cobalt(II) complexes is applied in the microemulsion polymerization of methyl and n-butyl methacrylate. In the presence of a catalytic chain transfer agent the rate of polymerization decreases with an increasing cobalt(II) catalyst loading. The course of the polymerization can be captured by the Morgan-Kaler model extended with radical desorption and aqueous phase termination. Proper molecular weight control (i.e. a molecular weight distribution with a dispersity of ∼2) is achieved, however, proves to be strongly dependent on the balance of cobalt(II) molecules and propagating radicals. The presence of a cobalt(II) catalyst enhances the likelihood of homogeneous nucleation, which decreases the colloidal stability in the methyl methacrylate microemulsion polymerizations. The n-butyl methacrylate microemulsion polymerizations on the other hand, display good colloidal stability and a remarkable increase in the nucleation efficiency.

A Study of Particle Nucleation in Dispersion Copolymerization of Methyl Methacrylate

AbstractThe dispersion polymerization of methyl methacrylate (MMA) and its copolymerization with the bio‐renewable monomer γ‐methyl‐α‐methylene‐γ‐butyrolactone (MeMBL) is studied in a methanol/water mixture. The addition of MeMBL to an MMA dispersion decreases both rate and particle size (PS), while rate decreases and PS increases when the fraction of methanol in the continuous phase is increased. The differing effects of comonomer and continuous phase composition can be attributed to monomer partitioning and critical chain length (jcrit) of the radicals formed in solution. A cobalt(II) catalytic chain transfer agent (CCTA) was used to determine a value of 20–30 monomer units for jcrit in MMA dispersion polymerization. A distinct population of small particles, about one‐tenth as large as the 1–2 µm mature particles, was observed throughout the entire course of the dispersion polymerization; it is these particles, most likely aggregates of nuclei continuously generated in solution, that are swept up by the mature particles during growth.magnified image

Mass transport by collisions in emulsion polymerization: why it is possible to use very hydrophobic catalysts for efficient molecular weight control

An alternative mass transport mechanism, based on collisions between different entities, is used to explain the performance of an extremely hydrophobic catalytic chain transfer agent (i.e.COPhBF) in emulsion polymerization. Mass transport in emulsion polymerization is generally accepted to proceed via the aqueous phase. COPhBF possesses no detectable water-solubility and would therefore be expected to be inefficient for molecular weight control in emulsion polymerization. However, proper molecular weight control using COPhBF has been demonstrated and results are presented that are consistent with the existence of an alternative mass transport mechanism in emulsion polymerization that circumvents the aqueous phase.

Catalytic Chain Transfer Mediated Emulsion Polymerization: Compartmentalization and Its Effects on the Molecular Weight Distribution

We present the first population balance calculations which encompass the complete molecular weight distribution (MWD) to discuss the implications of both radical and catalytic chain transfer agent (CCTA) compartmentalization in a catalytic chain transfer (CCT) mediated emulsion polymerization system. Compartmentalization effects are attributed to reduced frequencies of entry and exit of the CCTA (bis[(difluoroboryl)dimethylglyoximato]cobalt(II) or COBF). Two limiting scenarios were identified. In instances of fast CCTA entry and exit, monomodal MWDs are obtained governed by a global CCTA concentration. In instances of slow entry and exit, bimodal MWDs are obtained; one peak can be attributed to the generation of a bimolecular termination product produced in polymer particles devoid of CCTA, while a transfer-derived peak can be attributed to polymer particles containing one or more CCTA molecules. We present theoretical evidence that experimentally observed multimodal MWDs (Macromolecules 2009, 42, 7332−7341) originate from a reduced mobility of the CCTA and that when viscosity is high in the polymer particles, compartmentalization of the CCTA becomes important.

The effect of different catalytic chain transfer agents on particle nucleation and the course of the polymerization in ab initio batch emulsion polymerization of methyl methacrylate

The effect a Co(II) based catalytic chain transfer agent (CCTA) has on the course of the polymerization and the product properties of an emulsion polymerization is governed by the intrinsic activity and the partitioning behavior of the catalyst. The effect on the conversion time history, the molecular weight distribution and the particle size distribution is evaluated in batch emulsion polymerization of methyl methacrylate for three different CCTAs, which cover a range of intrinsic activities and partitioning behaviors. It was demonstrated that radical desorption from the particle phase to the aqueous phase preceded by chain transfer is the main kinetic event controlling the course of the polymerization and the product properties in terms of the particle size distribution. The experimental results show that the aqueous phase solubility of the CCTA is the key parameter controlling the course of the polymerization and the particle size distribution. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1038–1048, 2010

Evidence of Compartmentalization in Catalytic Chain Transfer Mediated Emulsion Polymerization of Methyl Methacrylate

Evidence of compartmentalization of the catalytic chain transfer agent in seeded emulsion polymerization of methyl methacrylate (MMA) is shown experimentally. The addition of bis[(difluoroboryl)dimethylglyoximato]cobalt(II) (COBF) to seed particles swollen below their maximum saturation concentration exhibited multimodal molecular weight distributions (MWD) which are attributed to a statistical distribution of COBF molecules over the polymer particles. The experimental observations suggest that there are two limits for catalytic chain transfer in emulsion polymerization: (i) at the earlier stages of the polymerization where a global COBF concentration governs the MWD and (ii) at the latter stages of the polymerization where a statistical distribution of COBF molecules governs the MWD. To the best of our knowledge, these results are the first to suggest evidence of compartmentalization in catalytic chain transfer mediated emulsion polymerization.

The effect of Co(II)‐mediated catalytic chain transfer on the emulsion polymerization kinetics of methyl methacrylate

AbstractThe effect the catalytic chain transfer agent, bis[(difluoroboryl) dimethylglyoximato] cobalt(II) (COBF), on the course of the ab initio emulsion polymerization of methyl methacrylate, and the product properties in terms of the molecular weight distribution were investigated. The emulsion polymerization kinetics have been studied with varying surfactant, initiator, and COBF concentrations. The experimentally determined average number of radicals per particle strongly depends on the concentration of COBF and proves to be in good agreement with the results of model calculations. The apparent chain transfer constant, determined up to high conversion, is in excellent agreement with the predicted value based on a mathematical model based on COBF partitioning and the Mayo equation. The results of this work enhance the fundamental understanding of the influence a catalytic chain transfer agent has on the course of the emulsion polymerization and the control of the molecular weight distribution. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5078–5089, 2009

Copolymerization of Styrene and Ethylene at High Temperature with Titanocenes Containing a Pendant Amine Donor

The copolymerization of ethylene and styrene at 120 °C was investigated with a series of titanocenes: Cp′TiCl2L [1, 2: Cp′ = cyclopentadienyl (Cp) versus 3, 4: Cp′ = {2-(dimethylamino)ethyl}cyclopentadienyl (CpN); 1, 3: L = Cl versus 2, 4: L = 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)], in combination with methylaluminoxane (MAO). At 120 °C, titanocenes bearing a pendant amine ligand (3 and 4) are effective catalysts for the ethylene-styrene (ES) copolymerization, whereas complexes lacking the pendant group (1 and 2) afford only mixtures of polyethylene (PE) and atactic polystyrene (aPS). At these temperatures, copolymerization with complexes 3 and 4 yields mixtures of ES copolymers and atactic polystyrene that are not readily separated by solvent extraction or gel permeation chromatography as both components of the mixture exhibit similar molecular weights and solubility in THF and acetone. The source of the atactic polystyrene was the autopolymerization of styrene as deduced by carrying out the copolymerization in the presence of a catalytic chain transfer agent, cobalt tetraphenylporphyrin (Co(tpp)). The addition of Co(tpp) caused a decrease in the molecular weight of the atactic polystyrene but had no effect on the molecular weight of the ES copolymer.

Tubular reactor synthesis of styrene–methacrylate copolymers in solution with supercritical carbon dioxide

Terpolymerizations of styrene, methyl methacrylate, and glycidyl methacrylate in solution with supercritical CO 2 were carried out in a tubular reactor up to almost complete monomer conversion. The residence time behavior of the set-up was investigated by injecting a tracer pulse and measuring the tracer concentration at the reactor exit via in-line FT-near infrared (NIR) spectroscopy. Molecular weight control during polymerization was achieved either by initiator concentration or by the addition of a catalytic chain-transfer agent. Number average molecular weights were adjusted to about 2500 g mol −1 to yield products which are suitable components for powder coating applications.

Control of Branching Vs. Cross‐Linking in Conventional Free Radical Copolymerization of MMA and EGDMA Using CoBF as a Catalytic Chain Transfer Agent

Solution copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) by conventional free radical methodology using a cobalt oxime (CoBF) as a catalytic chain transfer agent allows synthesis at high conversion of fully soluble branched poly (methyl methacrylate)s. Using CoBF levels of 8–32 ppm, mole ratios of MMA/EGDMA up to ∼100/10 can be employed in producing branched products without any macrogelation or microgel formation. Higher levels of EGDMA require higher levels of CoBF to inhibit cross‐linking, but the reactions are difficult to control and have poor reproducibility. An 1H NMR analysis confirms the incorporation of EGDMA at a level comparable to that of the comonomer feed and typically 50–70% of these residues are present as branching segments rather than as pendent partially reacted residues. Double detection (DD) and triple detection (TD) size exclusion chromatographs (SEC) have been used to evaluate the molar mass and branching character of the products. Molar mass data from both instruments show good agreement, and products with molar mass distribution curves very similar to those of previous products prepared using dodecanethiol (DDT) as a stoichiometric chain transfer agent can be produced. The maximum level of EGDMA that can be incorporated using CoBF is, however, lower than is the case when DDT is used. The analytical data from the TDSEC instrument allows the slice‐by‐slice generation of intrinsic viscosity (IV) data and corresponding data for the Zimm branching factor, g′, across the molar mass distribution. At a given molar mass of 130,000 g/mol, g′ correlates well with the level of EGDMA used in the polymerization feed, and is consistent with the 1H NMR data showing efficient incorporation of this branching comonomer. Dedicated to Professor John L. Stanford on the occasion of his 60th birthday.

Macromolecular design of the network structure of copolymers of ethylene glycol dimethacrylate with methyl methacrylate using the catalytic chain transfer reaction

Abstract It has been shown that catalytic chain transfer reaction is an effective instrument for the macromolecular design of the network structure of copolymers of ethylene glycol dimethacrylate with methyl methacrylate. Copolymers of various macromolecular architectures - from networks to soluble branched low-molecularweight oligomers - have been produced using cobalt porphyrin as catalytic chain transfer agent. The influence of cobalt porphyrin on the kinetics of copolymerization, structure and properties of final copolymers has been studied.

Catalytic Chain Transfer Copolymerization of Methyl Methacrylate and Butyl Acrylate

AbstractIn this work it is shown that catalytic chain transfer is a very efficient way of controlling molecular weight in the copolymerization of methyl methacrylate (MMA) and butyl acrylate (BA). The experimental data are compared to a previously developed model based on copolymerization kinetics and the mechanisms for catalytic chain transfer and for cobalt‐mediated living radical polymerization that can describe the observed transfer constants. Secondly, it is shown that the presence of a catalytic chain transfer agent does not affect the reactivity ratios within the concentration range studied. Finally, the effect of conversion and therewith composition drift on the catalytic chain transfer polymerization of MMA and BA is investigated and it is shown that under the conditions employed in the experiments a certain degree of macromer copolymerization is present at high partial conversions of MMA.Evolution of MWD with conversion for the CCT copolymerization of MMA and BA in toluene at 60 °C, [AIBN] = 6 × 10−3 mol · L−1.magnified imageEvolution of MWD with conversion for the CCT copolymerization of MMA and BA in toluene at 60 °C, [AIBN] = 6 × 10−3 mol · L−1.

Free-Radical Terpolymerization of Styrene and Two Methacrylates in a Homogeneous Phase Containing Supercritical CO2

Styrene−methyl methacrylate−glycidyl methacrylate terpolymers to be used as binders in powder coating materials were synthesized in a homogeneous fluid phase containing 20 wt % CO2. Reactions carried out at 120 °C and 350 bar yield almost complete monomer conversion. The molecular weights were around 3000 g·mol-1 with a polydispersity index of 1.7. Molecular weight control was achieved by n-dodecyl mercaptan or by a catalytic chain-transfer agent. Simulations of the cumulative and differential copolymer composition are in excellent agreement with experimentally derived values.

Synthesis of Well-Defined Macromonomers by Sequential ATRP−Catalytic Chain Transfer and Copolymerization with Ethyl Acrylate

Polymer chains with unsaturated end groups can be produced by the addition of catalytic chain transfer agents near the end of an atom-transfer polymerization. The polymers were analyzed by MALDI−TOF, NMR, and TGA experiments to determine the degree of unsaturation with good agreement. These polymers can subsequently be used as macromonomers in copolymerizations with monomers such as ethyl acrylate.

New applications of catalytic chain transfer polymerization to waterborne binders for automotive paint systems

Catalytic chain transfer polymerization (CCTP) is a conventional free radical polymerization technique that allows the preparation of macromonomers in a one step process. Acid functional macromonomers can be copolymerized with acrylate backbone monomers to form graft copolymers that, after neutralization with a base, are water dispersible. Low molecular weight oligomers from CCTP act as chain transfer agents themselves for methacrylate monomers via addition–fragmentation mechanism and lead to (semi) AB-block copolymers. Group transfer polymerization (GTP) is another polymerization technique to make AB-block copolymers but economically less attractive for functional comonomers since they interfere with the initiation mechanism. Graft and AB-block copolymer dispersions offer advantages in waterborne coatings compared to linear polymers of the same overall composition and molecular weight. Examples discussed in this paper are pigment dispersants and dispersion resins in which the backbone or A-segment of the copolymer has specific groups to anchor to the pigment surface and the side chains or B-segment give both charge and steric stabilization. AB-block copolymers with one block water soluble or dispersible also function as copolymerizable surfactants in an emulsion polymerization process and allow the synthesis of surfactant-free emulsions with low amounts of hydrophilic groups. The catalytic chain transfer agents (CCTAs) used in CCTP do have a high chain transfer activity for methacrylate monomers at very low concentrations so that low molecular weight oligomers (e.g. dimers) can be made. This chain transfer activity is lost in an emulsion polymerization process if the CCTA is (partly) water soluble. The paper will further demonstrate the use of dimers in the control of molecular weight in emulsion copolymerization.

Catalytic Chain Transfer Isomerization Reactions Involving 2-Phenylallyl Alcohol

A catalytic chain transfer agent, bis[(difluoroboryl)dimethylglyoximato]cobalt(II) (COBF), was utilized in free radical polymerizations involving 2-phenylallyl alcohol (PAA). In homopolymerization only oligomers were formed, and the predominant product was 2-phenylpropanal consistent with a radical-induced isomerization reaction. In copolymerizations with methyl methacrylate (PAA included at 5 and 10 mol %) the chain transfer constants increased significantly (compared to those obtained for methyl methacrylate homopolymerization). NMR analyses of the co-oligomer chains confirmed PAA incorporation into the copolymers and provided evidence for the presence of an aldehyde group resulting from chain transfer isomerization. Further analyses by matrix-assisted laser desorption ionization (MALDI) mass spectrometry indicated conclusively that a PAA monomer unit was incorporated into copolymer chains. The thermal degradation behavior of the oligomers was measured using thermogravimetric analysis (TGA). The thermograms clearly indicate that the PAA is situated at the chain ends of the majority of the polymer chains even at low concentrations of PAA in the initial monomer feed.