SET‐RAFT of MMA mediated by ascorbic acid‐activated copper oxide

In this work, cupric oxide (CuO) or cuprous oxide (Cu₂O) was used as the catalyst for the single electron transfer‐reversible addition‐fragmentation chain transfer (SET‐RAFT) polymerization of methyl methacrylate in the presence of ascorbic acid at 25 °C. 2‐Cyanoprop‐2‐yl‐1‐dithionaphthalate (CPDN) was used as the RAFT agent. The polymerization occurred smoothly after an induction period arising from the slow activation of CuO (or Cu₂O) and the “initialization” process in RAFT polymerization. The polymerizations conveyed features of “living”/controlled radical polymerizations: linear evolution of number‐average molecular weight with monomer conversion, narrow molecular weight distribution, and high retention of chain end fidelity. From the polymerization profile, it was deduced that the polymerization proceeded via a conjunct mechanism of single electron transfer‐living radical polymerization (SET‐LRP) and RAFT polymerization, wherein CPDN acting as the initiator for SET‐LRP and chain transfer agent for RAFT polymerization. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Polymérisation en milieu précipitant du styrène en solution dans différents alcools

Earlier studies from this laboratory on the polymerizations of acrylic acid and acrylonitrile under precipitating conditions have shown that the auto-acceleration is not caused by non-stationary conditions resulting from the precipitation of growing chains (“occlusion effect”) but by a “matrix effect”, an oriented association complex between the monomer and the polymer formed in the early stages of the reaction leading to assisted propagation. In the present work, a non-polar monomer-polymer system was selected in which molecular associations are unlikely. It was found that when polystyrene precipitates as a fine powder (in diluted monomer solutions in alcohols) auto-acceleration is observed but its extent drops with increasing rate of initiation and increasing temperature. Such situations do not arise in polymerizing systems in which a “matrix effect” operates. The study of the post-polymerization and of the swelling of polystyrene in styrene (10-propanol (90) mixtures) led to the conclusion that polystyrene in equilibrium with this mixture exhibits a glass transition temperature at ca 50°. The various results obtained in this study conform with the assumption of an occlusion effect. The growing chains being buried in the precipitated polymer, chain termination is severely restricted and becomes the determining step in the polymerization.     

Mechanistic Insight into Anion-Binding Catalytic Living Cationic Polymerization

Exploiting non-covalent interactions to catalyze challenging ionic polymerizations is an ambitious goal but is in its infancy. We recently demonstrated non-covalent anion-binding catalysis as an effective methodology to enable living cationic polymerization (LCP) of vinyl ethers in an environmentally benign manner. Here, we further elucidate the structure-reactivity relationships of the elaborately designed seleno-cyclodiphosph(V)azanes catalysts and the roles of anion-binding interactions by a combined theoretical DFT study and experimental study. The investigation suggests that the distinct cis-cyclodiphosph(V)azane framework combined with “selenium effect” and electron-withdrawing 3,5-(CF₃)₂-Phenyl substitution pattern in catalyst enables a critical contribution to accessing excellent stability, anion affinity and solubility under polymerization conditions. Thus, the catalyst could leverage anion-binding interactions to precisely control reversible and transient dormant-active species equilibrium, allowing it to dynamically bind, recognize and pre-organize propagating ionic species and monomer, thereby facilitating efficient chain propagation and minimizing irreversible chain transfer events under mild conditions. The more in-depth understanding of the mechanism for anion-binding catalytic LCP reported herein should help to guide future catalyst design and to extend this concept to broader polymerization systems where ionic species serve as crucial intermediates.     

Iron(III)‐mediated AGET ATRP of styrene using tris(3,6‐dioxaheptyl)amine as a ligand

A commercially available tris(3,6‐dioxaheptyl)amine (TDA‐1) was used as a novel ligand for activator generated by electron transfer atom transfer radical polymerization (AGET ATRP) of styrene in bulk or solution mediated by iron(III) catalyst in the presence of a limited amount of air. FeCl₃ · 6H₂O and (1‐bromoethyl)benzene (PEBr) were used as the catalyst and initiator, respectively; and environmentally benign ascorbic acid (VC) was used as the reducing agent. The polymerizations show the features of “living”/controlled free‐radical polymerizations and well‐defined polystyrenes with molecular weight M_n = 2400–36,500 g/mol and narrow polydispersity (M_w/M_n = 1.11–1.29) were obtained. The “living” feature of the obtained polymer was further confirmed by a chain‐extension experiment. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2002–2008, 2009     

AGET ATRP of water‐soluble PEGMA: Fast living radical polymerization mediated by iron catalyst

In this work, living radical polymerizations of a water‐soluble monomer poly(ethylene glycol) monomethyl ether methacylate (PEGMA) in bulk with low‐toxic iron catalyst system, including iron chloride hexahydrate and triphenylphosphine, were carried out successfully. Effect of reaction temperature and catalyst concentration on the polymerization of PEGMA was investigated. The polymerization kinetics showed the features of “living”/controlled radical polymerization. For example, M_n,GPC values of the resultant polymers increased linearly with monomer conversion. A faster polymerization of PEGMA could be obtained in the presence of a reducing agent Fe(0) wire or ascorbic acid. In the case of Fe(0) wire as the reducing agent, a monomer conversion of 80% was obtained in 80 min of reaction time at 90 °C, yielding a water‐soluble poly(PEGMA) with M_n = 65,500 g mol^?1 and M_w/M_n = 1.39. The features of “living”/controlled radical polymerization of PEGMA were verified by analysis of chain‐end and chain‐extension experiments. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

ARGET ATRP of methyl methacrylate with 2-(8-heptadecenyl)-4,5-dihydro-1H-imidazole-1-ethylamine (OLC) as both ligand and reducing agent in the presence of air

Copper(II)-mediated activators regenerated by electron transfer for atom transfer radical polymerization of methyl methacrylate (MMA) was successfully carried out in a limited amount of air in the presence of 2-(8-heptadecenyl)-4,5-dihydro-1H-Imidazole-1-ethylamine as ligand that served not only as ligand but also as reducing agents. Reduction of Cu(II) to Cu(I) by an excess amount of nitrogen-based ligand was followed by UV–visible spectroscopy. The kinetics of the polymerizations and effect of different polymerization conditions are investigated. It is found that the polymerization of MMA can be conducted well even if the amount of Cu(II) is as low as 1 mol% catalyst relative to initiator. The results of the polymerizations demonstrate the features of “living”/controlled free-radical polymerization, such as the number-average molecular weights being close to their corresponding theoretical values and increasing linearly with monomer conversion, and narrow molecular weight distributions. Chain extension of poly(methyl methacrylate)s with MMA was successful and demonstrated well-maintained end-group functionality.     

Anionic heterogeneous polymerization of methacrylonitrile by n-butyllithium

The anionic heterogeneous polymerization of methacrylonitrile by butyllithium in petroleum ether was investigated. The polymerization was of the “living” type, as seen from the linear dependence of the molecular weights on [MAN]/[BuLi]. This behavior was further supported by block polymerization experiments in which the monomer was added in two portions and the molecular weights obtained were directly proportional to the total monomer concentration. The initiator efficiency was low, and initiator consumption was only about 2%. This fact, together with the results of the block polymerizations showed that there was preferential addition of monomer to the growing chain ends rather than to the initiator. The molecular weights were independent of the rate of monomer addition. This as well as the “living” behavior of the polymerization of methacrylonitrile on a wide range of monomer and catalyst concentrations and the absence of chain transfer to monomer was essentially different from that of the similar heterogeneous polymerization of acrylonitrile by butyllithium previously investigated. This is due to the absence of an α-acidic hydrogen in methacrylonitrile.     

Studies on Phosphorus-Containing Polymers. V. Ring-Opening Polymerization of Tetrahydrofuran Catalyzed by Linear Phosphonitrilic Chloride

It was found that linear phosphonitrilic chloride could be used as a catalyst for ring-opening polymerization of tetrahydrofuran. Bulk polymerizations were carried out in a nitrogen atmosphere. After termination of polymerization, the reaction mixture was poured into water, thereby decomposing the catalyst. The product was dissolved in benzene and then subjected to lyophilization. The polymerization of tetrahydrofuran in the presence of linear phosphonitrilic chloride was found to be an equilibrium and a “living” polymerization. The polymerization product includes little phosphorus, and its infrared absorption spectrum agrees well with that of the polymer obtained with PF₅ catalyst. The results of the polymerization using epichlorohydrin as a promoter show that the number of active sites in the molecule of linear phosphonitrilic chloride is considerably smaller. Consequently it is conceivable that the catalytic activity of the linear phosphonitrilic chloride is attributed to its terminal ～～P⁺Cl₃PCl_?6 structure. Furthermore we presume that the polymerization of tetra-hydrofuran in the presence of this catalyst proceeds through a cationic ring-opening mechanism.     

Advances in the catalyses of polymerizations

Abstract

Nearly all technical processes for the production of polymers are carried out in the presence of catalysts. In the case of addition polymerization reactions, two mechanisms are possible: Start of the reaction via an initiator (e.g., peroxides) or start via a true catalyst (e.g., Ziegler/ Natta systems). In both areas remarkable progress has been made: Cationic “living” polymerizations of oxacycloalkanes, group transfer polymerization, metal-catalyzed alternating copolymerization of ethylene with carbon monoxide, and metallocene-catalyzed polymerizations of alpha-olefins. The polymerization of alpha-olefins with metallocene catalysts not only leads to the improvement of well-known polymers like polyethylene and polypropylene, but also enables the production of new polymers like syndiotactic polypropylene, syndiotactic polystyrene, and cycloolefin copolymers on an industrial scale.     

Iron‐mediated AGET ATRP of methyl methacrylate using metal wire as reducing agent

Methyl methacrylate (MMA) were successfully polymerized by atom transfer radical polymerization with activator generated by electron transfer (AGET ATRP) using copper or iron wire as the reducing agent at 90°C. Well‐controlled polymerizations were demonstrated using an oxidatively stable iron(III) chloride hexahydrate (FeCl₃·6H₂O) as the catalyst, ethyl 2‐bromoisobutyrate (EBiB) as the initiator, and tetrabutylammonium bromide (TBABr) or triphenylphosphine as the ligand. The polymerization rate was fast and affected by the amount of catalyst and type of reducing agents. For example, the polymerization rate of bulk AGET ATRP with a molar ratio of [MMA]₀/[EBiB]₀/[FeCl₃·6H₂O]₀/[TBABr]₀ = 500/1/0.5/1 using iron wire (the conversion reaches up to 82.2% after 80 min) as the reducing agent was faster than that using copper wire (the conversion reaches up to 86.1% after 3 h). At the same time, the experimental M_n values of the obtained poly(methyl methacrylate) were consistent with the corresponding theoretical ones, and the M_w/M_n values were narrow (～1.3), showing the typical features of “living”/controlled radical polymerization. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Polymerization of styrene with the VOCL3–AlEt2Br catalyst system

The rate of polymerization with the VOCl₃–AlEt₂Br catalyst system at 30°C. in n-hexane reached a maximum at an Al/V molar ratio of 1.5. At this ratio, the rate of polymerization was first-order with respect to catalyst and second-order with respect to monomer concentrations. The apparent activation energy calculated was 6.4 kcal./mole. Diethylzine was found to act as a chain transfer agent. However, the molecular weights of polymers obtained were low. The possibility of bromide-containing catalyst sites acting in the termination reaction has been investigated. The average valence of vanadium is discussed in relation to molecular weights.     

Transfer and Termination Reactions in “Living” Carbocationic Polymerizations

Abstract

The available data concerning the polymerization of three classes of monomers deemed to yield living polymers, vinyl ethers, styrenic monomers and isobutylene, are discussed from the point of view of transfer and termination reaction. In the case of vinylethers, linearity of [Mbar]_n with a yield up to 30,000 has been obtained, but when higher [Mbar]_n are planned, there is evidence for the occurrence of transfer reactions. In the case of isobutylene, indene, and p-Me-styrene, the linearity (up to [Mbar]_n ～ 10⁵) of [Mbar]_n with the amount of monomer polymerized which has been observed (but only at low temperature) is compatible with values of transfer constants to monomer measured in “conventional” systems. In these living systems, irreversible terminations are often not very important but may become significant toward the end of monomer consumption. The main termination process is reversible termination which may lead to narrow molecular weight distributions. The linearity of [Mbar]_n with yield is not conclusive evidence for the absence of transfer and termination and for the presence of particular active centers. The control of the polymerizations achieved up to now can be accounted for by the mechanisms of conventional cationic polymerizations, transfer reactions included.     

Open mechanistic problems of quasiliving carbocationic polymerization of olefins mediated by nucleophilic additives

This study is a comprehensive overview of the open problems and the existing views on the mechanism of quasiliving carbocationic polymerizations (QLCP) of olefins mediated by nucleophilic additives. The fundamental and general aspects of ideal living and quasiliving polymerizations involving other mechanisms, such as free radical, group transfer, ring-opening metathesis, ring-opening cationic and anionic processes, have been also analyzed and summarized. Quasiliving carbocationic polymerization of olefins in the presence of nucleophiles, which form complexes with the Lewis acid coinitiators, occur By reversible termination. Four different mechanisms have been discussed in this study: (1) reactivity leveling by nucleophiles (“electron donors”); (2) propagation by species with decreased ionicity (“stretched polarized bonds”) mediated by Lewis acid-nucleophile complexes (LA-Nu); (3) propagation by classical ion pair and free ion species; (4) proton scavenging by nucleophiles and 2,6-di-teri-butylpyridine proton trap. It is shown that mechanisms No. 1, 3 and 4 cannot explain all the existing findings, and although the experimental results can be interpreted with mechanism No. 2, the existence of “stretched polarized bonds” can be questionable. It is also concluded that compared to nonliving carbocationic polymerization, kinetic analysis indicates that the propagating species cannot be the same in quasiliving carbocationic polymerizations and in chain transfer dominated classical carbocationic polymerizations with ion pairs and free ions.     

Comparison of living polymerization mechanisms. Acrylates and carbocationic polymerization

The concept of a living polymerization is critically discussed. A system ranking various classes of “livingness” is proposed, and the importance of determining the real values of k_tr/k_p and k_t/k_p ratios is expounded. New living systems, including carbocationic polymerization and group transfer polymerization of acrylates are compared with classic ionic systems. The mechanism of propagation and the nature of the true active species are similar in both new and classic polymerizations. The role of various components which improve the “livingness” of the polymerizations is discussed and explained by dynamic equilibration between dormant and active species and suppression of side reactions.     

Preparation of carboxyl‐end‐group polyacrylamide with low polydispersity by ATRP initiated with chloroacetic acid

Atom transfer radical polymerization (ATRP) of acrylamide was successfully carried out with chloroacetic acid as initiator and CuCl/N,N,N′,N′‐tetramethylethylenediamine (TMEDA) as catalyst either in water at 80 °C or in glycerol–water (1:1 v/v) medium at 130 °C. In both cases, carboxyl‐end‐group polyacrylamide was obtained with lower polydispersity ranging from 1.03 to 1.44 depending on the polymerization condition. Polymerization kinetics showed that the polymerizations proceeded with a living/controlled nature and accelerated at a higher temperature. The effect of pH in the reaction system on the polymerizations was further studied, revealing that chloroacetic acid not only served as a functional initiator for the ATRP of acrylamde but also provided the acidic polymerization condition, which effectively protected the ATRP of acrylamide from the unexpected complexation and cyclization side‐reactions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3956–3965, 2007     

Some new developments in vinyl ether polymerization

Different combinations of acetals with trimethylsilyl iodide have been explored as new initiating systems for the vinyl ether polymerization. The resulting polymers are characterized by controlled molecular weights and narrow molecular weight distributions, confirming the living polymerization mechanism. Acetals can also be used as transfer agents in the polymerization of vinyl ethers. When using 1,1-diethoxyethane (DEE) as transfer agent and isobutyl vinyl ether (IBVE) as monomer, a transfer constant of 0.2 was obtained (at −40°C in toluene). This method, transposed to functional acetals, provides a new way to prepare polyvinyl ethers with one or two functional end groups. The cationic polymerization of isobutyl vinyl ether initiated with the combination triflic acid/thietane, where thietane acts as electron donating moderator, leads to star-shaped polyvinylether-polythietane block-copolymers (at −40°C in dichloromethane). The block-copolymer structure is obtained because the vinyl ether polymerization is stopped when the α-alkoxy thietanium ion (active species) is attacked by a thietane molecule, which is at the same time an initiation reaction for the thietane polymerization. The star-shaped structure of the block-polymer is the result of the intermolecular termination in the cationic polymerization of thietane. When using a bifunctional initiator system, a polymer network is obtained consisting of linear polyIBVE-segments interconnected by branched polythietane segments. These findings support the sulfonium ion structure of the active species in the cationic polymerization of vinyl ethers initiated by the acid-sulfide system.     

An Attempt to Adapt Superacid Chemistry for “Living” Carbenium Ion Polymerizations

The discovery that carbocations can be stabilized in super acid media, e.g., SbF₅-SO₂, etc., raises the possibility of “living” carbenium ion polymerization. Polymerization experiments with isobutylene and styrene carried out at high acid concentrations and in the virtual absence of nucleophile, i.e., under conditions conducive for living polymerization, failed to indicate a linear conversion vs molecular weight relationship and/or block copolymer formation. Additional model experiments with 2,4,4-trimethyl-1-pentene substantiate our conclusions that “living” carbocation polymerizations are unlikely to be produced by superacid chemistry.     

Anionic bulk polymerization of α-methylstyrene. Stopping of polymerization due to vitrification and equilibrium in bulk

Anionic living polymerization of α-methylstyrene containing a small amount of THF (less than 10%) was studied at temperatures between ?30°C and 50°C. At any temperature studied, a certain quantity of monomer remained without further polymerization. The effect of temperature and THF content on the final state was completely different in low and high temperature regions; at temperatures lower than ca. 20°C, the final monomer concentration decreased with increasing polymerization temperature and THF content. This is explained by the concept of “stopping of polymerization due to vitrification” of the polymerizing mixture. In fact, the final reaction mixture is really glassy in most cases and the red color of living polymer buried in the glass is discolored only very slowly when exposed to air. Detailed analysis of the results showed that the vitrification stopping holds only approximately. At temperatures higher than ca. 30°C, a normal equilibrium between propagation and depropagation holds, and the final monomer concentration increased with temperature. It is, however, far less than the equilibrium monomer concentration obtained in solution polymerization at the same temperature, and it increased appreciably with the increase in THF content. It is shown that the behavior of the equilibrium for the whole concentration range can be explained satisfactorily by a thermodynamic theory of ternary mixture.     

Nucleation-elongation polymerization under imbalanced stoichiometry

As a result of the helical structure of the polymeric product, the folding-driven polymerization of oligo(m-phenyleneethynylene) imines in solution should inherently show nucleation-elongation in chain growth. Here, we present evidence for this behavior based on results of polymerizations conducted under conditions of imbalanced stoichiometry. Because the polymerization proceeds via imine metathesis between a pair of bifunctional monomers of types A-A and B-B, the molar ratio of the polymerizing functional groups can be arbitrarily varied. Alternatively, stoichiometry can be controlled by the addition of a monofunctional oligomer. Similar results were obtained in both cases whereby the molecular weight distribution was significantly different from that expected for classical step-growth polymerizations. At equilibrium, high molecular weight polymers were observed to coexist with the monomer in excess. Thermodynamic equilibrium was established by showing that the same distribution was reached starting either from a monomer mixture or from high polymers to which one monomer was added. These results are in great contrast to the low molecular weight oligomers that were produced when the reaction was conducted by melt condensation of bifunctional aldehyde and amine monomers, a polymerization that apparently proceeds without the nucleation event. An equilibrium model that captures the features of nucleation-elongation under conditions of imbalanced stoichiometry qualitatively supports the monomer-polymer distribution observed experimentally.