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

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

Termination processes in group transfer polymerization

Two major termination processes in the GTP of methacrylates and acrylates have been studied. For methacrylates, intramolecular cyclization is the dominant, if not sole, termination reaction. Both cyclization and O/C-silyl isomerization of the chain end are observed for acrylates. Oligomerization experiments were used to study the relative rates of cyclization and propagation and the influence of temperature, catalyst and dp. Under typical GTP conditions for methacrylates, cyclization is at least two orders of magnitude slower than monomer addition.     

Synthesis of phosphinodiselenoic acid esters and their application as RAFT agents in styrene polymerization

Phosphinodiselenoic acid esters are synthesized from the reaction of chlorodiphenylphosphine and aryl- or alkyl-magnesium bromide in the presence of selenium powder. They are employed as RAFT agents in thermally initiated, styrene polymerization. The phosphinodiselenoic acid esters 6a and 6b showed some degree of control over the radical polymerization of styrene.     

Mechanistic aspects of group transfer polymerization

The experimental evidence supporting the involvement of enolate anions in group transfer polymerization(GTP) is reviewed. The results of silyl group exchange studies between living silyl ketene acetal-ended oligomers under typical GTP conditions are discussed. It is concluded that the observations of significant amounts of silyl group exchange in the presence of polymerizing monomer are not consistent with the originally proposed “associative mechanism” based on the GTP Criterion which precludes intermolecular silyl group exchange.     

Hyperbranched methacrylates by self-condensing group transfer polymerization

The synthesis of hyperbranched methacrylates was achieved by self-condensing group transfer polymerization of 2-(2-methyl-1-triethylsiloxy-1-propenyloxy)ethyl methacrylate ( 1 ). “Back-biting” is shown to be the predominant side reaction. In spite of this, the compact nature of the hyperbranched molecules was demonstrated through SEC-viscosity measurements and comparison with a linear analogue. The degree of branching can be controlled by copolymerization with conventional monomers, e.g. MMA, and the living polymers can be used as macroinitiators for star-shaped polymers.     

Arylthiosulfonium salts as transfer agents of the S-aryl group to double bonds

Conclusions A general method was developed for the preparation of arylthiosulfonium salts which are active transfer agents for the S-aryl group in reactions with alkenes, leading to the formation of-arylthioalkylsulfonium or S-arylepisulfonium salts.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 378–386, February, 1982.The authors express their gratitude to V. A. Chertkov for taking the low-temperature PMR spectra.     

The discovery and commercialization of group transfer polymerization

Group transfer polymerization (GTP) is a fundamentally new method for polymerization of acrylic monomers, discovered at DuPont over 20 years ago. It allows one to make block and other specialized polymer chain architecture at above ambient temperature. The method uses silyl ketene acetals as initiators and requires a nucleophilic catalyst. DuPont uses the process to make dispersing agents for pigmented inks and automobile finishes. The development of GTP from its discovery to introduction of commercial products is presented. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2855–2860, 2000     

Reversible addition‐fragmentation chain transfer polymerization of diisopropyl fumarate using various dithiobenzoates as chain transfer agents

Dialkyl fumarates as 1,2‐disubstituted ethylenes exhibit unique features of radical polymerization kinetics due to their significant steric hindrance in propagation and termination processes and provide polymers with a rigid chain structure different from conventional vinyl polymers. In this study, we carried out reversible addition‐fragmentation chain transfer polymerization of diisopropyl fumarate (DiPF) in bulk at 80 °C using various dithiobenzoates with different leaving R groups as chain transfer agents to reveal their performance for control of molecular weight, molecular weight distribution, and chain end functionality of the resulting poly(DiPF) (PDiPF). 2‐(Ethoxycarbonyl)‐2‐propyl dithiobenzoate ( DB1 ) and 2,4,4‐trimethyl‐2‐pentyl dithiobenzoate ( DB2 ) underwent fragmentation and reinitiation at a moderate rate and consequently led to the formation of PDiPF with well‐controlled chain structures. It was confirmed that molecular weight of PDiPF produced by controlled polymerization with DB1 and DB2 agreed with theoretical one and molecular weight distribution was narrow. Dithiobenzoate and R fragments were introduced into the polymer chain ends with high functionality as 95% by the use of DB1 . In contrast, polymerizations using 1‐(ethoxycarbonyl)benzyl dithiobenzoate ( DB3 ), 1‐phenylethyl dithiobenzoate ( DB4 ), and 2‐phenyl‐2‐propyl dithiobenzoate ( DB5 ) resulted in poor control of molecular weight, molecular weight distribution, and chain end structures of PDiPF. Fragmentation and reinitiation rates of the used benzoates as chain transfer agents significantly varied depending on the R structures in an opposite fashion; that is, introduction of bulky and conjugating substituents accelerated fragmentation, but it retarded initiation of DiPF polymerization. It was revealed that balance of fragmentation and reinitiation was important for controlled polymerization of DiPF. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3266–3275     

Free-radical polymerization of methyl methacrylate in the presence of dithiobenzoates as reversible addition-fragmentation chain transfer agents

The pseudoliving radical polymerization of methyl methacrylate in bulk mediated by dithiobenzoates with various leaving groups as reversible addition-fragmentation chain-transfer agents has been studied. It has been shown that polymerization proceeds under conditions of the low steady-state concentration of radical intermediates; as a result, the steady-state of the process is rapidly achieved even at low conversions. Retardation of polymerization observed at high concentrations of reversible addition-fragmentation chain-transfer agents is apparently associated with the occurrence of chain termination reactions involving intermediates, as evidenced by the model reaction. The autoacceleration of polymerization is suppressed with an increase in the concentration of reversible addition-fragmentation chain-transfer agents. An efficient approach to the synthesis of a narrow-dispersed PMMA with the controlled molecular mass has been suggested.     

Controlled radical polymerization of acrylonitrile in the presence of trithiocarbonates as reversible addition-fragmentation chain transfer agents

Specific features of pseudo-living radical polymerization of acrylonitrile in dimethyl sulfoxide solution in the presence of low-molecular-weight and polymeric trithiocarbonates as reversible addition-fragmentation chain transfer agents were studied.     

Molecular architecture control in acrylic polymers by group transfer polymerization

The living polymerization of methacrylates by a repeated silyl Michael addition reaction (GTP)^a operates at room temperature and permits a high degree of molecular architecture control. Chelic, telechelic, block, comb, star, loop, and ladder polymers have been synthesized. For nucleophilic catalysts other than bifluoride, a dissociative mechanism is now proposed.     

Chain transfer in ethylene polymerization. VII. Very reactive and depletable transfer agents

The study of chain transfer in the free-radical polymerization of ethylene was extended to very reactive transfer agents, particularly aldehydes and mercaptans The chaintransfer constant C_s for aldehydes was insensitive to temperature changes but was strongly reduced as pressure was increased. Mercaptans were found to deplete during polymerization (C_s > 1.0). The conventional Mayo equation was integrated to allow accurate calculation of C_s for depleting transfer agents.     

Characterization of hydrophilic networks synthesized by group transfer polymerization

Group transfer polymerization was used to synthesize several series of hydrophilic random and model networks. Cationic random networks were prepared both in bulk and in tetrahydrofuran (THF) using a monofunctional initiator and simultaneous polymerization of monomer and branch units, while a bifanctional initiator was employed in THF for the synthesis of model networks comprising basic or acidic chains. Upon polymerization of the monomer, the latter initiator gives linear polymer chains with two “living” ends, which are subsequently interconnected to a polymer network by the addition of a branch unit. Homopolymer network star polymers were also synthesized in THF by a one‐pot procedure. The synthesis involved the use of a monofunctional initiator and the four‐step addition of the following reagents: (i) monomer, to give linear homopolymers; (ii) branch unit, to form “arm‐first” star polymers; (iii) monomer, to form secondary arms and give “in‐out” star polymers; and, finally (iv) branch unit again, to interconnect the “in‐out” stars to networks. Different networks were prepared for which the degree of polymerization (DP) of the linear chains between junction points was varied systematically. For all networks synthesized, the linear segments, the “arm‐first” and the “in‐out” stars were characterized in terms of their molecular weight (MW) and molecular weight distribution (MWD) using gel permeation chromatography (GPC). The degrees of swelling of both the random and model networks in water were measured and the effects of DP, pH, and monomer type were investigated.     

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

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

Synthesis of end-functionalized polystyrenes with organosilicon end-capping reagents via living cationic polymerization

The end-functionalization of living polymers with bases (methanol, benzylamine, diethyl sodiomalonate, and sodium methoxide) and organosilicon compounds [X ? Si(CH₃)₃;X ? : CH₂?C(CH₃)COO? , CH₃COO? , CH₂?CHCH₂? , C₆H₅? ] was investigated in the living cationic polymerization of styrene initiated with the 1-phenylethyl chloride/SnCl₄/nBu₄NCl system in CH₂Cl₂ at ?15°C. The four bases and C₆H₅SiMe₃, independent of their structures, were apparently incapable of reacting with the living end and invariably led to polystyrenes with the ω-end chlorine [～ ～ ～ CH₂CH(Ph)Cl] originated from the initiating system. The number-average end-functionality (F?_n) of the chloride, determined by ¹H-NMR, was close to unity (F?_n > 0.9). The presence of chlorine in the polymer was also confirmed by elemental analysis. In contrast, the quenching by the trimethylsilyl compounds with X = methacryloxy, acetoxy, and allyl gave ω-end-functionalized polystyrenes with the corresponding terminal groups (X) for which the F?_n values were close to unity (F?_n > 0.9). The effects of the structure of silyl compounds on end-capping are also discussed. © 1994 John Wiley & Sons, Inc.     

Cationic ring-opening transfer polymerization of spiro ortho esters initiated by carbon black

The ring-opening transfer polymerization of spiro ortho esters (SOE) initiated by carbon black was investigated. In the absence of carbon black, no polymerization occurred at all. In the presence of channel black containing carboxyl group, the ring-opening transfer polymerization of SOE was initiated at 50-70°C. to give polyether ester, namely alternating copolymer of epoxide and lactone. The rate of polymerization of 1,4,6-trioxaspiro[4.4]nonane and 1,4,6-trioxaspiro[4.5]decane was considerably small compared with that of 1,4,6-trioxaspiro[4.6]undecane. The activation energy of the polymerization of 2-chloromethyl-1,4,6-trioxaspiro[4.6]undecane was estimated to be 6.0 kcal/mol. The initiating activity of carbon black increased with an increase in carboxyl group content of carbon black. Furnace black that contained no carboxyl group was unable to initiate the polymerization. Furthermore, the carbon black lost the initiating ability of the polymerization upon the blocking of carboxyl group on the surface by the treatment with potassium hydroxide or diazomethane. Based on these results, it was concluded that carboxyl group on carbon black plays an important role in the initiation. During the polymerization, a part of the polymer formed was grafted onto carbon black: the grafting ratio was 10–30%. The mechanisms of initiation and grafting were discussed.     

Surface-initiated group transfer polymerization mediated by rare earth metal catalysts

We present the first example of a surface-initiated group transfer polymerization (SI-GTP) mediated by rare earth metal catalysts for polymer brush synthesis. The experimentally facile method allows rapid grafting of polymer brushes with a thickness of >150 nm in <5 min at room temperature. We show the preparation of common poly(methacrylate) brushes and demonstrate that SI-GTP is a versatile route for the preparation of novel polymer brushes. The method gives access to both thermoresponsive and proton-conducting brush layers.