End‐group fidelity of copper(0)‐meditated radical polymerization at high monomer conversion: an ESI‐MS investigation

Copper(0)‐mediated radical polymerization (single electron transfer‐living radical polymerization) is an efficient polymerization technique that allows control over the polymerization of acrylates, vinyl chloride and other monomers, yielding bromide terminated polymer. In this contribution, we investigate the evolution of the end‐group fidelity at very high conversion both in the presence and in the absence of initially added copper (II) bromide (CuBr₂). High resolution electrospray‐ionization mass spectroscopy (ESI‐MS) allows determination of the precise chemical structure of the dead polymers formed during the polymerization to very high monomer conversion, including post polymerization conditions. Two different regimes can be identified via ESI‐MS analysis. During the polymerization, dead polymer results mainly from termination via disproportionation, whereas at very high conversion (or in the absence of monomer, that is, post‐polymerization), dead polymers are predominantly generated by chain transfer reactions (presumably to ligand). The addition of CuBr₂ significantly reduces the extent of termination by both chain transfer and disproportionation, at very high monomer conversion and under post‐polymerization conditions, offering a convenient approach to maintaining high end‐group fidelity in Cu(0)‐mediated radical polymerization. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Theoretical study of chain transfer in anionic polymerization. Transfer by the mechanism of side-group substitution

A new theoretical consideration of chain transfer to monomer in the anionic polymerization of hydrocarbon monomers is presented. It is shown that the kinetic scheme used in theoretical studies reported previously contradicts the widespread views on the chemical mechanism of carbanionic reactions. It is suggested that the most probable path of the transfer reaction is the proton abstraction from the side group of the monomer; the terminal double bond of the monomer molecule remains unchanged, and therefore the intermediate species can participate in succeeding reactions as a macromonomer. The molecular characteristics of polymer formed in processes with monomer transfer by side-group substitution are determined. At high conversion, the polymer formed in such a process is shown to possess a number-average degree of polymerization, P̄_n, approaching the theoretical value for living polymers, and a P̄_w exceeding it the more the higher the intensity of transfer. Furthermore, it shows a broad molecular weight distribution and a fairly noticeable degree of branching. These results considerably differ from those previously reported.     

Living carbocationic polymerization. IV. Living polymerization of isobutylene

Truly living polymerization of isobutylene (IB) has been achieved for the first time by the use of new initiating systems comprising organic acetate-BCl³ complexes under conventional laboratory conditions in various solvents from −10 to −50°C. The overall rates of polymerization are very high, which necessitated the development of the incremental monomer addition (IMA) technique to demonstrate living systems. The living nature of the polymerizations was demonstrated by linear versus grams polyisobutylene (PIB) formed plots starting at the origin and horizontal number of polymer molecules formed versus amount of polymer formed plots. obeys [IB]/[CH₃COOR^t · BCl₃]. Molecular weight distributions (MWD) are very narrow in homogeneous systems whereas somewhat broader values are obtained when the polymer precipitates out of solution . The MWDs tend to narrow with increasing molecular weights, i.e., with the accumulation of precipitated polymer in the reactor. Traces of moisture do not affect the outcome of living polymerizations. In the presence of monomer both first and second order chain transfer to monomer are avoided even at −10°C. The diagnosis of first and second order chain transfer has been accomplished, and the first order process seems to dominate. Forced termination can be effected either by thermally decomposing the propagating complexes or by nucleophiles. In either case the end groups will be tertiary chlorides. The living polymerization of isobutylene initiated by ester · BCl₃ complexes most likely proceeds by a two-component group transfer polymerization.     

Synthesis of Reactive-Ended Acrylic Polymers by Group Transfer Polymerization: Initiation with Silyl Ketene Acetals

Living methacrylate polymers are obtained at room temperature and above by initiation with ketene silyl acetals in the presence of a soluble bifluoride catalyst. During the polymerization, a trialkylsilyl group is transferred from the living chain end to incoming monomer. The new procedure has thus been named group transfer polymerization (GTP). Monodisperse polymers with predetermined molecular weights as high as 100,000 can be obtained by adjusting the monomer/initiator ratio. Telechelic poly(methyl methacrylate) with hydroxy or carboxy ends can be obtained by using an initiator containing a protected hydroxy or carboxy group and coupling the resulting living polymer.     

Reversible addition–fragmentation chain transfer polymerization of alkyl‐2‐cyanoacrylates: An assessment of livingness

Alkyl 2‐cyanoacrylates (CAs) are primarily used as instant adhesives, including those sold under the Loctite brand. The adhesive action can be inhibited with acid stabilizers allowing radical polymerization to be employed. The following article details the first attempted controlled/living radical polymerization of alkyl CAs: Reversible addition fragmentation chain transfer (RAFT) polymerization mediated by a poly(methyl methacrylate) dithiobenzoate macroRAFT agent for three different CA monomers (ethyl 2‐cyanoacrylate, n‐butyl 2‐cyanoacrylate, and 2‐phenylethyl cyanoacrylate) allowed the preparation of the first block copolymers of this challenging but commercially important monomer class. Nevertheless, GPC with UV detection indicated significant loss of the RAFT end‐group for all three CAs limiting control/living character. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1397–1408     

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.     

Perfluorocyclobutyl-containing Amphiphilic Block Copolymers Synthesized by RAFT Polymerization

Amphiphilic block copolymers containing hydrophobic perfluorocyclobutyl‐based (PFCB) polyacrylate and hydrophilic poly(ethylene glycol) (PEG) segments were prepared via reversible addition‐fragmentation chain transfer (RAFT) polymerization. The PFCB‐containing acrylate monomer, p‐(2‐(p‐tolyloxy)perfluorocyclobutoxy)‐phenyl acrylate, was first synthesized from commercially available compounds in good yields, and this kind of acrylate monomer can be homopolymerized by free radical polymerization or RAFT polymerization. Kinetic study showed the 2,2′‐azobis(isobutyronitrile) (AIBN) initiated and cumyl dithiobenzoate (CDB) mediated RAFT polymerization was in a living fashion, as suggested by the fact that the number‐average molecular weights (M_n) increased linearly with the conversions of the monomer, while the polydispersity indices kept less than 1.10. The block polymers with narrow molecular weight distributions (M_w/M_n≦1.21) were prepared through RAFT polymerization using PEG monomethyl ether capped with 4‐cyanopentanoic acid dithiobenzoate end group as the macro chain transfer agent (mPEG‐CTA). The length of the hydrophobic segment can be tuned by the feed ratio of the PFCB‐based acrylate monomer and the extending of the polymerization time. The micellization behavior of the block copolymers in aqueous media was investigated by the fluorescence probe technique.     

Living carbocationic polymerization. IV. Living polymerization of isobutylene

Truly living polymerization of isobutylene (IB) has been achieved for the first time by the use of new initiating systems comprising organic acetate-BCl₃ complexes under conventional laboratory conditions in various solvents from ?10 to ?50°C. The overall rates of polymerization are very high, which necessitated the development of the incremental monomer addition (IMA) technique to demonstrate living systems. The living nature of the polymerizations was demonstrated by linear M _n versus grams polyisobutylene (PIB) formed plots starting at the origin and horizontal number of polymer molecules formed versus amount of polymer formed plots. DP _n obeys [IB]/[CH₃COOR^t · BCl₃]. Molecular weight distributions (MWD) are very narrow in homogeneous systems (M _w/M _n = 1.2–1.3) whereas somewhat broader values are obtained when the polymer precipitates out of solution (M _w/M _n = 1.4–3.0). The MWDs tend to narrow with increasing molecular weights, i.e., with the accumulation of precipitated polymer in the reactor. Traces of moisture do not affect the outcome of living polymerizations. In the presence of monomer both first and second order chain transfer to monomer are avoided even at ?10°C. The diagnosis of first and second order chain transfer has been accomplished, and the first order process seems to dominate. Forced termination can be effected either by thermally decomposing the propagating complexes or by nucleophiles. In either case the end groups will be tertiary chlorides. The living polymerization of isobutylene initiated by ester. BCl₃ complexes most likely proceeds by a two-component group transfer polymerization.     

Modeling and Experimentation of RAFT Solution Copolymerization of Styrene and Butyl Acrylate,Effect of Chain Transfer Reactions on Polymer Molecular Weight Distribution

Chain transfer reactions widely exist in the free radical polymerization and controlled radical polymerization, which can significantly influence polymer molecular weight and molecular weight distribution. In this work, the chain transfer reactions in modeling the reversible addition–fragmentation transfer (RAFT) solution copolymerization are included and the effects of chain transfer rate constant, monomer concentration, and comonomer ratio on the polymerization kinetics and polymer molecular weight development are investigated. The model is verified with the experimental RAFT solution copolymerization of styrene and butyl acrylate, with good agreements achieved. This work has demonstrated that the chain transfer reactions to monomer and solvent can have significant impacts on the number‐average molecular weight (M_n) and dispersity (Ð).     

Mechanistic Investigation of Catalyst‐Transfer Suzuki–Miyaura Condensation Polymerization of Thiophene–Pyridine Biaryl Monomers with the Aid of Model Reactions

We have investigated the requirements for efficient Pd‐catalyzed Suzuki–Miyaura catalyst‐transfer condensation polymerization (Pd‐CTCP) reactions of 2‐alkoxypropyl‐6‐(5‐bromothiophen‐2‐yl)‐3‐(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)pyridine ( 12 ) as a donor–acceptor (D –A) biaryl monomer. As model reactions, we first carried out the Suzuki–Miyaura coupling reaction of X–Py–Th–X′ (Th=thiophene, Py=pyridine, X, X′=Br or I) 1 with phenylboronic acid ester 2 by using tBu₃PPd⁰ as the catalyst. Monosubstitution with a phenyl group at Th‐I mainly took place in the reaction of Br–Py–Th–I ( 1 b ) with 2 , whereas disubstitution selectively occurred in the reaction of I–Py–Th–Br ( 1 c ) with 2 , indicating that the Pd catalyst is intramolecularly transferred from acceptor Py to donor Th. Therefore, we synthesized monomer 12 by introduction of a boronate moiety and bromine into Py and Th, respectively. However, examination of the relationship between monomer conversion and the M_n of the obtained polymer, as well as the matrix‐assisted laser desorption ionization time‐of‐flight (MALDI‐TOF) mass spectra, indicated that Suzuki–Miyaura coupling polymerization of 12 with (o‐tolyl)tBu₃PPdBr initiator 13 proceeded in a step‐growth polymerization manner through intermolecular transfer of the Pd catalyst. To understand the discrepancy between the model reactions and polymerization reaction, Suzuki–Miyaura coupling reactions of 1 c with thiopheneboronic acid ester instead of 2 were carried out. This resulted in a decrease of the disubstitution product. Therefore, step‐growth polymerization appears to be due to intermolecular transfer of the Pd catalyst from Th after reductive elimination of the Th‐Pd‐Py complex formed by transmetalation of polymer Th–Br with (Pin)B–Py–Th–Br monomer 12 (Pin=pinacol). Catalysts with similar stabilization energies of metal–arene η²‐coordination for D and A monomers may be needed for CTCP reactions of biaryl D–A monomers.     

The relationship between catalyst precursors and chain end groups in homogeneous propene polymerization catalysis

The chain transfer to monomer reactions promoted by primary and secondary growing chains in the propene polymerization promoted by ansa‐zirconocenes and postmetallocene precursors are studied by using DFT methods. From the theoretical results it comes out that the prevalence of propene insertion over β‐hydrogen transfer to the monomer decreases drastically in the presence of a secondary chain. Furthermore, we explained the reason why C₂‐symmetric metallocene catalysts promote the selective formation of cis but‐2‐enyls end group after a 2,1 inserted unit whereas for octahedral bis(phenoxy‐imine)titanium‐based catalysts, chain release promotes exclusively the formation of allyl terminated chain end. These results might be useful to design ligand precursors able to obtain not only high M_n PP polymers but also tuned chain end groups to build new polymer architectures. Overall, a more general picture of the enantioselectivity of the chain transfer to monomer processes is reported. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 699–708, 2010     

Physically controlled,free‐radical polymerization of vaporized fluoromonomer on solid surfaces

Gas/vapor‐deposition polymerization (GDP) of vinyl monomer is expected to exhibit a unique polymerization behavior different from its polymerization in the liquid phase. Free‐radical GDP of 2,2,3,3,3‐pentafluoropropyl methacrylate (FMA) was carried out with a conventional free‐radical initiator (azobisisobutyronitrile) on substrate surfaces. A linear relationship between the number‐average molecular weight and polymer yield was observed, and the consecutive copolymerization of methyl methacrylate (MMA) and FMA led to the formation of block copolymer P(MMA‐block‐FMA). These results suggested that the GDP process on substrate surfaces has a living nature. During the process, the active species at growing chain ends may be immobilized on the deposit surface and restricted from the chain‐transfer reactions, resulting in a continuation of the propagation reaction. The GDP on substrate surfaces is therefore a physically controlled polymerization process. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2621–2630, 2004     

Vinyl polymerization. 416. Polymerization of vinyl monomer initiated by polyethyleneglycol

The polymerization of vinyl monomer initiated by polyethyleneglycol (PEG) in aqueous solution was carried out at 85°C with shaking. Acrylonitrile (AN), methyl methacrylate (MMA), and methacrylic acid were polymerized by PEG–300 (M?_n = 300), whereas styrene was not. The effects of the amounts of monomer and PEG, the molecular weight of PEG, and the hydrophobic group at the end of PEG molecule on the polymerization were studied. The selectivity of vinyl monomer and the effect of the hydrophobic group are discussed according to “the concept of hard and soft hydrophobic areas and monomers.” The kinetics of the polymerization was investigated. The overall activation energy in the polymerization of AN was estimated as 37.9 kJ mol^?1. The polymerization was effected by a radical mechanism.     

Determination of Monomer Transfer Constants in Emulsion Polymerization

The monomer transfer constant, C_m can be determined from the chain length distribution (CLD) under conditions in which the monomer transfer reaction rate is much larger than the other chain termination processes. Such reaction conditions are feasible in emulsion polymerization where bimolecular termination reactions are relatively less important. We conducted theoretical investigations aimed at finding the necessary reaction conditions to apply the CLD method to emulsion polymerization. The number of polymer chains per polymer particle needs to be large enough in order to keep the effects of unknown chain lengths to a minimum, i.e., the unknown chains formed during the nucleation period and those which stop growing when the polymerization is stopped for sampling. In emulsion polymerization, the polymer concentration at the polymerization locus is higher than the corresponding bulk polymerization as long as monomer droplets exist, and the polymer transfer reaction may possess significant effects under conditions where monomer transfer reactions are important. The Monte Carlo (MC) simulation results have shown that although the CLD profile becomes broader due to the polymer transfer reactions, they do not significantly change the slope, from which C_m is determined. According to the present simulation results, the CLD method is considered applicable even when the polymer transfer reaction cannot be neglected. The MC simulation method can be used to find the experimental conditions where the CLD method is applicable.     

Polymerisation anionique de l'acroleine—I. Polymerisations amorcees par des organo-sodiques dans le tetrahydrofuranne

In tetrahydrofuran, with Na⁺ as counter-ion, the anionic polymerization of acrolein involves numerous transfer reactions to monomer and to polymer; on the other hand, termination of growing chains does not occur. The use of initiators, like carbanions or oxanions, does not affect the polymerization rate. The kinetic order of the reaction is unity for monomer and unity for initiator; these results indicate that the living ends are not associated at the studied concentrations of initiator. Without stating precisely the mechanism of the transfer reactions, we have proposed a kinetic scheme.In tetrahydrofuran, with Na⁺ as counter-ion, the anionic polymerization of acrolein involves numerous transfer reactions to monomer and to polymer; on the other hand, termination of growing chains does not occur. The use of initiators, like carbanions or oxanions, does not affect the polymerization rate. The kinetic order of the reaction is unity for monomer and unity for initiator; these results indicate that the living ends are not associated at the studied concentrations of initiator. Without stating precisely the mechanism of the transfer reactions, we have proposed a kinetic scheme.     

Chain transfer and living functionalization of polymethacrylates by group transfer polymerization using ethyl 2-Phenyl-2-butenoate

A living functionalization method has been investigated for group transfer polymerization (GTP) of poly(alkyl methacrylates) using ethyl 2-phenyl-2-butenoate (EPB). The end-capping reactions of EPB to living trimethylsilyl ketene acetal-ended poly(methyl methacrylate) (PMMA) chain ends have been systematically studied and characterized by SEC, VPO, UV-visible spectrosocopy, ¹H and ¹³c NMR spectroscopy. The results of sequential monomer addition, varying stoichiometry and copolymerization indicate that EPB effects efficient chain end functionalization only at stoichiometric concentrations; chain transfer reactions (chain transfer constant = 0.4) occur with excess EPB and during copolymerization with MMA. Chain transfer reactions (chain transfer constant = 0.1) also occur when copolymerizing ethyl 2-methyl-2-butenoate with MMA.     

Initiation and propagation steps in the equibinary polymerization of butadiene by bis(h3-allylnickel trifluoroacetate)

Polymerization of butadiene by bis(h³-allylnickel trifluoroacetate) in benzene and o-dichlorobenzene solvents yields an equibinary 1,4-polybutadiene, containing equal amounts of cis and trans isomers. Initiation proceeds by addition of the allylic moiety of the initiator to a butadiene molecule. The rate of initiation is high enough to ensure complete consumption of the catalyst for a monomer/catalyst molar ratio of about 10 at 5°C. The propagation exhibits the characteristics of a “living” polymerization: the molecular weight is proportional to the conversion, and at the end of the reaction, the average degree of polymerization is equal to the monomer/catalyst molar ratio. Living polybutadienyl-nickel trifluoroacetate is able to reinitiate not only butadiene polymerization but also allene polymerization. However, for high [monomer]/[catalyst] ratios, conversion-dependent transfer reactions limit the molecular weight to 7000 in benzene and to 70,000 in bulk polymerization in the presence of small amounts of o-dichlorobenzene.     

Polymer end group modifications and polymer conjugations via “click” chemistry employing microreactor technology

This study presents the development of microreactor protocols for the successful continuous flow end group modification of atom transfer radical polymerization precursor polymers into azide end‐capped materials and the subsequent copper‐catalyzed azide alkyne click reactions with alkyne polymers, in flow. By using a microreactor, the reaction speed of the azidation of poly(butyl acrylate), poly(methyl acrylate), and polystyrene can be accelerated from hours to seconds and full end group conversion is obtained. Subsequently, copper‐catalyzed click reactions are executed in a flow reactor at 80 °C. Good coupling efficiencies are observed and various block copolymer combinations are prepared. Furthermore, the flow reaction can be carried out in only 40 min, while a batch procedure takes several hours to reach completion. The results indicate that the use of a continuous flow reactor for end group modifications as well as click reactions has clear benefits towards the development and improvement of well‐defined polymer materials. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1263–1274     

Macromolecular engineering of polylactones and polylactides. X. Selective end-functionalization of poly(D,L)-lactide

Functional aluminum alkoxides, such as Et_3–pAl(O? CH₂? X)_p, where p = 1,3 and X = a functional group, are very effective initiators for the (D, L)-lactide polymerization in toluene at 70°C. The coordination-insertion type of polymerization is living. Linear polyesters of a predictable molecular weight and a narrow molecular weight distribution are obtained within the period of time required for the total monomer conversion. The functional group (X) associated with the active alkoxy group of the initiator is selectively and quantitatively attached to one chain end, whereas the second end group is systematically a hydroxyl function resulting from the hydrolysis of the living growing site. Asymmetric telechelic polylactides are thus obtained in a perfectly controlled way. A kinetic study has shown that the polymerization is first order in both the monomer and initiator. © 1993 John Wiley & Sons, Inc.     

Iniferter concept and living radical polymerization

Iniferters are initiators that induce radical polymerization that proceeds via initiation, propagation, primary radical termination, and transfer to initiator. Because bimolecular termination and other transfer reactions are negligible, these polymerizations are performed by the insertion of the monomer molecules into the iniferter bond, leading to polymers with two iniferter fragments at the chain ends. The use of well‐designed iniferters would give polymers or oligomers bearing controlled end groups. If the end groups of the polymers obtained by a suitable iniferter serve further as a polymeric iniferter, these polymerizations proceed by a living radical polymerization mechanism in a homogeneous system. In these cases, the iniferters (C S bond) are considered a dormant species of the initiating and propagating radicals. In this article, I describe the history, ideas, and some characteristics of iniferters and living radical polymerization with some iniferters that contain dithiocarbamate groups as photoiniferters and several compounds as thermal iniferters. From the viewpoint of controlled polymer synthesis, iniferters can be classified into several types: thermal or photoiniferters; monomeric, polymeric, or gel iniferters; monofunctional, difunctional, trifunctional, or polyfunctional iniferters; monomer or macromonomer iniferters; and so forth. These lead to the synthesis of various monofunctional, telechelic, block, graft, star, and crosslinked polymers. The relations between this work and other recent studies are discussed. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2121–2136, 2000