Immortal SET–LRP mediated by Cu(0) wire

Immortal SET–LRP Mediated by Cu(0) Wire

XUAN JIANG, BRAD M. ROSEN, VIRGIL PERCEC

Department of Chemistry, Roy & Diana Vagelos Laboratories, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323

Received 29 March 2010; accepted 2 April 2010

DOI: 10.1002/pola.24059

Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Cu(0)-wire/Me6-TREN is a well established catalyst

for living radical polymerization via SET–LRP. Here, it is dem-

onstrated that this polymerization is not just living, but it is in

fact the first example of immortal living radical polymerization.

The immortality of SET–LRP mediated with Cu(0) wire was dem-

onstrated by attempting, in an unsuccessful way, to irreversible

interrupt multiple times the polymerization via exposure to O2

from air. SET–LRP indeed stopped each time when the reaction

mixture was exposed to air. However, the SET–LRP reaction,

was restarted each time after resealing the reaction vessel and

reestablishing the catalytic cycle with the same Cu(0) wire, to

produce the same conversion as in the conventional uninter-

rupted SET–LRP process. Despite the interruption by O2, the

reactivated SET–LRP had a good control of molecular weight,

molecular weight evolution, and molecular weight distribution,

with perfect retention of chain-end fidelity. VC 2010 Wiley Periodi-

cals, Inc. J Polym Sci Part A: Polym Chem 48: 2716–2721, 2010

KEYWORDS: immortal; kinetics (polym.); living polymerization;

radical polymerization; SET–LRP

INTRODUCTION Single-electron transfer living radical poly-merization (SET–LRP)1–4 catalyzed by Cu(0) has been de-monstrated to be an efficient method for the synthesis ofpolymers with linear and more complex topology under mildconditions. In SET–LRP various forms of copper metal, suchas powder,1–7 wire,8–12 or nascent Cu(0) produced from thein situ disproportionation of Cu(I)X,13–17 can be used as het-erogeneous catalysts capable of mediating the polymerizationof acrylates,3,6,8–11,18–23 methacrylates,3,24,25 acrylamides,14–17

and vinyl chloride1,3,4,12,26 with excellent control of molecularweight evolution and distribution, and perfect retention ofchain-end functionality.7,22,23 SET–LRP has been used in thesynthesis of mechanophore-linked polymers,27–30 dendriticmacromolecules,31 stars,32,33 branched14 and graft copoly-mers.13,15–17 The heterogeneous, outer-sphere single electrontransfer process of SET–LRP has also been applied todevelop new synthetic methodologies, such as single-electrontransfer radical addition fragmentation chain transfer poly-merization (SET–RAFT)24,34 and single-electron transfernitroxide-radical coupling (SET–NRC).35,36

As defined by its inventors,37,38 ‘‘immortal polymerization isthe polymerization that gives polymers with a narrow molecu-lar weight distribution, even in the presence of a chain trans-fer reaction, because of its reversibility, which leads to the re-vival of the polymers once dead, that is, the immortal natureof the polymers.’’37 Immortal polymerization was originallydeveloped for the ionic ring-opening polymerization (ROP) ofepoxide/epsulfide,39–44 lactones,45–52 lactides,53–57 and cycliccarbonates58–61 in the presence of chain-transfer agents, typi-cally alcohols. In this case, to conduct a successful immortal

ROP, a rapid and reversible exchange between metal alkoxideand alcohol is required. As the exchange is sufficiently rapid inrelation to propagation, the molecular weight distribution ofthe final polymers remains as narrow as in the absence ofalcohols. Therefore, the polymerization tolerates chain-transferagents without sacrificing any control. In this report, weexpand the concept of immortal polymerization to include anyliving radical polymerization which can be resuscitated afterexposure to classic inhibitors, such as O2, which would typ-ically mediate an irreversible termination, and still give goodcontrol of molecular weight evolution and distribution. In thisimmortal SET–LRP, the dead polymer chains can be reinitiatedthrough simple deoxygenation and the living polymerization isresumed, even after multiple cycles of O2 exposure.

Critical to any transition metal-catalyzed LRP, is the equilib-rium between the active radical chains (Pn

�) and the dormantchains (Pn�X).2,62,63 This equilibrium is achieved through dis-tinct activation and deactivation mediated by different oxida-tion states of the metal catalyst. Many transition metal cata-lysts, such as Cu(0) and Cu(I)X, are easily oxidized and theiruse in LRP requires stringently anerobic conditions. Any ad-ventitious air introduced into the reaction mixture will inter-rupt the catalytic cycle, since the lower oxidation state of themetal required for activation will be oxidized to an less activeCu2O, or inactive Cu(II), higher oxidation state species, therebyprematurely terminating the polymerization. Therefore, rigor-ous deoxygenation of the reaction mixture and a well-sealedreaction vessel are necessary to conduct most transition-metalcatalyzed LRP. In SET–LRP, a Cu-based transition metal-cata-lyzed LRP, deoxygenation is typically achieved through multiple

Correspondence to: V. Percec (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 2716–2721 (2010) VC 2010 Wiley Periodicals, Inc.

2716 INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

freeze-pump-thaw cycles. However, our laboratory recentlydemonstrated that the SET–LRP of MA can be performed inthe presence of air by adding a small amount of the reducingagent, hydrazine hydrate.18 Cu(0), is in fact a very effective O2

scavenger, that is oxidized to Cu2O in this process. Cu2O candirectly mediate activation in LRP (eq 1),2,3,64–71 but does somore slowly than Cu(0). In addition, Cu2O can also dispropor-tionate to Cu(0), (eq 2), but this process is slower than thecorresponding disproportionation of Cu(I)Br. This is due to thefact that Cu2O is insoluble and, therefore, its disproportiona-tion involves a heterogeneous process, while Cu(I)X is solubleand its disproportionation takes place via a homogeneous pro-cess. Thus, poorly deoxygenated SET–LRP reactions are accom-panied by a sizable induction period as Cu2O from the surfaceof Cu(0) is gradually converted to more reactive species viadisproportionation. The addition of hydrazine hydrate, can cir-cumvent this induction by regenerating the active catalystthrough a rapid reduction of Cu2O to Cu(0).18

Cu2Oþ R� X ! CuðIÞXþ CuðIIÞOþ R� (1)

Cu2O ! Cuð0Þ þ CuðIIÞO (2)

In the earliest reports on SET–LRP, Cu(0) powder was used asthe catalyst in a 1:1 M ratio of [Cu(0)]/[ligand].5,6,19–23 How-ever, it was observed that some of the Cu(0) powderremained in the reaction mixture even at the end of polymer-ization.5 Residual Cu(0) is the result of a self-controlled regen-eration of nascent Cu(0) through the disproportionation of insitu generated Cu(I)X/N-ligand. The residual Cu(0) couldpotentially serve as an O2 scavenger without the need of addi-tional reducing agents, as long as the amount of O2 intro-duced into the system is relatively small. Later, Cu(0) powderwas replaced by Cu(0) wire as a recyclable catalyst for SET–LRP.8 Because of the decreased surface area to mass ratio ofCu(0) wire versus Cu(0) powder, relatively large amounts ofCu(0) wire are typically used, and, therefore, a large excess ofcopper catalyst is present in the polymerization mixture. The-oretically, SET–LRP catalyzed with Cu(0) wire should be morerobust than the corresponding polymerization catalyzed withCu(0) powder since the bulk Cu(0) provides not only the ini-tial source of activating species but also scavenges O2, and itsabundance provides the source of new activators necessary toreestablish the catalytic cycle after exposure to air.

In this report, we demonstrate that the SET–LRP mediatedwith Cu(0) wire is immortal. Various attempts to kill theSET–LRP of MA through exposure to O2 during the polymer-ization failed. Although the O2 oxidized the active Cu(O) cata-lyst to Cu2O, it only temporarily disrupts the polymerization.Upon deoxygenation via one freeze-pump-thaw cycle, thepolymerization was resuscitated and the SET–LRP continuedwithout broadening the molecular weight distribution orlosing the polymer chain-end fidelity.

RESULTS AND DISCUSSION

SET–LRP Interrupted Once by Exposure to O2

Previously, rigorous deoxygenation was necessary to achievea SET–LRP without an induction period. The apparent rate

constant of propagation, kappp , decreases slightly if O2 is intro-duced into the reaction during sampling or if the reactionvessel is not perfectly sealed. Recent studies showed that theSET–LRP can be conducted without freeze-pump-thaw cyclesin the presence of reducing agents such as hydrazine hydrate(N2H4�H2O).

18 It was proposed that O2 was scavenged by theCu(0) wire to form Cu2O, which can then be reduced back toCu(0) by hydrazine hydrate. Compared to the SET–LRP ofMMA in the absence of N2H4, SET–LRP of MMA in the pres-ence of 0.1 equiv N2H4 to initiator exhibited a similar kappp ,but a higher initiator efficiency (Ieff).

72

During SET–LRP, nascent Cu(0) is generated in situ from thedisproportionation of Cu(I)X/L formed during the activationand deactivation processes. The nascent Cu(0) species is notonly a powerful catalyst for SET–LRP but also a reducingagent that scavenges O2. To test the tolerance of the SET–LRPsystem to air and how fast it can recover after exposure toair, a series of O2 tolerance tests under SET–LRP conditionswere conducted. The first test performed was to bubble airinto a standard SET–LRP reaction performed under the reac-tion condition: MA ¼ 1.0 mL, DMSO ¼ 0.5 mL, [MA]0 ¼7.4 mol/L, [MA]0/[MBP]0/[Me6-TREN]0 ¼ 222/1/0.1 with12.5 cm of 20 gauge Cu(0) wire at 50% conversion.9–11 After10 min, 48% conversion of MA was achieved according toNMR (kappp ¼ 0.072 min�1), at which point the Schlenk tubewas opened, and air was bubbled through the reaction mix-ture for 5 min. The polymerization ultimately stopped at aslightly higher conversion, 52% [Fig. 1(a)], due to some poly-merization during the delay between sampling and the intro-duction of O2. After resealing the Schlenk tube, one freeze-pump-thaw cycle followed by refilling with N2 was performedto remove the air above the surface of reaction mixture. It isunlikely that all of the dissolved O2 in the mixture was removedafter one degassing cycle. Upon re-initiation, a slower apparentrate constant, kappp ¼ 0.061 min�1, was observed from 65 to88% conversion. Upon exposure to air, the nascent Cu(0) cata-lyst could be oxidized to Cu2O and the catalytic cycle inter-rupted. The surface of Cu wire, however, is unlikely to becomefully oxidized during this short period of time. Once the air wasdisplaced by N2, the catalytic cycle was quickly restored byCu(0) wire activation of dormant chains. Once the reaction mix-ture is deoxygenated, the Cu2O generated via oxidation canmediate activation (eq 1) and eventually disproportionate (eq2). However, since Cu2O is insoluble in the reaction mixture itsdisproportionation is slower than that of Cu(I)X species and,therefore, the reactivation of the SET–LRP is expected to beslower. It is apparent that the Cu2O present in the reaction mix-ture after deoxygenation has a similar retarding effect to theexternal addition of Cu(II)X2/Me6-TREN to reaction mixturedemonstrated in previous reports.3,20 This trend is in stark con-trast to previous experiments, where the Cu(0) catalyst wastemporarily removed from the reaction mixture and then rein-troduced to the reaction mixture without introducing any oxy-gen. In these previous experiments, the values of kappp beforeand after the interruption were nearly identical.5

In a second test, a 0.5 mL portion of MeOH was added tothe reaction mixture after exposure to O2 at �50%

ARTICLE

IMMORTAL SET–LRP MEDIATED BY CU(0) WIRE, JIANG, ROSEN, AND PERCEC 2717

conversion. Usually, methanol is used to precipitate mostpoly(acrylates) and, therefore, to irreversibly interrupt theirpolymerization. However, for SET–LRP, methanol is an excel-lent solvent and also a good solvent for PMA. Therefore, kappp

of the second stage was 0.037 min�1. In this case, kappp wasmuch slower than that obtained from mere O2 exposure dueto the effect of dilution. In both tests, the polymerizationsceased after being exposed to air, and yet the polymer chainwas not subjected to termination via O2 incorporation as aperoxy-chain end. It is possible that Cu(0) effectively removesO2 before it can form the polymeric peroxide or that the re-versible deactivation of the propagating radical species isfaster than their irreversible reaction with O2. In both O2 toler-ance tests a continuous decrease in polydispersity (Mw/Mn),and a linear molecular weight evolution with conversion wereobserved. In addition, the perfect retention of chain-end fidel-ity even after exposure to O2 exposure encouraged furthertests of the robustness of SET–LRP process.

SET–LRP Repeatedly Interrupted by Exposure to O2

To further establish that SET–LRP mediated with Cu(0) wireis indeed immortal, the polymerization was exposed to airmultiple times in the course of the reaction. To withdrawthe sufficient amount of sample required to monitor thereaction kinetics, a relatively slow SET–LRP reaction was

necessary. Therefore, the reaction conditions from Figure 2were used for this purpose (MA ¼ 2.0 mL, DMSO ¼ 1.0 mL,[MA]0 ¼ 7.4 mol/L, [MA]0/[MBP]0/[Me6-TREN]0 ¼ 444/1/0.1 with 4.5 cm length of 20 gauge Cu(0) wire). During thepolymerization, the reaction mixture was sparged with airfour times at different conversions (22%, 35%, 49%, and66%). Five kinetic domains were observed with distinct kappp

(0.014 min�1, 0.016 min�1, 0.017 min�1, 0.025 min�1, and0.017 min�1, separately). The kappp was relatively low at firstand it reached a maximum value in the middle of the reac-tion (corresponding to 50–70% conversion) before finallydecreasing toward the end. The instantaneous rate con-stants were calculated using eq 3:

kapp:xp ðinstantaneousÞ ¼� ln ½M�x

½M�o � ln ½M�x�1½M�o

� �

ðtx � tx�1Þ ; ðx ¼ 1; 2;3…Þ(3)

as shown in Figure 2(c). The overall kappp for the SET–LRP reac-tion is 0.025 min�1. However, the single point rate constant,kapp;xp , follows a similar trend to that observed in Figure 2(a). Byplotting the changes in kappp and kapp;xp [Fig. 2(e)], it is clear thatthe kappp in the middle of polymerization, at conversions 50–70%, is larger than in other regions. At 30% conversion, kappp

FIGURE 1 Kinetic plots of SET–LRP of

MA initiated MBP in DMSO at 25 �Cusing 12.5 cm of 20 gauge Cu(0) wire

as catalyst with (a and b) an exposure

to air flow or (c and d) an exposure to

air flow and addition of 0.5 mL of

MeOH during the middle of the reac-

tion. Reaction conditions: [MA]0/[MBP]0/

[Me6-TREN]0 ¼ 222/1/0.1, [MA]0 ¼ 7.4

mol/L, MA ¼ 1.0 mL, DMSO ¼ 0.5 mL.

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA


from Figure 2(a) (kappp ¼ 0.014 min�1 at 0–22% conversion)was similar to those from Figure 2(c) (kapp;1p ¼ 0.012 min�1 at9% and kapp;2p ¼ 0.015 min�1 at 22%). Although kappp increased(kappp ¼ 0.025 min�1 at 49–66% conversion) after exposure toair, it was lower than that in SET–LRP without air exposure atsimilar conversion (kapp;3p ¼ 0.034 min�1 at 57% and kapp;4p ¼0.031 min�1 at 69%) due to the accumulation of Cu2O that dis-proportionates slower than Cu(I)X. The Mn and Mw/Mn fromthe polymerization with multiple air exposures are exactly thesame as the results from the O2-free reaction [Fig. 2(d)], whichfurther supports the absence of undesired irreversible termina-tion in SET–LRP mediated with Cu(0) wire. These results dem-onstrate that SET–LRP can tolerate air to a certain extent and is,therefore, a more robust living radical polymerization thanother LRP processes. For instance, the ATRP is using Cu(I)X as

catalyst, which will be oxidized to Cu(II) upon exposure to air.The catalytic cycle cannot be reestablished with Cu(II) speciesand the polymerization cannot be reinitiated.

CONCLUSIONS

SET–LRP of MA mediated with Cu(0) wire is immortal sincethe excess Cu(0) wire can serve as a nearly endless scav-enger of O2 in the reaction and can also reestablished thecatalytic cycle after air exposure. The insoluble Cu2O gener-ated during exposure to air disproportionates slower than thesoluble Cu(I) species and continue the SET–LRP process. The O2

tolerance tests of SET–LRP of MA in DMSO at 25 �C clearlyshowed the robustness of this system. The dormant polymerchains were reinitiated and continued to grow to high

FIGURE 2 Kinetic plots of SET–LRP of

MA initiated MBP in DMSO at 25 �Cusing 4.5 cm of 20 gauge Cu(0) wire as

catalyst (a and b) with four time expo-

sures to air flow during the middle of

reaction and (c and d) without air expo-

sure; (e) plot of rate constants obtained

from Figure 2(a,c) versus conversion of

monomers. Reaction conditions: [MA]0/

[MBP]0/[Me6-TREN]0 ¼ 444/1/0.1, [MA]0

¼ 7.4 mol/L, MA ¼ 1.0 mL, DMSO ¼0.5 mL.

ARTICLE


conversion without any irreversible termination even afterperiodically bubbling air into the reaction mixture. In all O2

tolerance tests, the polymerization proceeded with linear evo-lution of molecular weight, perfect chain end functionality,and narrow molecular weight distribution. The immortal na-ture of SET–LRP mediated with Cu(0) wire is expected to pro-vide extraordinarily simple approaches to block copolymeriza-tion through sequential monomer addition and to thesynthesis of more complex topologies and architectures.

EXPERIMENTAL

MaterialsMethanol (MeOH) (Fisher, certified ACS, 99.9%), and methyl2-bromopropionate (MBP) (99%, Acros) were used asreceived. Dimethyl sulfoxide (DMSO) (Fisher, certified ACS,99.9%) was distilled under reduced pressure before use.Methyl acrylate (MA) (99%, Acros) was passed over a short-column of basic Al2O3 before use to remove the radical in-hibitor. Hexamethylated tris(2-aminoethyl)amine (Me6-TREN)was synthesized as described in the literature.73

Techniques1H NMR spectra (500 MHz) were recorded on a Bruker DRX500NMR instrument at 20 �C in CDCl3 with tetramethylsilane (TMS)as internal standard. Gel permeation chromatographic (GPC)analysis of the polymer samples were done on a Perkin–ElmerSeries 10 high-performance liquid chromatography, equippedwith an LC-100 column over (30 �C), a Nelson Analytical 900 Se-ries integration data station, a Perkin–Elmer 785A UV-vis detec-tor (254 nm), a Varian star 4090 refractive index (RI) detector,and three AM gel (500 Å, 5 lm; 104 Å, 5 lm; and 105 Å, 10 lm)columns. THF (Fisher, HPLC grade) was used as eluent at a flowrate of 1 mL/min. The number-average (Mn) and weight-average(Mw) molecular weights of the PMA samples were determinedwith PMMA standards purchased from American PolymerStandards. As the hydrodynamic volume of PMA is the same asof PMMA, no correction is needed in the determination ofMn.

Typical Procedure for O2 Tolerant Test UnderSET–LRP ConditionThe monomer (MA, 1.00 mL, 11.1 mmol), solvent (DMSO,0.5 mL), initiator (MBP, 5.6 lL, 0.050 mmol), catalyst (12.5 cmof gauge 20 copper wire, wrapped around a Teflon-coatedstirbar), and ligand (Me6-TREN, 1.4 lL, 5.0 lmol) wereadded to a 25 mL Schlenk tube in the following order:Cu(0), ligand, solvent, monomer, initiator. After six freeze-pump-thaw cycles, the Schlenk tube was filled with nitrogen,and the reaction mixture was placed in an oil bath thermo-stated at 25 �C 6 0.1 �C with stirring. During the freeze-pump-thaw process, the Cu(0) wire and stirbar were heldabove the reaction mixture using a small magnet. After thereaction was carried out for 10 min, the Schlenk tube wasopened by removing the glass stopper and the reaction mix-ture was bubbled with air flow through a SS needle for5 min. A sample was taken after air exposure and the Schlenktube was sealed again. One freeze-pump-thaw cycle was con-ducted to remove the air on the top of frozen reaction mix-ture and then Schlenk tube was refilled with N2. The side armof the tube was purged with nitrogen before it was opened

for samples to be removed at predetermined times, with anairtight syringe. Samples were dissolved in CDCl3, and theconversion was measured by 1H NMR spectroscopy. The Mn

and Mw/Mn values were determined by GPC with PMMAstandards (conversion: 88%, MGPC

n ¼ 19,539, Mw/Mn ¼ 1.21).

Financial support by the National Science Foundation (DMR-0548559 and DMR-0520020) and the P. Roy Vagelos Chair atPenn are gratefully acknowledged.

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Immortal SET–LRP mediated by Cu(0) wire

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