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EPR Study of Atom Transfer RadicalPolymerization (ATRP) of Styrene

Krzysztof Matyjaszewski* and Atsushi Kajiwara

Depart m ent of Chemi st ry, Carnegi e M el l on U ni versi t y,4400 Fi f t h A venue, P i t t sbur gh, P ennsyl vani a 15213 

Received August 28, 1997 

Revised M anu scri pt Received D ecember 9, 1997 

Contr olled/“living” polymeriza tion methods offer thebest wa y to prepar e well-defined (co)polymers (con-trol led molecular weight , polydispersi t ies, terminalfunctionalities, chain architecture composition) in sys-tems w here the cont ribution of side reactions is sma ll.1-3

Ext ension of at om tra nsfer ra dical a ddition (ATRA)4,5

to atom tra nsfer ra dical polymeriza tion (ATRP ) providesa new a nd efficient w ay to conduct contr olled/“living”ra dica l polymerization.6 With a variety of alkyl ha lides,R-X (X   )   C l o r B r) , a s t h e i n i t i a t o r a n d a t ra n s i t i o nmet a l species complexed by suit a ble liga nd (s), CuX/2,2′-bipyridine, as the catalyst , ATRP of vinyl monomerss u ch a s s t y re n e a n d (m e t h )a c ry l a t e s p ro ce eds i n acont rolled/“living ” fa shion.7 The result ing polymers

ha ve degrees of polymerizat ion predetermined by ∆[M]/[I]o up to  M  n   ∼ 105 and low polydispersities, 1.05< M w /M n  <   1.5. For example wh en 1-phenylethyl chloride8a

or arenesulfonyl chloride8b i s u s e d a s a n i n i t i a t o r a n dCuC l/4,4′-diheptyl-2,2′-bipyrid ine or 4,4′-di(5-nonyl)-2,2′-bipyridine (dNbipy) complex is used as the catalyst ,s t y re ne i s p ol y m eri z ed b y re pe t i t iv e a t o m t ra n s f e rradical additions to yield a well-defined polymer with anar row molecular weight distribution (M w /M n  )  1.05).

According to kinetic a nd mechan ist ic studies of st y-rene ATRP, i t was proposed that the polymerizat ionproceeds by monomer addition to free radicals which arereversibly generat ed by a n a tom tra nsfer process fromdormant polymer chains with hal ide end groups.9 I n

t h e s e rea c t i on s , a s m a l l a m ou n t of C uI I

species serveas a deactivator which moderates rates and is respon-sible for ma inta ining low polydispersi t ies. The CuII

species can be separately added to the system or canbe formed spontaneously as a result of th e so-cal ledpersistent ra dical effect .10 A r o ug h e st i ma t e of t h eamount of formed CuBr 2/dNbipy species from kinet icstudies was   ∼5%, ba sed on t he C uB r/dNbipy cat a lyst.9

Electron pramagnetic resonance (EPR) spectroscopyis a very useful tool to investigate paramagnetic spe-cies.11 S t ru c t u res , con ce n t ra t i on s , a n d dy n a m i cs ofp a ra m a g n et i c c om p ou n ds ca n b e o bt a i n e d f rom E P Rm ea s u r em en t s i n t h e s t u dy of r a d i ca l p ol y me ri za -tions.12,13 EP R ha s been used to determine concentra -tions of free TEMPO in nitroxide-mediated polymeri-

zation of styr ene.14,15 Furthermore, EPR spectroscopyca n b e u s ed t o i n v es t i g a t e t h e ch e m is t ry of p a ra m a g -n et i c m e t a l com pl ex es . E P R ca n p ot e nt i a l ly y i el dinformation on the local structure, coordination struc-ture, symmetry, concentrat ion, and aggregated struc-ture of para magnetic copper(II) species.16 This com-munication reports the direct determination by EPR ofcopper(II) species in styrene ATRP.17

A typical scheme for an ATRP system is shown inS c he m e 1. A h a l og en a t e d i n i t i a t o r re a c t s w i t h a di a -magnetic copper(I) complex with the rate constant ofa c t i va t i o n (k a ) t o f or m a n i ni ti a t in g r a d ica l a n d a

para magnetic copper(II) species. The organ ic ra dicalthen ini t iates a radical polymerizat ion which is con-trolled by the reversible deactivat ion of the propaga tingradical with paramagnetic copper(II) species with therat e consta nt   k d. Polymeric radicals propaga te with thera t e con s t a n t   k p, a n d i rre v e rs i b l y t e rm i n a t e w i t h t h era t e c o n s t a n t   k t   via coupling (product P n +m ) a nd /ordisproportionat ion (products P dn   a n d P H m ). E v e ry a c tof termination is accompanied by the irreversible forma-tion of excess deactiva tor (copper(II )). In th e invest iga -

tion of this r eaction by EP R spectroscopy, initiating a ndpropaga ting rad icals, a s w ell as copper(II) species ar epara magnetic and EP R active. In principle, al l of thesespecies could be observed by EPR spectroscopy; however,u n f ort u n a t e ly (or f ort u n a t e ly ) o n ly t h e p a ra m a g n e t iccop pe r(I I ) s p eci es ca n b e ob s erv ed du e t o i t s h i g hconcentra tion relat ive to the organic ra dicals. Initia tingra dicals usual ly ha ve short l i fet imes; they a re presentin extremely low concentrat ions and react easi ly withmonomer to form propagating radicals which are alsopresent in low concentra t ions. The concentra t ion oforganic radicals in these systems is usua lly in the rangeof 10-8 to 10-7 mol/L.12,13 The concentra tions of copper-(II) species in this system are above 10-3 mol/L (videi nf r a ) w h i ch i s 104 or 105 t i mes h ig her t h a n t h e

ini t iat ing a nd propaga ting ra dicals according t o persis-tent radical effect.10 Thus , th e copper(II ) species is t hepredominant species observed by EP R in ATRP syst ems.

Time-dependent EPR signals of copper species in theATR P of s t y re n e a re s h o w n i n F i g ure 1 . Th e s i g n a lfeature present after 20 min heating is considered tobe a typical axial symmetric copper(II) signal .16 Th eprelimina ry s tud ies of the str ucture of copper(II) speciesin t he presence of copper(I) indica te some intera ctionsb et w e e n t h e m , p os s ib ly v i a h a l og e n b ri dg in g . Th econcentrat ion of copper(II) species was est imated bydouble integrat ion of t hese signals. The t ime depen-dence of copper(II) concentra t ions in the ATRP ofstyrene, in toluene, initiated by 1-phenylethyl bromide

an d benzyl bromide is shown in F igure 2 along wit h theresult of bulk polymerizat ion of styrene ini t iated by1-phenylethyl bromide.

In the case of the solution polymerizat ion ini t iatedby 1-phenylethyl bromide, the concentra tion of copper-(II) ini t ial ly increased very ra pidly a nd nea rly rea cheda steady-sta te concentra t ion. This concentra t ion mayb e re g u la t e d n ot on l y b y t h e ATR P e q u il ib ria b u taddit ional ly by slow genera tion of ra dicals result ingfrom t he therma l sel f-ini t iat ing process. The “steady-state” concentration was about 2.5-2.8 mmol/L. In t hecase of the benzyl bromide ini t iated system, the con-

Scheme 1. Reaction Scheme of ATRP

548   M acromolecul es  1998, 31 ,  548-550

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centration of copper(II) increased gradually reaching a“ st e a d y s t a t e ” on l y a f t er 4-5 h . Th e E P R r es ul tssupport the kinetic results of these polymerizat ionswhich showed that benzyl bromide is a slow ini t iator.The higher concentra tion of copper(II) species in bulkis due t o higher ini t ial concentra t ions of both ini t iatorand copper(I), since in all experiments the same [M]o:[RX]o:[Cu(I)]o   rat ios were used.

The percentage of Cu II formed from Cu I w a s ca l c u-l a t e d, a n d t h e re s u lt s a re s h ow n i n F i g u re 3. I n a l l of

the cas es of the ATRP of styrene, approximat ely 4-6%of the copper(I) formed copper(II) species in the polym-erizat ion system w ith 94-96%of copper(I) species stillremaining in the monovalent state.

A syst em wit h copper(II ) a dded before polymeriza tion(10 mol %CuBr 2  relat ive to CuBr) was examined. Thetime dependence of copper(II) concentra tions, a long wit hthe results of the sa me system wit hout copper(II) speciesat the beginning, are shown in Figure 4. When CuBr 2was added independently, the copper(II) concentrationsta bi lized at a steady-sta te concentra t ion of 12.5%,which is consistent with the slower polymerization rate.9

Apparently, some initial radical coupling could occurrdespite excess copper(II) present.

In summary, we observed EPR signal of copper(II)species in the ATRP syst em. The stea dy sta te concen-tration of copper(II) species is approximately 4-6%ofthe origina l copper(I) species. These results a re con-sistent with our previous kinetic dat a.9

Acknowledgment.  The auth ors are grat eful to Dr.Michael P . Hendrich, Department of Chemistry, Car n-egie Mellon U niversity, for his h elp with mea surementsof EPR spectra. This work is part ly supported by theIndustria l Sponsors of the ATRP Consortium a t C MU.A.K. acknowledges the Ministry of Education, Science,and Culture, J apa n, for f inancial support .

Figure 1.   EPR spectra of the polymerization mixture mea-sured a t 25 ° C a fter 0, 20, 40, 60, 120, and 180 min a t 110 °C.St yr ene/1-phen ylet hyl bromid e/Cu B r/dNbipy ()100/1/1/2) intolu ene (50 vol %).

Figure2.   Time-dependent concentr a tions of copper(II) speciesfor ATRP initiated by 1-phenylethyl bromide (O) and b enz y lbromide (4) i n t olue ne sol ut i on and b ulk pol y m er i zat i oninitiated by 1-phenylethyl bromide (b).

Figure 3.   Time-dependent proportion of copper(II) speciesformed from copper(I) species by ATRP initiated by 1-phenyl-ethyl bromide (O) and benzyl bromide (4) in toluene solut ionan d bulk polymerizat ion initiated by 1-phenylethyl bromide(b).

Figure 4.   Time-dependent proportion of copper(II) species forATRP initia ted by 1-phenylethy l bromide (O) a n d t h e s a m esy st e m c ont ai ni ng i ni t i a l l y 0 . 0 1 M of C uB r 2/dNb ipy (1) in

toluene solution.

M acromolecul es, Vol. 31, N o. 2, 1998    Commu nica t ions t o t he E dit or   549

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References and Notes

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(15) Fukuda, T.; Terauchi, T.; Goto, A.; Ohno, K. ; Tsujii , Y . ;M i y a m ot o , T. ; K o b a t a k e , S . ; Y a m a d a , B .   M acrom olecules 1996,   29 , 6393.

(16) (a) Hath aw ay, B . ; Billing, D. E.  Coord. Chem. Rev . 1970,  5 ,1 43 . (b ) H a t h a w a y , B . ; D u g g a n , M . ; M u r p h , A .; M u l la n e ,J .; P ower, C.; Walsh, A.; Walsh, B .  Coord. Ch em. Rev . 1981,36 , 267.

(17) EPR spectra were recorded on a Bruker ESP-300 X-bandEPR spectrometer. A 0.2 mL samples of the polymerizationmixtures were taken from th e polymerizat ion systems andput into an EPR tube (o.d. 4 mm) under argon. The mixture

was degassed three times by freeze-pump-tha w cycles ands ea l e d u n d e r v a c u um . S p ec t r a w e r e r e c or d e d a t r o omtemperature after polymerization at controlled temperaturef or a g i ve n t i m e . I t i s r e c og n i z ed t h a t c on c en t r a t i o n o fgrowing ra dicals is much higher a t polymerizat ion temper-ature ([P •] ∼ 1 0-7 M a t 1 10 ° C ) t h a n a t r o om t e m p e r a t u r e([P •] , 10-8 M). How ever, this does not effect concentr at ionof Cu II species wh ich can cha nge by less t ha n 0.01%, since[C u II ] > 1 0-3 M. Concentrations of copper(II) species weree st i m a t e d b y d ou b le i n t e gr a t i o n o f s p ec t r a . S p ec t r a o fcopper(II) trifluoroacetylacetonate in the same media at thesame t emperat ure were used as s tan dards. Typical polym-erizat ion syst em is styr ene/initia tor/CuB r/dNbipy ()10 0/1/1/2) wit h 50%of toluene or bulk. P olymeriza tion t empera-ture was 110 °C for s tyrene.

MA971299H

550   Commu nica t ions t o t he E dit or   M acromolecul es, Vol. 31, N o. 2, 1998 

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