Holder, Simon J.

Jones, Richard G.

Parker, James

English

Kent Academic Repository

Direct Evidence for the Interaction of theMechanisms of Thermally Initiated and AtomTransfer Radical PolymerizationJames Parker, Richard G. Jones,* andSimon J. HolderCentre for Materials Research, School of Physical Sciences,University of Kent, Canterbury, Kent, CT2 7NR U.K.Received March 21, 2000Revised Manuscript Received September 6, 2000Introduction. Autopolymerization of styrene1-5 re-mains relatively unimportant in standard free radicalpolymerization, but Matyjaszewski, Fukuda, and Hawkerhave noted that it is of great importance at the elevatedtemperatures used in living free radical polymerizations(LFRP) mediated by nitroxyls such as TEMPO.6-8Autopolymerization of styrene in ATRP systems hasbeen observed by Angot et al.9 and Matyjaszewski etal.10 during syntheses of star polymers of styrene at highmonomer concentrations and at temperatures of 100 °Cor higher. In both cases, sides peaks at lower molecularweights were observed in the size exclusion chromato-grams of the products that were attributed to linearpolystyrene arising from the thermal initiation of themonomer.In a recent paper we published the synthesis of a poly-(methyphenylsilane)-graft-polystyrene copolymer.11 Thiswas achieved by the bromomethylation of poly(meth-ylphenylsilane) (PMPS),12 with the resulting bromom-ethylated poly(methylphenylsilane) (BrMePMPS)13 be-ing used as a macromolecular initiator for the bulkATRP of styrene at 110 °C using Cu(I)Br and bipyridine(bipy) as a catalyst system.14,15Although thermal initiated polymerization has beennoted in both LFRP and ATRP reactions, its interactionwith these systems has only been shown for LFRP.16In this paper we show how a detailed study of thisgrafting reaction has yielded valuable informationconcerning the interaction of the mechanisms of ATRPand the underlying autopolymerization.Results and Discussion. PMPS12 and BrMePMPS13were synthesized according to literature procedures. Thepolystyrene grafts were grown by ATRP.11,17 The courseof the reaction was followed by 1H NMR spectroscopyand size exclusion chromatography. The reaction timeuntil gelation for the ATRP reaction was found to varygreatly with graft chain length, from 7 h for a calculateddegree of polymerization (calculated DP) of 70 to 72 hfor a calculated DP of 200. It should also be notedthat the reaction time required for a grafting reactionto high molecular weight is significantly longer than forthe equivalent homopolymerization using a Cu(I)Br(bipy) catalyst system.14 The plot of Mn versus conver-sion for the low molecular weight grafting reaction(calculated DP  70) is linear, indicating that thepolymerization has living character (Figure 1). The nearperfect overlay of the size exclusion chromatograms ofa typical low molecular weight graft copolymer recordedat 334 nm (characteristic of the PMPS ó-conjugatedsilicon backbone) and at 254 nm using a UV detectordemonstrates that all of the material present containscatenated silicon atoms (Figure 2). In contrast, thetypical overlay for a high molecular weight copolymersystem (calculated DP  200), shown in Figure 3,displays a secondary peak with a shoulder at lowermolecular weight in the 254 nm chromatogram, whichis not present in the 334 nm chromatogram. This is* Address for correspondence.Figure 1. A plot of Mn versus % conversion for the synthesisof a low molecular weight graft copolymer (calculated DP 70).Figure 2. An overlay of the size exclusion chromatograms(SEC) for a low molecular weight graft copolymer (calculatedDP  70) acquired by UV detection at 334 (âââ) and 254 nm(s).Figure 3. An overlay of the SEC for a high molecular weightgraft copolymer (calculated DP  200) acquired at 254 (s)and 334 nm (- - -) and after photolysis (âââ).9166 Macromolecules 2000, 33, 9166-916810.1021/ma000507u CCC: $19.00 © 2000 American Chemical SocietyPublished on Web 11/21/2000attributable to polystyrene that is not attached tocatenated silicon atoms. It is thus apparent that thereis a side reaction producing polystyrene that is notbound to a polysilane chain.18 Given the high temper-ature and the extended reaction times, we have drawnthe conclusion that it is polystyrene produced followingautoinitiation through a Mayo-type thermal mecha-nism.3,4To investigate further the formation of the low mo-lecular weight fraction in the course of the overallpolymerization, several such products were photolyzedin order to release the polystyrene grafts,19 as can beseen from Figure 3. Low molecular weight material (Mn 100-1000) is formed, which is consistent with thedegradation of the parts of the polysilane backbone towhich grafts were not attached. In addition, a single,almost monomodal peak is observed at the same posi-tion as the secondary peak of the parent system. Fromthe close overlay of the low molecular weight tails ofthese peaks in the two chromatograms it is evident thatthis represents a greater amount of polymer than thenongrafted polystyrene that was present before pho-tolysis. The low polydispersity index, 1.4, for this peakincluding its still perceptible shoulder20 indicates thatthe thermal and ATRP reactions produce polymerchains of very similar lengths.21 Such a value is notunreasonable for high molecular weight polymer formedthrough ATRP. It follows that the growth of the auto-initiated polystyryl chains must also be controlledwithin the ATRP mechanism.Copper(II) bromide will convert polystyryl radicals totheir dormant form whether or not they originate fromthermal or Cu(I)Br initiated processes. This would beanalogous with the reverse ATRP process noted byMatyjaszewski.22 Under the conditions of a normalATRP polymerization, Cu(I)Br and RBr are present inequimolar amounts, in this case the latter species beingthe -PhCH2Br groups attached to the polysilane back-bone. Cu(II)Br2 should be present in only very smallamounts. However, the Cu(I)Br used as the catalyst inthese reactions was only 98% pure with the majorimpurity being thought to be Cu(II)Br2. To test whetherthis was sufficient to moderate the thermally initiatedpolymerization, a reaction was carried out under identi-cal conditions to the ATRP reaction, except that thebrominated polysilane was excluded. Following isola-tion, the product proved to be a high molecular weight(Mn  250 000), broad distribution (Mw/Mn ) 2.5)polymer. There had been no obvious moderation of thepolymerization by any Cu(I) species produced within thesystem. It follows, therefore, that the only rationale forthe photochemical observations is an interaction of thethermal and ATRP polymerizations through a commonintermediate and that this must be the propagating freeradical. In order for the autoinitiated chains to comeunder the ATRP control, a sufficient excess of Cu(II)-Br2 would have to be available within the reactionsystem. A mechanism for the production of such anexcess has been noted previously.23 Any occurrence ofirreversible bimolecular termination involving radicalsfrom the Cu(I)Br initiated process would make availablethe Cu(II)Br2 required for bringing thermally initiatedradicals under the control of the ATRP. While the effecton the molecular weight distribution in the low molec-ular weight grafting reaction is imperceptible, given thesignificant concentrations of propagating free radicalsformed through autoinitiation and the far longer reac-tion times, the effect is far more obvious in the highmolecular weight grafting reaction. These processes areindicated in Scheme 1, within which no distinction isdrawn between radicals, R¥, originating within theCu(I) or Mayo-type processes. Such processes are en-tirely analogous to the persistent radical effect describedand modeled by Fischer24 for the TEMPO moderatedbulk polymerization of styrene and extended to ATRPreactions by Matyjaszewski et al.25,26In conclusion, while the effects of the autopolymer-ization of styrene have been observed previously,9,10 webelieve that this fortuitous study has produced the mostcogent evidence to date of the interaction of the mech-anisms of ATRP and the underlying thermal polymer-ization through a persistent radical effect. Although wehave only demonstrated the effect for a single metal-mediated system, we believe that it is quite general inall such systems involving high reaction temperatures.Acknowledgment. The authors thank the JapanChemical Innovation Institute for funding through theIndustrial Science and Frontier Program supported bythe New Energy and Industrial Technology Develop-ment Organisation (NEDO).Supporting Information Available: A typical polymer-ization procedure. This material is available free of charge viathe Internet at http://pubs.acs.org.References and Notes(1) Simon, E. Ann. 1839, 31, 265.(2) Dostal, H.; Mark, H. Trans. Faraday Soc. 1936, 32, 54.(3) Mayo, F. R. J. Am. Chem. Soc. 1968, 90, 1289.(4) Mayo, F. R. Polym. Prepr. (Am. Chem. Soc., Div. Polym.Chem.) 1961, 22, 55.(5) Chong, Y. K.; Rizzardo, E.; Solomon, D. H. J. Am. Chem.Soc. 1983, 105, 7761.(6) Greszta, D.; Matyjaszewski, K. Macromolecules 1996, 29,7661.(7) Fukuda, T.; Terauchi, T.; Goto, A.; Ohno, K.; Tsujii, Y.;Miyamoto, T. Macromolecules 1996, 29, 6393.(8) Hawker, C. J. Acc. Chem. Res. 1997, 30, 373.(9) Angot, S.; Murthy, K. S.; Taton, D.; Gnanou, Y. Macromol-ecules 1998, 31, 7218.(10) Matyjaszewski, K.; Miller, P. J.; Pyun, J.; Kickelbick, G.;Diamanti, S. Macromolecules 1999, 32, 6526.(11) Jones, R. G.; Holder, S. J. Macromol. Chem. Phys. 1997, 198,3571.(12) Jones, R. G.; Budnick, U.; Holder, S. J.; Wong, W. K. C.Macromolecules 1995, 26, 8036.(13) Holder, S. J.; Swain, A. C.; Jones, R. G.; Went, M. J.;Benfield, R. E. Polymer Prepr. (Am. Chem. Soc., Div. Polym.Chem.) 1995, 36, 312.(14) Wang, J. S.; Matyjaszewski, K. Macromolecules 1995, 28,7901.(15) Wang, J. S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117,5614.Scheme 1. Probable Mechanism for Atom TransferRadical PolymerizationMacromolecules, Vol. 33, No. 25, 2000 Communications to the Editor 9167(16) Dourges, M.-A.; Charleux, B.; Vairon, J.-P. Macromolecules1999, 32, 2495.(17) PMPS: Mn ) 11 700, Mw ) 165 700, and Mw/Mn ) 14.BrMePMPS: Mn ) 5200, Mw ) 8500, and Mw/Mn ) 1.6.BrMe substitution ) 33% of phenyl groups by 1H NMR. LowDP graft copolymer: Mn ) 9700, Mw ) 37 200, and Mw/Mn) 3.8. High DP graft copolymer (includes both peaks): Mn) 60 800, Mw ) 150 300, and Mw/Mn ) 2.4. SEC’s wereacquired against linear polystyrene standards with a UVdetector and as such do not take into account hydrodynamicdifferences between the linear and graft copolymers. Atypical polymerization procedure is given as SupportingInformation.(18) It should also be noted that the monomodal distribution forthe low DP copolymers and the multimodal distribution forhigh DP copolymers did not depend on the level of substi-tution in the parent BrMePMPS. These distributions werealso observed for copolymers with about 40% and 20%bromomethylation, indicating that the extended reactiontimes were not due to crowding of initiator sites leading toinefficient initiation. A fuller discussion of initiator efficiencyand the relative activity of the grafting sites and chains willbe published in a full paper at a later date.(19) Polysilanes undergo UV photodegradation to low molecularweight fragments.(20) Thought to be a small polystyrene fraction formed throughbimolecular termination of propagating free radicals.(21) The presence of low molecular weight initiating species couldpossibly account for this result, but the precursor polymerswere reprecipitated three times and a monomodal distribu-tion for the lower molecular weight copolymers would seemto refute this argument.(22) Xia, J.; Matyjaszewski, K. Macromolecules 1997, 30, 7692.(23) Matyjaszewski, K.; Patten, T. E.; Xia, J. J. Am. Chem. Soc.1997, 119, 674.(24) Fischer, H. Macromolecules 1997, 30, 5666.(25) Shipp, D. A.; Matyjaszewski, K. Macromolecules 1999, 32,2948.(26) Matyjaszewski, K. Macromolecules 1998, 31, 4710.MA000507U9168 Communications to the Editor Macromolecules, Vol. 33, No. 25, 2000

117, 5614. Scheme 1. Probable Mechanism for Atom Transfer Radical Polymerization Macromolecules,

BrMePMPS: Mn ) 5200, Mw ) 8500, and Mw/Mn ) 1.6. BrMe substitution ) 33% of phenyl groups by 1H NMR. Low DP graft copolymer:

Communications to the Editor 9167

Faraday Soc.

Polymer Prepr.

Polysilanes undergo UV photodegradation to low molecular weight fragments.

Thought to be a small polystyrene fraction formed through bimolecular termination of propagating free radicals.

Direct evidence for the interaction of the mechanisms of thermally initiated and atom transfer radical polymerization

http://kar.kent.ac.uk/2562/1/Polysilane_Graft_Polystyrene_ATRP_Macromolecules_2000.pdf

Direct evidence for the interaction of the mechanisms of thermally initiated and atom transfer radical polymerization

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