Mesogen-jacketed liquid crystalline polymers via stable freeradical polymerization

Stefania Pragliola1, Christopher K. Ober1*, Patrick T. Mather2, Hong G. Jeon3

1 Cornell University, Materials Science and Engineering, Ithaca, NY 14853-1501, U.S.A.2 Air Force Research Laboratory, AFRL/MLBP, 2941 P St., Wright Patterson AFB, OH 45433-7750 U.S.A.3 Systran Corp., Materials Directorate, AFRL/MLBP, 2941 P St., Wright Patterson AFB, OH 45433-7750U.S.A.

(Received: January 15, 1999; revised: April 23, 1999)

SUMMARY: Stable free radical polymerization has been used in the controlled synthesis of poly(2,5-bis[(4-butylbenzoyl)oxy]styrene), PBBOS. This “mesogen-jacketed liquid crystalline polymer”, which has mesoge-nic units attached directly to the backbone in a side-on mode, has been found to exhibit thermotropic liquidcrystallinity similar to more conventional main-chain architectures. Stable free radical polymerization ofPBBOS consistently produced molecular weight distributions below the theoretical limiting polydispersity of1.5 calculated for a conventional free radical polymerization process. Surprisingly, a comparison of the syn-thesis of polystyrene to the polymerization of PBBOS under nearly identical conditions showed that thePBBOS polymerized with a significantly higher reaction rate and monomer conversion efficiency. The nema-tic phase of these polymers was determined to be stable over the temperature range spanning the polymerglass transition temperature up to the temperature for thermal decomposition. The molecular arrangement ofthe PBBOS polymers was examined by wide-angle X-ray diffraction and is described here.

IntroductionStable (or controlled) free radical polymerization (SFRP)a

has gained in importance over the last few years because itcan be applied to the controlled radical synthesis of bothblock copolymers and homopolymers. Based on initiatingsystems of either 2,2,6,6-tetramethyl-piperidinyl-1-oxy(TEMPO) and benzoyl peroxide (BPO) or an initiator suchas 1-phenyl-1-(29,29,69,69-tetramethyl-19-piperidinyloxy)-

ethane (1) (see Fig. 1), the SFRP process has been demon-strated to work extremely well for styrene and its deriva-tives1–5). As an example of the diversity of materials so farproduced, star polymers have been prepared using multi-site initiators. Unusual monomers such as vinylbenzylchloride have been polymerized with little branching.

Other variations of controlled radical polymerizationusing activated halogens have now been developed that

Macromol. Chem. Phys.200, No. 10 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 1999 1022-1352/99/1010–2338$17.50+.50/0

Fig. 1. Schemeof synthesis of poly(2,5-bis[(4-butylbenzoyl)-oxy]styrene)(PBBOS) via stablefreeradical polymerization

a Stablefree radical polymerization,living free radical polymerizationandcontrolledradicalpolymerization aretermsfor radicalpolymerizationin thepresenceof a radicaltrappingspecies.

2338 Macromol.Chem.Phys.200,2338–2344(1999)

Mesogen-jacketedliquid crystallinepolymersvia stablefreeradicalpolymerization 2339

successfully produce narrow molecular weight distribu-tion methacrylates6–8). Examples of side group LC poly-(methacrylates)formedby atomtransferradicalpolymer-ization havealso beenpreparedand studiedby Pugh etal.8) TheSFRPprocedurehasbeensuccessfully appliedtostyrene-basedmonomersfor the synthesisof side groupliquid crystalline polymers with the mesogenic unitslongitudinally attachedto the flexible main chain9). Inthesematerials, the side group was attached to a vinylphenolgroup, a monomerthat hadbeenpreviously poly-merizedby SFRPin otherstructures.

In this paper, we report the synthesis via SFRPof themesogenjacketed liquid crystalline polymer, poly(2,5-[(4-butylbenzoyl)oxy]styrene),PBBOS (3), using (1) asinitiator. PBBOS has been previously synthesizedviaconventional radicalpolymerizationby Zhouet al.10) Themonomeris interestingbecauseit is basedon vinylhydro-quinone, a monomerthat hasbeenlargely unexplored inrecenttimes. The resulting polymer hasbeenreportedtobe a side group LC polymer with mesogenic unitsattached directly to the backbone in a side-onmode toform a “mesogen-jacketed liquid crystalline polymer”(MJLCP)11,12). For this classof polymers, the presenceofa flexible spacerbetweenthe backbone and the meso-genic units is unnecessary for mesophaseformation, incontrastto the need for such a spacerin conventional(end-attached) side group LCPs13). The steric crowdingbetweenthebackbone andtherigid andbulky mesogenicunits actsto stiffen the chainbackbone, leading to poly-mersthat behaveassemi-rigid main chain liquid crystal-line polymers(MCLCPs)with persistencelengthsin therangeof 15 nm14,15). Consequently, MJLCPsare distinct

from both conventional main chain and side group LCpolymersandform a uniqueclassof mainchainLC poly-mersthatcanbesynthesizedby conventionalchainpoly-merization(e.g., radicalpolymerization). Theuseof con-trolled radical polymerization to synthesize a MJLCP,suchas PBBOS, permits the synthesisof new polymerswith low polydispersity, and potentially rod-coil blockcopolymers. In this paper, we presenttheSFRPsynthesisof a mesogenjacketedLC polymer, PBBOS, along with adescription of someof its physicalproperties.

Experimental part

Synthesis

The monomer, 2,5-bis[(4-butylbenzoyl)oxy]styrene,BBOS,((2), Fig. 1), wassynthesizedby reactionof 2-vinyl-1,4-dihy-droxybenzeneand 4-butylbenzoylchloride accordingto themethoddescribedby Zhouet al.10)

1H NMR (CDCl3): d = 0.95ppm, 6H for CH3a, 1.35ppm,

4H for CH2b, 1.65ppm,4H for CH2

c, 2.70ppm,4H for CH2d,

5.10–5.90ppm, 2H for = CH2e, 6.60–6.95ppm, 1H for

1CHf = and 7.00–8.30ppm, 11H for the phenylenerings(Fig. 2).

BBOSpossessesa smecticphase(Fig. 3) betweenits melt-ing point (608C) and its clearing temperature(958C) (seeDSC thermogramin Fig. 4). The higher melting point ob-tainedfor thiscompound,comparedwith theliteraturevaluespresented in ref.10), is likely attributableto its higherpurity.

Thepolymerizationswerecarriedout in bulk, first heatingthe monomer, 2,5-bis[(4-butylbenzoyl)oxy]styrene(2), andinitiator (1) (purchasedfrom Binrad,Inc.) in a molar ratio of100/0.3at 808C underinert atmospherefor 5 min and then

Fig. 2. 1H NMR spectrumof 2,5-bis[(4-butylbenzoyl)oxy]styrene(BBOS) at roomtemperaturein CDCl3

2340 S.Pragliola,C. Ober, P. T. Mather, H. G. Jeon

heatingthe mixture to 1188C. Reactiontimes between0.5and20h wereused.PBBOS(3) wasprecipitatedin methanolanddriedin a vacuumovenat roomtemperature.

Characterization

A VarianXL 200MHz NMR wasusedwith chloroform-d assolventto obtainNMR spectraof all the compounds.Ther-mal analysiswascarriedout undera nitrogenatmospherebymeansof PerkinElmerDSC7andTGA7 instrumentsusingaheatingrateof 108C/min.Theliquid crystallinepropertiesofall compoundswere characterizedusing a Nikon HFX-DXpolarizing microscopewith a Mettler FP80 heating stage.Measurementof molecularweight and polydispersityof allpolymer sampleswere carriedout by gel permeationchro-matography(GPC)equippedusingfour WatersUltrastyragelHT columns(connectedin series)operatingat 318C. THFwasusedasthesolventandtheGPCwasoperatedat 0.3ml/min. Solution concentrationsof 1.0 mg/mL and solutionvolumesof 20 lL wereemployed.GPCdatawerecollectedusinga Waters490programmablemultiwavelengthdetector,and molecular weights were calculatedfrom GPC elutionvolumedatausingmonodispersepolystyrenestandards.

Wide-angleX-ray scattering(WAXS) experimentswereconductedwith a Rigaku rotating anode X-ray generatoroperatedat 40 kV and 260mA, usinga coppertarget andagraphitemonochromator. A parallel bundle of about 5–10fibers drawnfrom the nematicmelt wasplacedon a pinholecollimator in a vertical orientation,and the diffraction pat-terns were recordedon a phosphoricimage plate using aStatton cameraat room temperature.Diffraction patternswereobtainedby scanningthephosphoricimageplateusinga Molecular DynamicsStorm 820 image-platereader. Thediffractionpeakpositions(d-spacings)andwidthswerecali-bratedwith silicon powder, andtheintensitieswerecorrectedfor absorption,polarization,andscatteringgeometryfactors.Intensityprofilesasa functionof theBraggangle(2h) wereobtainedusing ImageTool software.UTHSCSA ImageToolis beta-versionsoftwareavailableby anonymousFTP frommaxrad6.uthscsa.edu.

Resultsand discussion

Synthesis

The monomer, 2,5-bis[(4-butylbenzoyl)oxy]styrene,BBOS, hasbeenshownto form semi-rigid LC polymersby radical polymerization. We observed that BBOS is avery reactivemonomerwhen polymerized under stablefreeradicalconditionsthatuse1 asinitiator. Tab.1 showsthe molecular weight, polydispersity, andmonomercon-version data for different samples of PBBOS obtainedusing different reaction times underbulk polymerizationconditions. Molecular weights were determined by gelpermeation chromatography on samples taken directlyfrom thereactionmixtureandarereportedaspolystyreneequivalent molecular weights. Polydispersityvalueswerecalculated as the ratio of M

—w to M

—n. Conversions were

obtainedby comparing therelativeareasfor themonomerand polymer UV peaks on each chromatogram. Asexpectedfor SFRpolymerization conditions,eachsampleof PBBOS had a narrow molecular weight distribution(Fig. 5). In every case, it wasalways below the theoreti-cal limiting polydispersity of 1.5 calculatedfor a conven-tional free radical process19). In contrast,only extremelypolydispersesamplesPBBOS (PDIL 3) were obtainedby Zhou et al. by conventional free radical polymeriza-tion usingAIBN as initiator. Moreover, we were readily

Fig. 3. A photomicrographof 2,5-bis[(4-butylbenzoyl)oxy]-styrene(BBOS)takenat 848C showinga smecticmesophase

Fig. 4. DSC curve of 2,5-bis[(4-butylbenzoyl)oxy]styrene(BBOS)(N2, 108C/min)

Tab.1. Polymerization of PBBOS (molar ratio monomer(BBOS)/initiator:1000/3)

Sample M—

n M—

w PDI Timein h

Conv.in %

lnM0/Mt

S1 4800 6770 1.41 0.5 17 0.19S2 18600 23600 1.27 2.5 34 0.42S3 31200 40500 1.30 4 48 0.65S4 48000 61000 1.27 15 60 0.65S5 51000 66000 1.29 20 67 1.10

Mesogen-jacketedliquid crystallinepolymersvia stablefreeradicalpolymerization 2341

able to control both the resulting molecular weight ofeachPBBOS sampleandmonomerconversion by lengthof reaction time (Fig. 6).

As typical of SFRPreactions, it wasobservedthat themolecular weight of thegrowing polymerchaingraduallyincreasedwith time while maintaining its narrow molecu-lar weight distribution (Fig. 7). The rate of polymeriza-tion wasquite fastduring the initial 4 h of reaction when50% conversion of monomeris reached. In the subse-quentreactionperiod, the molecular weight of the poly-mercontinuesto graduallyincrease,andafter 20 h, mole-cular weightsof L50000 Dalton anda conversion equalto L70%canbeobtained.Theseresultsarequite unusualif comparedto thoseobtainedfor polystyrenesynthesizedvia SFRPundernearly identicalconditions.High molecu-lar weight polystyrene (M

—n A 10000 Dalton) can be

obtained at similar rates in high yield by SFRPonly ifparticular additives, which help to increasethe rate ofpolymerization, arepresentin thereaction mixture20–23).

It wasobservedthat this approach wasunnecessaryforformation of PBBOS. High molecular weightsand con-versionscanbe reachedduring the first hoursof reactionwithout any additives and at lower temperature thanrequired for simple styrene polymerization. Thesehighrates of polymerization occur at temperatures at least158C lower than usually reportedfor PS(seeFig. 7 and8). Moreover, it was observedthat very high molecular

Fig. 5. GPC curves of poly(2,5-bis[(4-butylbenzoyl)oxy]styr-ene) (PBBOS) samples (see Tab.1) showing the incrementalincreaseof molecularweightwith time

Fig. 6. Plot of M—

n (0) and%-conversion(f) for poly(2,5-bis[(4-butylbenzoyl)oxy]styrene) (PBBOS)versustime of reaction (seealsoTab.1)

Fig. 7. Plot of M—

n versusconversion in a comparisonbetweenpolystyrene(PS) and poly(2,5-bis[(4-butylbenzoyl)oxy]styrene)(PBBOS) synthesized via SFRPunder nearly identical condi-tions. (g) PBBOS,(0) PS (synthesizedat 1238C, no additives.Seealsoref.1)), (f) PS(synthesizedat 1308C andusingcamphor-sulfonic acid(CSA) asadditive.Seealsoref.20)), (h) PS(synthe-sized at 1258C andusingFMPTSasadditive.Seealsoref.21))

Fig. 8. Percentconversion versustime of reaction,a compari-son between polystyrene(PS) and poly(2,5-bis[(4-butylbenz-oyl)oxy]styrene)(PBBOS)synthesizedvia SFRPundernearlyidentical conditions.(g) PBBOS,(0) PS(synthesized at 1238C,no additives. Seealso ref.1)), (f) PS(synthesizedat 1308C andusing CSA asadditive.Seealso ref.20)), (h) PS(synthesized at1258C andusingFMPTSasadditive.Seealsoref.21))

2342 S.Pragliola,C. Ober, P. T. Mather, H. G. Jeon

weights of PBBOS, greater than 100000 Dalton, andpolydispersities lessthan1.2 canbe realizedby decreas-ing the quantity of initiator in the mixture of reaction.Tab.2 shows the polystyrene equivalent molecularweights and polydispersity resultsobtainedfor samplesof PBBOS[S6andS7]synthesizedunderthesamecondi-tions describedabove (conversion L20%), but using amolarratio betweenmonomerandinitiator of 1000/1.

Theseresultsdemonstrateseveral unusual featuresofthis monomerand the importantdifferences in the reac-tion stateof BBOScomparedto styrene.In searching forexplanationsof this higherrateof reactivity, it wasnotedthat liquid crystallinity might play a role.Polymerizationswerecarriedout at temperaturesabovethoseof theinitialLC state of the monomer, but polymerization quicklyinducedformation of a new polymeric mesophase.Thispreordering mayenablemoreefficient polymerization,asthe monomers would be lined up. Preordering effectshavebeenobservedin manyLC systems including someof thosereportedrecently by Percecet al.25) Since themonomersidegroups lie roughly orthogonal to the poly-merchain direction,backbonegrowthoccursperpendicu-lar to themesogenic groupdirector. It is alsopossible thattheelectronwithdrawingcharacterof thebenzoategroupson the monomerprovidea morereactivestatefor SFRP.The effect of severe steric hindranceon the TEMPO ispresentlyunknown. The eventual slowing down of thepolymerization rateis probably dueto the increasedvisc-osity of thesesemirigidrodpolymers.

We expectthat BBOS is a very reactive monomerforblock copolymerization. Preliminary results appear toconfirm this view. Presently, weareexploringthepossibi-lity of synthesizing block copolymers of poly(acetoxy-styrene-b-2,5-bis[(4-butylbenzoyl)oxy]styrene), startingfrom TEMPO-terminatedpoly(acetoxystyrene). We haveobservedthat p-acetoxystyreneis a monomerwith reac-tivity fasterthan styrene.A recent report by Zhou et al.describesthe preparationof block copolymers basedonstyrenewith a BBOS-like monomer26).

Thermalcharacterization

Initial characterization hasrevealed that all the samplesof PBBOS were not crystalline, but insteadare glassy,mesophase-forming polymerswith a glasstransition tem-perature L1308C. Above this temperature, PBBOSbecomesbirefringent and a nematic mesophasecan be

observed(Fig. 9). It wasimpossible to observeanyclear-ing transition, becausethe polymer decomposesbeforereaching its nematic-isotropic transition temperature(“clearingpoint”). Additionally, a 10% weight loss wasobserved at 3808C by TGA. Thesethermal dataarecon-sistentwith thedatapreviouslypublishedby Zhouet al.10)

X-ray dataanalysis

The X-ray diffraction patterns obtained for fibers ofPBBOS,S8(M

—n = 87 K) andS9(M

—n = 39 K) drawnfrom

the nematicmelt areshown in Fig. 10 in which the fiberaxis is vertical. Two characteristicreflections appear inthepatterns,onebeinga strong,orientedarcon theequa-tor, and the secondbeing a diffuse and lightly orienteddiffraction signal on the meridian.Previously, Xu et al.16)

reported similar X-ray results on their shear-orientedsamples.The relatively sharp diffraction attributed to thedistance betweenpolymer backbones is highly intensearound the equator, indicating that the chainsareprefer-entially oriented along the draw direction. The outerdif-fraction peak,attributedto thespatialcorrelationbetweenmesogenicunits,is weaklyconcentratedaroundthemeri-dian, indicating that the axesof mesogenstend to orientperpendicular to the polymer chains.The orientationalorder parameters (pP2P) have been determined17,18) andcompared betweenS8 (Fig. 10a) and S9 (Fig. 10b) byscanningazimuthally both the inner andouter diffractionrings. The inner diffraction peak for S8 yields pP2P =0.50 l 0.02, while the same peak for S9 (lower MW)yields pP2P = 0.49l 0.02,indicating no significanteffectof molecular weight. In contrast, the orientational orderparametersmeasured from theouter diffraction peaksareof much lower value, indicating that the mesogensarerelatively lessoriented. In particular, we havefound thatfor the outer diffraction peak pP2P = 0.34l 0.02 for S8,while pP2P = 0.31l 0.02 for S9. In light of the lack ofMW-dependencefor the inner-peak pP2P data,this outer

Tab.2. Polymerization of PBBOS (molar ratio monomer(BBOS)/initiator:1000/1)

Sample M—

n M—

w PDI

S6 107000 113000 1.06S7 108000 122000 1.13

Fig. 9. Photomicrographof poly(2,5-bis[(4-butylbenzoyl)oxy]-styrene) (PBBOS) taken at 1488C showing a nematic meso-phase

Mesogen-jacketedliquid crystallinepolymersvia stablefreeradicalpolymerization 2343

peak data indicates a degreeof orientational indepen-dencefor the mesogenic units with respect to the mainchains.

Fig. 11 shows the plots of intensityversus2h for bothhigh (S8) and low (S9) molecular weight polymers. Theequatorialspacings (i and ii ) provideinformation regard-ing interchain packing distance which is slightlydecreased from L18 A (2h = 4.9o) for sample S9 toL17.2 A (2h = 5.1o) for sample S8, indicating a moreordered and compact structure developed in the highmolecular weight polymer. The meridional spacings (iii

and iv in Fig. 11) which represent the intermoleculardis-tance betweenthe mesogenic units showdiffusescatter-ing at 2h = 19.8o (d L 4.5 A), causedby the liquid-likeassociationof thealignedmesogenic groups.Comparisonof high and low molecular weight polymersrevealsthatthe meridional intensity profiles are exactly coincidentwith each other as shown in Fig. 11. This interestingresult suggeststhat the compactnessof lateralpacking ofpolymer backbonesdoesnot seriouslyaffect the correla-tion length betweenmesogens,an observation consistentwith pP2P measurementsdiscussedabove. Moreover, thisobservation suggests nematicbiaxiality in PBBOS withsome independenceof backbone and mesogendirectors.The ability provided by SFRPformation of thesepoly-mersto control molecular weight, molecular weight dis-tribution and polymer chain architecture makes thesematerials useful modelsof main chain LC polymersforfutureinvestigationsin polymerphysics.

Acknowledgement: The authors would like to thank theAFOSRMURI programfor supportof this research.Partialsup-port by the National ScienceFoundation,Division of MaterialsResearch and use of the facilities of the Cornell Center forMaterials Research are also appreciated. Finally, CKO thanksthe Xerox Research Centre of Canadafor a gift usedto supportthis work.

1) M. K. Georges, R. P. N. Veregin, P. M. Kazmaier, G. K.Hamer, Macromolecules26, 2987(1993)

2) M. K. Georges, R. P. N. Veregin, P. M. Kazmaier, G. K.Hamer, TrendsPolym.Sci.2, 66 (1994)

Fig. 10. X-ray diffractionpatternsobtainedfrom (a) high molecularweightpolymer(S8)and(b) low molecular weightpoly-mer (S9)by drawinga fiber from the nematicmelt. The contrast of eachpatternhasbeendigitally enhancedfor viewing pur-pose,while dataanalysishasbeenperformed onunenhanceddiffractionpatterns. Thefiber axisis vertical

Fig. 11. WAXS profiles of high (i) and low (ii) molecularweight polymersalong the equatorandWAXS profiles of high(iii) andlow (iv) molecularweightpolymersalongthemeridian

2344 S.Pragliola,C. Ober, P. T. Mather, H. G. Jeon

3) C. J.Hawker, J. Am.Chem. Soc. 116, 11314(1994)4) C. J.Hawker, Acc.Chem.Res.30, 373(1997)5) G. G. Barclay, C. J.Hawker, H. Ito, A. Orellana,P. R. P. Mal-

enfant,R. F. Sinta,Macromolecules31,1024 (1998)6) E. T. Patten,J.Xia, T. Abernathy, K. Matyjaszewski, Science

272, 866(1996);A. Kajiwara, K. Matyjaszewski, Macromol.RapidComm. 19, 319(1998)

7) M. Steenbock,M. Klapper, K. Mullen, Macromol. Chem.Physic199, 763(1998)

8) A. Kasko, A. Heintz, C. Pugh, Macromolecules31, 256(1998)

9) C. Bignozzi, C. K. Ober, M. Laus, “LC Side Chain-CoilDiblock Copolymers By Living Free Radical Polymeriza-tion”, Macromol.Symp.,in press

10) Q. F. Zhou, X. Wan, X. L. Zhu, F. Zhang,X. D. Feng,Mol.Cryst.Liq. Cryst. 231, 107(1993)

11) Q. F. Zhou, H. M. Li, X. D. Feng,Macromolecules20, 233(1987)

12) Q. F. Zhou, X. L. Zhu, Z. Q. Wen, Macromolecules22, 491(1989)

13) H. Finkelmann, H. Ringsdorf, H. J. Wendorff, Makromol.Chem.179, 273(1978)

14) F. Hardouin, S. Mery, M. F. Achard,L. Noirez, P. Keller, J.Phys.II 1, 511 (1991)

15) Q. F. Zhou,Polym.Bull. (Chinese)3, 160(1991)

16) G. Xu, J. Hou, S. Zhu, X. Yang,M. Xu, Q. Zhou, Polymer35, 5441(1994);Q. F. Zhou in “Liquid CrystallinePolymerSystems:TechnologicalAdvances”, A. I. Isayev, T. Kyu andS. Z. D. Cheng,Eds.,American Chemical Society, Washing-ton DC, 1996,pp.344–357

17) L. E. Alexander, “X-Ray Diffraction Methods in PolymerScience”, Huntington,New York 1979

18) W. Haase,Z. X. Fan,H. J. Muller, J. Chem.Phys. 89, 3317(1988)

19) G. G. Odian,“Principles of Polymerization”, JohnWiley &Sons, NewYork 1981,pp.280–281

20) M. K. Georges, R. P. N. Veregin, P. M. Kazmaier, G. K.Hamer, M. Serbian, Macromolecules27, 7228(1994)

21) P. G. Odell, R. P. N. Veregin, L. M. Mishawka, M. K.Georges,Macromolecules30, 2232(1987)

22) E. Malmstrom, R. D. Miller, C. J. Hawker, Tetrahedron 53,15225(1997)

23) B. Keoshkerian, M. Georges, M. Quinlan, R. Veregin, B.Goodbrand,Macromolecules31, 7559(1998)

24) M. C. Bignozzi,M. Laus,C. K. Ober, unpublishedresults25) V. Percec, C.-H. Ahn, B. Barboiu, “Polymeric Materials:

ScienceandEngineeringPreprints”, 1997,vol. 77, p. 10726) X. H. Wan,Y. F. Tu, D. Zhang,Q. F. Zhou,ChineseJ. Polym.

Sci. 16(4) 377(1998)