The polymorphism and polymerisation of two related acids have been investigated under high pressure conditions. Acrylic acid crystallises as a new polymorph at 0.65 GPa whilst methacrylic acid crystallises in a new polymorph at a higher pressure of 1.5 GPa. Both these new polymorphs exhibit similar hydrogen bonding motifs to the low temperature phases, however, the molecular packing differs significantly

Oswald, Iain

Urquhart, Andrew

English

Name not available

Polymorphism and polymerisation of acrylic and methacrylic acid at high pressure

Iain D. H. Oswald

Andrew J. Urquhart

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CrystEngComm

University of Strathclyde Institutional Repository

Strathprints Institutional RepositoryOswald, Iain and Urquhart, Andrew (2011) Polymorphism and polymerisation of acrylic andmethacrylic acid at high pressure. CrystEngComm, 13 (14). pp. 4503-4507. ISSN 1466-8033Strathprints is designed to allow users to access the research output of the University of Strathclyde.Copyright c© and Moral Rights for the papers on this site are retained by the individual authorsand/or other copyright owners. You may not engage in further distribution of the material for anyprofitmaking activities or any commercial gain. You may freely distribute both the url (http://strathprints.strath.ac.uk/) and the content of this paper for research or study, educational, ornot-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to Strathprints administrator:mailto:strathprints@strath.ac.ukhttp://strathprints.strath.ac.uk/Polymorphism and polymerisation of acrylic and methacrylic acid at highpressure†Iain D. H. Oswald* and Andrew J. UrquhartReceived 8th March 2011, Accepted 10th May 2011DOI: 10.1039/c1ce05295kThe polymorphism and polymerisation of two related acids havebeen investigated under high pressure conditions. Acrylic acidcrystallises as a new polymorph at 0.65 GPa whilst methacrylic acidcrystallises in a new polymorph at a higher pressure of 1.5 GPa.Both these new polymorphs exhibit similar hydrogen bonding motifsto the low temperature phases, however, the molecular packingdiffers significantly.Polyacrylic acid (PAA) and polymethacrylic acid (PMA) are used ina wide range of research and industrial applications, ranging fromsuperabsorbent materials to drug delivery vehicles. TraditionallyPAA and PMA are synthesised using both thermal and photo-polymerisation methods from the corresponding monomers acrylicand methacrylic acid. Over the past ten years, Raman spectroscopyhas been used to investigate the polymerisation of simple unsaturatedcompounds such as acrylamide1 and ethylene2 through the applica-tion of pressure. Further studies have investigated the effect ofpressure and laser irradiation on compounds such as acetylene,butadiene and propene.3 Recently Murli and Song used RamanSpectroscopy to investigate the application of pressure to a monomercapable of hydrogen bonding, acrylic acid. Murli and Song investi-gated the induced initiator-free polymerization of acrylic acid bycompressing pure acrylic acid to approximately 8 GPa.4 Theyobserved two crystalline phases (I and II) of acrylic acid with Phase Irepresenting a low-temperature configuration whilst Phase II wassuggested to enhance molecular interactions towards polymerisation,albeit that the crystal structure for this phase was not reported.Murliand Song stated that ‘In situ high-pressure X-ray or neutron diffractionmeasurements would be helpful to monitor the changes in bond lengthsof acrylic acid as a function of compression.’4 With this in mind, wehave undertaken to characterise the crystal structures of acrylic acid(A) and its derivative methacrylic acid (B) (Scheme 1) under high-pressure conditions using X-ray single-crystal diffraction in order toprovide further insight into this system.The low-temperature structure of acrylic acid has previously beendetermined by Higgs and Sass5 and later by Boese et al.6 Thisstructure is composed of planes of acrylic acid dimers in space groupIbam. The planes are displaced by 7.705 A along the [1/21/2 0]direction with an interlayer separation of 3.103 A (1/2 the c-axislength). The packing of themolecules follows an ABAB arrangementwith the carboxylic acid groups situated above one another (Fig. 1).Our initial investigation of the high-pressure structural behaviourof acrylic acid focussed on the use of a Merrill–Bassett DiamondAnvil Cell (DAC) to crystallise the liquid at elevated pressure.7 Thistechnique has successfully been used to characterise novel poly-morphs of materials, especially those that show a kinetic barrier toconversion on increasing pressure.8 By heating the sample in situ athigh pressure one is able to overcome this barrier by accessing thenew polymorph through the melt. Using this method but with onlymoderate heating Phase I was isolated at 0.4 GPa (Table S1†). Thetemperature was restricted due to the issue of thermal polymerisation.The cell parameters of Phase I at high pressure and low-temperatureare remarkably similar. Interestingly, at high pressure Phase I showsan increase in the interlayer separation over the low-temperaturephase determined at 125 K. One might expect that this directionwould have been the most compressible as there are no hydrogenbonds along this direction and given its response to temperature(1.6% reduction from 138 K to 125 K). The b-direction is the onlydirection to show any compression and involves a reduction in theintermolecular spacing between the CH2 groups of neighbouring aciddimer (Table S2†).The growth of single crystals of acrylic acid at higher pressures wasnot possible as its melting point was too high in order to grow a singlecrystal directly from the melt. Heating to the usual temperatures of423K induced polymerisation and so a new technique was requiredScheme 1 The schematic diagrams of (A) acrylic and (B) methacrylicacids.Strathclyde Institute of Pharmacy and Biomedical Sciences, University ofStrathclyde, 161 Cathedral Street, G4 0RE Glasgow, UK. E-mail: iain.oswald@strath.ac.uk; Fax: +44(0)141 552 2562; Tel: +44(0)141 5482157† Electronic supplementary information (ESI) available: The electronicsupplementary information contains experimental details includingcrystallographic information as well as further Raman spectra. CCDCreference numbers 816976–816980. For ESI and crystallographic datain CIF or other electronic format see DOI: 10.1039/c1ce05295kThis journal is ª The Royal Society of Chemistry 2011 CrystEngComm, 2011, 13, 4503–4507 | 4503Dynamic Article LinksC<CrystEngCommCite this: CrystEngComm, 2011, 13, 4503www.rsc.org/crystengcomm COMMUNICATIONDownloaded by University of Strathclyde on 29 October 2012Published on 25 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CE05295KView Online / Journal Homepage / Table of Contents for this issuein order to explore further the high-pressure behaviour. Using a novelloading technique, inspired by the work of Weck et al. (see ESI†),9a sample of acrylic acid was crystallised from 4 : 1methanol : ethanolat a high pressure of 0.65 GPa. Using Raman and single-crystalX-ray diffraction this phase was determined to be a novel polymorphof acrylic acid. In order to avoid future confusion with the phasesimplied by the Raman spectra in the paper ofMurli and Song, we aredesignating this phase, Phase II.At 0.65 GPa, acrylic acid crystallises with the same dimer inter-action inmonoclinic space group,P21/c (see Table S2†). Phase II alsoshows some displacement of the layers but in this instance by 4.161Aalong the [2 0 1] direction. However, Phase II possesses an ABCpacking arrangement with the acid dimer sandwiched between twoalkene moieties of the layers above and below (Fig. 1). One of theother major differences is the corrugated nature of the layers in PhaseII compared to the planar nature in the other phase. Despite thismajor difference the separation between discrete dimers remainsconsistent with the low-temperature phase (3.107 A).In order to verify that the initial product from the high-pressurerecrystallisation was the same as the annealed product, the Ramanspectrum for both the initial and final products was taken. Fig. 2shows the fingerprint region of the Raman spectrum for each phaseand the change with respect to pressure. One can clearly see thedifference in the region of 500–550 cm1 where there is one mainpeak at 520 cm1 in Phase I whilst there are two distinct peaks inPhase II at 510 and 533 cm1. The stretch at 520 cm1 inPhase I has been previously attributed to the out-of-plane motion ofthe carboxylic acid group.10 Kulbida et al. also observed a weakintensity at 535 cm1 which they assigned to be the wagging motionof the CO2 moiety; this peak is barely visible as a weak shoulder inthe spectrum at 0.2 GPa (546 cm1). The intensity of boththese peaks is significantly increased and more clearly separated forPhase II.We attribute this change to the differences in surrounding envi-ronment of the CO2 group. The CO2 group in Phase I is sandwichedbetween CO2 groups from the layers above and below whilst inPhase II the surrounding CO2 groups are replaced by the alkenemoieties. Furthermore the change in topographical nature of thelayers may also contribute to the increased motion of the moleculein Phase II.Comparison of our data against that ofMurli and Song shows thatthey did not observe Phase II during the slow direct compressionfrom ambient pressure to 4.5 GPa; this phase was not observed evenafter three months. However, in further experiment they rapidlycompressed the sample to 3.3 GPa obtaining similar data to thosereported here, especially in the region between 100 and 200 cm1(Fig. S1†).During the initial attempts to grow a single crystal from the melt itwas observed that acrylic acid underwent a reconstructive phasetransition if it was heated at a pressure greater than 0.5 GPa. If onedecompresses this sample one is able to retain this polymorph to 0.38GPa. Unfortunately, repeated decompression experiments resulted inthe melting of the sample and so we were not able to observe whetherPhase II was retained to the melting point.We have also investigated for the first time the structural behaviourof methacrylic acid at both low temperature and high pressure.Methacrylic acid is a liquid at ambient pressure (mp 14 C) and soa crystal was grown from the polycrystalline powder encased ina glass capillary (o.d. 0.3 mm) by cycling the temperature close to themelting point.11 The low-temperature phase (Phase I) of methacrylicacid crystallises in monoclinic P21/c with the molecules adoptinga trans conformation as opposed to the cis confirmation in acrylicacid. This is in line with observations of Badawi et al.who calculatedthat the trans-form was 2 to 3.5 kJ mol1 more stable than thecis-form depending on whether DFT-B3LYP/6-311 + G** orMP2/6-311 + G** level of theory was used.12 The molecules interactthrough the carboxylic acid group to form dimers (O1/O2 2.622(1)A) (Table S2†). The dimers are stacked to form tilted columns alongthe a-direction with a separation between dimers of 3.42 A (Fig. 3a).Fig. 1 Packing arrangement of Phase I (upper) and Phase II (lower) ofacrylic acid. The colours represent different layers. Note the ABABpacking in Phase I and the ABC packing in Phase II.4504 | CrystEngComm, 2011, 13, 4503–4507 This journal is ª The Royal Society of Chemistry 2011Downloaded by University of Strathclyde on 29 October 2012Published on 25 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CE05295KView OnlineThe only further potential contact made between the columns isa long interaction between C3H3/O1 which is beyond the sum ofthe van der Waals radii for the two atoms (2.67 A, 157).During the high-pressure study of methacrylic acid we encoun-tered a similar problem to the acrylic acid study where the meltingpoint was too high for crystal growth from the melt and thermalpolymerisation occurred. This prevented the growth of a new phaseusing the traditional techniques, however, it did give an insight intothe behaviour of Phase I crystallised at high pressure. At 0.3 GPa,the cell parameters are longer than were found at 123 K which wefound surprising given that, at least in the direction of the stacking,the separation between the molecules was greater than the low-temperature form and the sum of the van der Waals for the twogroups involved. One would have expected that this directionwould have been compressed at this pressure. Comparison of theunit cell parameters obtained at high pressure with those collectedat a temperature closer to ambient conditions (273 K) showeda 4% reduction in volume with little change in the hydrogen-bonding interaction that holds the dimers together (2.631(8) A).The interlayer distance between the dimer units is observed to belonger (3.54 A).Following the successful isolation of a new polymorph of acrylicacid crystallised from the pressure-transmitting medium we set abouttrying to repeat this experiment with methacrylic acid. A 50 : 50mixture of methacrylic acid and 4 : 1 methanol : ethanol wasprepared and loaded into a DAC. The pressure was increased untilthe methacrylic acid crystallised from solution. A single crystal wasgrown through the modification of pressure and modest heating. At1.5 GPa a new set of cell parameters were identified and a novelpolymorph of methacrylic acid was structurally characterised. WeFig. 2 The Raman spectra of Phase I (upper) and Phase II (lower) of acrylic acid on increasing pressure. Note the difference in the spectra between 500and 550 cm1.This journal is ª The Royal Society of Chemistry 2011 CrystEngComm, 2011, 13, 4503–4507 | 4505Downloaded by University of Strathclyde on 29 October 2012Published on 25 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CE05295KView Onlineshall designate this polymorph, Phase II. The crystallographic datacan be found in Table S1†.Fig. 3b shows the interactions of the molecules within a layer ofPhase II. As one can observe the dimer interactions remain and aresimilar in length to the ambient-pressure phase (O1/O2 2.621(1) A)(Table S2†). The major change in this phase is the orientation of theneighbouring groups. In Phase I, the ethylene groups are positionedclose to the carboxylic acid groups whereas in Phase II the methylgroups are observed to be close to the acid group (2.43 A H5/O2normalised distance). Alongwith this change there is a rotation of thegroups so that a corrugated plane of molecules is formed instead ofthe isolated dimers in Phase I with a reduction in mean plane sepa-ration to 3.19 A (Fig. 3b cf. Fig. 3a).The Raman spectra of the two phases are shown in Fig. 4 togetherwith their response to pressure. There is a clear difference between theRaman spectra of the two phases. One can observe that in the region400 cm1 there are two bands present although in Phase II there isa slight shoulder at higher wavenumber on the peak at 370 cm1.These bands are associated with the wagging vibration of the methyland ethylene tail group (C3C2C4) around the C1–C2 bond. Moreobvious difference lies in the region between 950 and 1010 cm1 whichdescribes the CH2 and CH3 rocking motion, respectively.12 In PhaseII the 950 cm1 vibration is split further so that a C–C stretchingfrequency of themethyl group is now observed as well as the waggingof the ethylene hydrogen atoms about the C2–C3 bond.12Compression of methacrylic acid also induces the polymerisationdespite the presence of themethyl group close to the ethylene group.13Fig. S1† shows the Raman spectrum for the polymer materialproduced from the direct compression of methacrylic acid down-loaded to ambient pressure. One can observe that the spectra are verysimilar to one another suggesting that the product from thecompression of either phase is the same. Acrylic acid also shows thatthe polymer produced from either polymorph is similar (Fig. S2†). Inthis case however there are peaks still present from some residualmonomer (1635 cm1).In conclusion we have shown that acrylic and methacrylic acidform novel crystalline phases at high pressure. These structures arecomposed of the same dimer motifs that are present in the crystalstructures determined at ambient pressure, however, the manner inwhich they are packed differs significantly. We have also shown thatdirect compression of either polymorph of acrylic or methacrylic acidcan yield a polymeric substance. In fact, theRaman spectra show thata similar polymer can be obtained from either Phase I or Phase II ofacrylic acid or methacrylic acid. The polymer that we have formedFig. 3 The different packing arrangements in methacrylic acid at (a) 123 K and (b) 1.5 GPa. Red atoms—oxygen; grey atoms—carbon; white atoms—hydrogen.4506 | CrystEngComm, 2011, 13, 4503–4507 This journal is ª The Royal Society of Chemistry 2011Downloaded by University of Strathclyde on 29 October 2012Published on 25 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CE05295KView Onlinefrom acrylic acid appears to be different to that isolated byMurli andSong in their paper.AcknowledgementsThe authors would like to thank Simon Parsons for the copies of theprograms SHADE and Eclipse as well as the loan of the DiamondAnvil Cell. We would also like to thank the EPSRC for funding (EP/E036244/1) (AJU).Notes and references1 M. Bradbury, S. Hamann and M. Linton, Aust. J. Chem., 1970, 23,511.2 D. Chelazzi, M. Ceppatelli, M. Santoro, R. Bini and V. Schettino, J.Phys. Chem. B, 2005, 109, 21658; M. Citroni, M. Ceppatelli, R. Biniand V. Schettino, J. Chem. Phys., 2005, 123, 9.3 M. Ceppatelli, M. Santoro, R. Bini and V. Schettino, J. Chem. Phys.,2000, 113, 5991; M. Citroni, M. Ceppatelli, R. Bini and V. Schettino,Science, 2002, 295, 2058; M. Citroni, M. Ceppatelli, R. Bini andV. Schettino, J. Chem. Phys., 2005, 123, 194510.4 C. Murli and Y. Song, J. Phys. Chem. B, 2009, 114(35), 9744.5 M. A. Higgs and R. L. Sass, Acta Crystallogr., 1963, 16, 657.6 R. Boese, D. Bl€aser, I. Steller, R. Latz and A. B€aumen, ActaCrystallogr., Sect. C: Cryst. Struct. Commun., 1999, 55, IUC9900006.7 L. Merrill and W. A. Bassett, Rev. Sci. Instrum., 1974, 45, 290.8 I. D. H. Oswald and W. A. Crichton, CrystEngComm, 2009, 11, 463.9 G. Weck, P. Loubeyre and R. LeToullec, Phys. Rev. Lett., 2002, 88,035504.10 A. Kulbida,M. N. Ramos, M. Rasanen, J. Nieminen, O. Schrems andR. Fausto, J. Chem. Soc., Faraday Trans., 1995, 91(11), 1571.11 J. E. Davies and A. D. Bond, Acta Crystallogr., Sect. E: Struct. Rep.Online, 2001, 57, o947.12 H. M. Badawi, M. A. A. Al-Khaldi, S. S. A. Al-Abbad andZ. H. A. AI-Sunaidi, Spectrochim. Acta, Part A, 2007, 68(3), 432.13 S. Beuermann, M. Buback, P. Hesse and I. Lacik, Macromolecules,2006, 39, 184–193.Fig. 4 The Raman spectra of Phase I (upper) and Phase II (lower) of methacrylic acid on increasing pressure.This journal is ª The Royal Society of Chemistry 2011 CrystEngComm, 2011, 13, 4503–4507 | 4507Downloaded by University of Strathclyde on 29 October 2012Published on 25 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CE05295KView Online

Oswald, Iain D. H.

Urquhart, Andrew J.

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Polymorphism and polymerisation of acrylic and methacrylic acid at high pressure

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