- Email: [email protected]
New unsaturated surfactants for the dispersion polymerisation of methyl methacrylate in supercritical carbon dioxide
European Polymer Journal 37 (2001) 1347±1351
www.elsevier.nl/locate/europolj
New unsaturated surfactants for the dispersion polymerisation of methyl methacrylate in supercritical carbon dioxide M.R. Giles *, S.M. Howdle School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK Received 1 September 2000; accepted 17 November 2000
Abstract This paper describes the synthesis of a siloxane based stabiliser by ring opening metathesis polymerisation of (bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane using Ru(PCy3 )Cl2 (CHPh) as the initiator. The resulting polymer was then used as a stabiliser for the free radical polymerisation of methyl methacrylate in supercritical carbon dioxide (scCO2 ). The poly(methyl methacrylate) (PMMA) formed using poly((bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane) has high molecular weight and a narrow polydispersity. Under optimised conditions of dispersion polymerisation in scCO2 , discrete spherical particles of PMMA are formed with an average diameter of 2.7 lm. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Methyl methacrylate; Supercritical; Carbon dioxide; Unsaturated stabiliser; Dispersion polymerisation
1. Introduction The use of supercritical carbon dioxide (scCO2 ) as a polymerisation medium has increased rapidly over the last ®ve years [1]. Carbon dioxide oers a number of advantages over conventional solvents as it is environmentally benign, relatively inexpensive, has tuneable physical properties (density and dielectric constant) and, on the release of pressure, returns to a gas leaving no solvent residues [2]. The polymerisation of acrylates is industrially important and there has been extensive research into the use of scCO2 as a solvent. Poly(methyl methacrylate) (PMMA) is insoluble in scCO2 and the addition of a stabiliser is required to form a stable dispersion. The ®rst reported polymerisation of methyl methacrylate (MMA) in scCO2 was by DeSimone et al., who utilised poly(1,1-dihydroper¯uorooctyl acrylate) (poly(FOA)) homopolymer as the steric stabiliser [3]. A
* Corresponding author. Tel.: +44-115-951-3486; fax: +44115-951-3058. E-mail address: [email protected] (M.R. Giles).
number of other ¯uorinated and siloxane polymers have also been identi®ed as eective in scCO2 . Poly(dimethylsiloxane) monoacrylate (PDMS) macromonomer has been employed in the polymerisation of MMA [4±6]. These macromonomers however have the disadvantage of being copolymerised into the resultant material. Lepilleur and Beckman [7], Yates et al. [8] and others [9] have used block or graft copolymers as eective stabilisers with one section being ``CO2 philic'' (usually ¯uorinated or siloxane based) and the other section being ``CO2 phobic''. The size of the grafted side chain and backbone have been shown to in¯uence the steric stabilising properties, with an appropriate balance being required to form a dispersion [7]. In this paper we describe a new approach to such stabilisers in which the backbone is unsaturated and the side chains contain a CO2 soluble functionality, leading to the CO2 ``philic'' and ``phobic'' sections required for an eective stabiliser. The only previous reference to unsaturated surface active moieties in scCO2 concerns the proposed use of acetylenic alcohols as surfactants [10]. Our stabiliser was prepared by ring opening metathesis polymerisation (ROMP) in conventional
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 2 5 8 - 5
1348
M.R. Giles, S.M. Howdle / European Polymer Journal 37 (2001) 1347±1351
solvents. ROMP is a versatile polymerisation technique leading to materials in high yield, with excellent control of molecular weight and polymer architecture [11] and, in this case, controlled synthesis of a polymer containing an unsaturated backbone. The recent development of well-de®ned ruthenium carbene catalysts by Grubbs and coworkers has extended the range of functional groups that can be tolerated [12,13]. The Grubbs catalyst [(Ru(Cl)2 (PCy)2 (CPhH)] also has the advantage of not being air and moisture sensitive and was therefore used for this study. The stabiliser reported here was produced by the ROMP of (bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane using Ru(Cl)2 (PCy)2 (CPhH). To the best of our knowledge, this is also the ®rst reported polymerisation of this particular monomer using a well-de®ned ruthenium carbene catalyst. However, in this paper we shall concentrate only on its potential use as a stabiliser. The polymer formed comprises a ``CO2 phobic'' unsaturated backbone and a ``CO2 philic'' grafted siloxane functionality. The material was therefore tested as a stabiliser for the free radical polymerisation of MMA in scCO2 . 2. Experimental (Bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane (Fluka), Ru(P(Cy)3 )2 Cl2 (CHPh) (Strem) and MMA (ICI inhibited with 2 ppm of Topanol (a mixture of hindered amines)) were used as received unless stated. The stabiliser was formed by the synthetic route outlined in Fig. 1. In a typical ROMP, 1.5 g (bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane and 5 mg Ru(P(Cy)3 )2 Cl2 (CHPh) were each dissolved in 5 ml aliquots of degassed THF. The two solutions were mixed rapidly to give a light purple solution, which
turned a straw yellow colour within 10 s. Within 2 min the reaction thickened to form a gel. After 30 min the solvent was removed under reduced pressure to give a brown material, the colouration being caused by catalyst residues. The polymerisation of (bicyclo[2.2.1]hept-5-en2-yl)triethoxysilane using Ru(P(Cy)3 )2 Cl2 (CHPh) produced a gelled, presumably cross-linked, material which exhibited a high degree of swelling in chloroform and in THF, expanding up to ®ve times in volume. A small portion of this material proved to be soluble in chloroform and gel permeation chromatography of the soluble fraction revealed the soluble material to have a molecular weight of Mn 144,800 and a polydispersity of 1.73. The predicted molecular weight as calculated from the initial concentration of initiator and monomer was Mn 118,000. Free radical polymerisations of MMA (Fig. 2) were performed in a 60 ml stainless steel autoclave (NWA GmbH) using high purity carbon dioxide (BOC Gases, SFC grade) which was passed through a drying column. In a typical polymerisation, the autoclave was loaded with reactants and sealed. The cell was then pressurised (3000 psi) with high grade N2 , to leak test the equipment and to degas the reactants. The cell was ®lled with CO2 , stirred and heated up to the reaction temperature (65°C) using a preheated block. The correct pressure was obtained by adding additional CO2 . Molecular weight data were obtained by gel permeation chromatography with chloroform as the solvent (Aldrich) at 30°C using Polymer Laboratories Plgel 5 lm Mixed-D columns and refractive index detector. Calibration was accomplished with PMMA narrow standards (Polymer Laboratories). Both the sample analysis and the calibration were conducted at a ¯ow rate of 1 ml min 1 . Scanning electron microscopy (SEM) data were collected using a JEOL 6400 SEM. Samples were mounted on an aluminium stub using an adhesive
Fig. 1. ROMP of (bicyclo([2.2.1]hept-5-en-2-yl) triethoxysilane by Ru(P(Cy)3 )2 Cl2 (CHPh).
Fig. 2. Free radical polymerisation of MMA.
M.R. Giles, S.M. Howdle / European Polymer Journal 37 (2001) 1347±1351
1349
carbon tab and were gold coated. 1 H-NMR and 13 C NMR data were collected using a Bruker 300 MHz spectrometer. FTIR data was collected on a Perkin Elmer System 2000 spectrometer.
3. Result and discussion The free radical polymerisation of MMA using AIBN was ®rst performed in the absence of any added stabiliser and produced PMMA in 52% yield with a broad molecular weight distribution (entry 1, Table 1). A polymerisation under identical conditions was also carried out in the presence of (bicyclo[2.2.1]hept-5-en2-yl)triethoxysilane in order to con®rm that this monomer species was inactive as a stabiliser and did not in any way in¯uence the polymerisation of MMA. The resulting PMMA also showed a broad molecular weight and was produced in low yield (entry 2, Table 1). The morphology of these materials was foamed, indicating no stabilisation. In order to test out the ROMP prepared stabiliser [poly((bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane)] the synthesised material, was then tested as a stabiliser for the free radical polymerisation of MMA. Initially, the polymerisation was performed in the presence of 2 wt.% poly((bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane). The reaction conditions are given in entry 3, Table 1. The PMMA produced was a ¯uy white solid collected in a 79% yield. The SEM images (Fig. 3) show that although particles appear to have formed initially, they have
Fig. 3. SEM image showing the material formed by 2% stabiliser 1.
subsequently aggregated to formed strings and other structural motifs. This type of behaviour has been seen with other stabiliser systems and has been attributed to there being insucient stabiliser present in the polymerisation [14,15]. The polymerisation was repeated under identical conditions, with the addition of 4 wt.% of stabiliser compared to monomer. Under these conditions the polymerisation is almost quantitative and high molecular weight material is produced (entry 4, Table 1). The molecular weight of the polymer has increased to Mn 82,000 and the molecular weight distribution remains constant. An SEM image of the resultant material (Fig. 4) shows discrete spherical particles with a
Table 1 Polymerisation of MMA Entry
% AIBNa
% Surfactanta
Average temperature (°C)
Average pressure (psi)
Yield (%)
Mn b
Mw b
PDIb
Particle sizec
Appearanced
1
1
±
66
3100
52
35 800
124 200
3.5
na
2e
1
±
66
2900
45
40 000
123 600
3.1
na
Oil/white solid Oil/white solid
3
1
2
67
3150
79
69600
182100
2.6
5.2
4
1
4
68
3200
97
81 700
219 800
2.7
2.7
5f
1
4
66
3100
91
37 000
146 100
3.9
na
a
Percentage weight/weight relative to monomer. Determined by gel permeation chromatography. c As determined by SEM. d Appearance of the material directly after venting. e On the addition of 4 wt.% monomer of (bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane. f On the addition of 4% with respect to monomer of extracted poly((bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane). b
Fluy white solid Fluy white solid Fluy white solid
1350
M.R. Giles, S.M. Howdle / European Polymer Journal 37 (2001) 1347±1351
Fig. 4. SEM image showing particles formed by 4% stabiliser 2.
diameter of 2.7 lm. The particle size distribution is also narrow as shown in Fig. 5, where data has been collected for 100 particles. A decrease in particle size has also been observed with increasing stabiliser concentration, an observation which is consistent with other stabiliser systems in scCO2 and also aqueous and non-aqueous media [4]. The ROMP of metathesis of (bicyclo[2.2.1]hept-5-en2-yl)triethoxysilane using WCl6 and Bui2 AlCl3 has been previously reported by Makovetsky et al. [16]. The material they produced was highly cross-linked as indicated by its low solubility in conventional solvents. Our experiments similarly indicate that there is substantial cross-linking in the ROMP prepared polymeric material. In fact, only a small proportion of the material was soluble in scCO2 . When the gel was exposed to scCO2 , in a microscale infrared spectroscopic cell [17] new features
appeared in the FTIR spectrum which clearly demonstrated the presence of a soluble species, but the majority of the material remained insoluble. The soluble species is most likely the soluble fraction observed in the GPC measurements described in the experimental section. Exhaustive extraction of the gel with chloroform for 24 h and then with scCO2 at 270 bar for 4 h revealed that 13% of the material could be extracted. Despite several attempts, we were unable to isolate this material directly due to small quantities involved. However, we were able to test the stabiliser activity of the remainder of the exhaustively extracted gel by running a polymerisation of MMA (entry 5, Table 1). These results clearly show that the stabilisation is substantially reduced; the molecular weights and polydispersity are closer to those obtained without any stabiliser. The SEM of the PMMA obtained shows that aggregated particles were formed, con®rming that the extracted material is much less eective as a stabiliser. We believe that it is only the soluble fraction of the material which shows activity as a stabiliser in scCO2: Clearly, the soluble material is very eective at even very low concentrations; only 13 wt.% of the 4 wt.% added (entry 4, Table 1) is the soluble portion. No amount of exhaustive extraction can fully remove all of the soluble material from the gel, hence the high residual yield obtained (entry 5, Table 1).
4. Conclusions The soluble portion of poly((bicyclo[2.2.1]hept-5-en2-yl)triethoxysilane) has been demonstrated to act as an eective stabiliser at levels approaching only 0.4 wt.% with respect to monomer in the free radical polymeri-
Fig. 5. Distribution of particle sizes for 100 particles.
M.R. Giles, S.M. Howdle / European Polymer Journal 37 (2001) 1347±1351
sation of MMA. The stabiliser contains a ``CO2 philic'' pendant siloxane group and a ``CO2 phobic'' unsaturated backbone. This we believe is the only example of such a stabiliser architecture and indicates another area for the development of new stabilisers. Work is currently under way to devise methods for extraction and collection of the soluble fraction for further detailed analysis. We will also investigate the eect of varying the molecular weight of the stabiliser and of changing reaction pressure/temperature and hence density on the PMMA obtained from the dispersion polymerisation.
Acknowledgements We gratefully acknowledge the EPSRC for support (MRG GR/K76023). We also acknowledge the Royal Society for a University Research Fellowship (SMH). We thank also Dr. K.H. Pickel (NWA GmbH) and Mr. K. Stanley for technical assistance, Miss R.M.T. Grif®ths, Prof. M. Poliako and Dr. M.W. George for their help and advice.
References [1] Kendall JL, Canelas DA, Young JL, DeSimone JM. Chem Rev 1999;99:543±63. [2] McHugh MA, Krukonis VJ. Supercritical ¯uid extraction: principles and practice. Boston: Butterworth; 1994.
1351
[3] Desimone JM, Guan Z, Elsbernd CS. Science 1992;257: 945±7. [4] Shaer KA, Jones TA, Canelas DA, DeSimone JM, Wilkinson SP. Macromolecules 1996;29:2704±6. [5] OÕNeill ML, Yates MZ, Johnston KP, Smith CD, Wilkinson SP. Macromolecules 1998;31:2838±47. [6] OÕNeill ML, Yates MZ, Johnston KP, Smith CD, Wilkinson SP. Macromolecules 1998;31:2848±56. [7] Lepilleur C, Beckman EJ. Macromolecules 1997;30:745± 56. [8] Yates MZ, Li G, Shim JJ, Maniar S, Johnston KP, Lim KT, Webber S. Macromolecules 1999;32:1018±26. [9] Yong TM, Hems WP, Van Nunen JLM, Holmes AB, Steinke JHG, Taylor PL, Segal JA, Grin DA. Chem Commun 1997:1811±12. [10] Wilkinson SP, Robeson LM. Surfactants for use in liquid/ supercritical CO2 . 1998. [11] Ivin KJ. Ole®n metathesis and metathesis polymerization. San Diego: Academic Press; 1997. [12] Schwab P, France MB, Ziller JW, Grubbs RH. Angew Chem-Int Ed 1995;34:2039±41. [13] Schwab P, Grubbs RH, Ziller JW. J Am Chem Soc 1996;118:100±10. [14] Christian P, Howdle SM. Macromolecules 2000;33:237± 9. [15] Giles MR, Hay JN, Howdle S, OÕConnor SJ, Winder RJ. Macromolecules 1999;32:1996±9. [16] Makovetsky KL, Finkelshtein ES, Ostrovskaya IY, Portnykh EB, Gorbacheva LI, Goldberg AL, Ushakov NV, Yampolsky YP. J Mol Catal 1992;76:107±21. [17] Howdle SM, Poliako M. In: Kiran E, editor. NATO advanced study institute supercritical ¯uids-fundamentals for application, vol. 273. Dordrecht: Kluwer; 1994. p. 527± 37.
www.elsevier.nl/locate/europolj
New unsaturated surfactants for the dispersion polymerisation of methyl methacrylate in supercritical carbon dioxide M.R. Giles *, S.M. Howdle School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK Received 1 September 2000; accepted 17 November 2000
Abstract This paper describes the synthesis of a siloxane based stabiliser by ring opening metathesis polymerisation of (bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane using Ru(PCy3 )Cl2 (CHPh) as the initiator. The resulting polymer was then used as a stabiliser for the free radical polymerisation of methyl methacrylate in supercritical carbon dioxide (scCO2 ). The poly(methyl methacrylate) (PMMA) formed using poly((bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane) has high molecular weight and a narrow polydispersity. Under optimised conditions of dispersion polymerisation in scCO2 , discrete spherical particles of PMMA are formed with an average diameter of 2.7 lm. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Methyl methacrylate; Supercritical; Carbon dioxide; Unsaturated stabiliser; Dispersion polymerisation
1. Introduction The use of supercritical carbon dioxide (scCO2 ) as a polymerisation medium has increased rapidly over the last ®ve years [1]. Carbon dioxide oers a number of advantages over conventional solvents as it is environmentally benign, relatively inexpensive, has tuneable physical properties (density and dielectric constant) and, on the release of pressure, returns to a gas leaving no solvent residues [2]. The polymerisation of acrylates is industrially important and there has been extensive research into the use of scCO2 as a solvent. Poly(methyl methacrylate) (PMMA) is insoluble in scCO2 and the addition of a stabiliser is required to form a stable dispersion. The ®rst reported polymerisation of methyl methacrylate (MMA) in scCO2 was by DeSimone et al., who utilised poly(1,1-dihydroper¯uorooctyl acrylate) (poly(FOA)) homopolymer as the steric stabiliser [3]. A
* Corresponding author. Tel.: +44-115-951-3486; fax: +44115-951-3058. E-mail address: [email protected] (M.R. Giles).
number of other ¯uorinated and siloxane polymers have also been identi®ed as eective in scCO2 . Poly(dimethylsiloxane) monoacrylate (PDMS) macromonomer has been employed in the polymerisation of MMA [4±6]. These macromonomers however have the disadvantage of being copolymerised into the resultant material. Lepilleur and Beckman [7], Yates et al. [8] and others [9] have used block or graft copolymers as eective stabilisers with one section being ``CO2 philic'' (usually ¯uorinated or siloxane based) and the other section being ``CO2 phobic''. The size of the grafted side chain and backbone have been shown to in¯uence the steric stabilising properties, with an appropriate balance being required to form a dispersion [7]. In this paper we describe a new approach to such stabilisers in which the backbone is unsaturated and the side chains contain a CO2 soluble functionality, leading to the CO2 ``philic'' and ``phobic'' sections required for an eective stabiliser. The only previous reference to unsaturated surface active moieties in scCO2 concerns the proposed use of acetylenic alcohols as surfactants [10]. Our stabiliser was prepared by ring opening metathesis polymerisation (ROMP) in conventional
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 2 5 8 - 5
1348
M.R. Giles, S.M. Howdle / European Polymer Journal 37 (2001) 1347±1351
solvents. ROMP is a versatile polymerisation technique leading to materials in high yield, with excellent control of molecular weight and polymer architecture [11] and, in this case, controlled synthesis of a polymer containing an unsaturated backbone. The recent development of well-de®ned ruthenium carbene catalysts by Grubbs and coworkers has extended the range of functional groups that can be tolerated [12,13]. The Grubbs catalyst [(Ru(Cl)2 (PCy)2 (CPhH)] also has the advantage of not being air and moisture sensitive and was therefore used for this study. The stabiliser reported here was produced by the ROMP of (bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane using Ru(Cl)2 (PCy)2 (CPhH). To the best of our knowledge, this is also the ®rst reported polymerisation of this particular monomer using a well-de®ned ruthenium carbene catalyst. However, in this paper we shall concentrate only on its potential use as a stabiliser. The polymer formed comprises a ``CO2 phobic'' unsaturated backbone and a ``CO2 philic'' grafted siloxane functionality. The material was therefore tested as a stabiliser for the free radical polymerisation of MMA in scCO2 . 2. Experimental (Bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane (Fluka), Ru(P(Cy)3 )2 Cl2 (CHPh) (Strem) and MMA (ICI inhibited with 2 ppm of Topanol (a mixture of hindered amines)) were used as received unless stated. The stabiliser was formed by the synthetic route outlined in Fig. 1. In a typical ROMP, 1.5 g (bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane and 5 mg Ru(P(Cy)3 )2 Cl2 (CHPh) were each dissolved in 5 ml aliquots of degassed THF. The two solutions were mixed rapidly to give a light purple solution, which
turned a straw yellow colour within 10 s. Within 2 min the reaction thickened to form a gel. After 30 min the solvent was removed under reduced pressure to give a brown material, the colouration being caused by catalyst residues. The polymerisation of (bicyclo[2.2.1]hept-5-en2-yl)triethoxysilane using Ru(P(Cy)3 )2 Cl2 (CHPh) produced a gelled, presumably cross-linked, material which exhibited a high degree of swelling in chloroform and in THF, expanding up to ®ve times in volume. A small portion of this material proved to be soluble in chloroform and gel permeation chromatography of the soluble fraction revealed the soluble material to have a molecular weight of Mn 144,800 and a polydispersity of 1.73. The predicted molecular weight as calculated from the initial concentration of initiator and monomer was Mn 118,000. Free radical polymerisations of MMA (Fig. 2) were performed in a 60 ml stainless steel autoclave (NWA GmbH) using high purity carbon dioxide (BOC Gases, SFC grade) which was passed through a drying column. In a typical polymerisation, the autoclave was loaded with reactants and sealed. The cell was then pressurised (3000 psi) with high grade N2 , to leak test the equipment and to degas the reactants. The cell was ®lled with CO2 , stirred and heated up to the reaction temperature (65°C) using a preheated block. The correct pressure was obtained by adding additional CO2 . Molecular weight data were obtained by gel permeation chromatography with chloroform as the solvent (Aldrich) at 30°C using Polymer Laboratories Plgel 5 lm Mixed-D columns and refractive index detector. Calibration was accomplished with PMMA narrow standards (Polymer Laboratories). Both the sample analysis and the calibration were conducted at a ¯ow rate of 1 ml min 1 . Scanning electron microscopy (SEM) data were collected using a JEOL 6400 SEM. Samples were mounted on an aluminium stub using an adhesive
Fig. 1. ROMP of (bicyclo([2.2.1]hept-5-en-2-yl) triethoxysilane by Ru(P(Cy)3 )2 Cl2 (CHPh).
Fig. 2. Free radical polymerisation of MMA.
M.R. Giles, S.M. Howdle / European Polymer Journal 37 (2001) 1347±1351
1349
carbon tab and were gold coated. 1 H-NMR and 13 C NMR data were collected using a Bruker 300 MHz spectrometer. FTIR data was collected on a Perkin Elmer System 2000 spectrometer.
3. Result and discussion The free radical polymerisation of MMA using AIBN was ®rst performed in the absence of any added stabiliser and produced PMMA in 52% yield with a broad molecular weight distribution (entry 1, Table 1). A polymerisation under identical conditions was also carried out in the presence of (bicyclo[2.2.1]hept-5-en2-yl)triethoxysilane in order to con®rm that this monomer species was inactive as a stabiliser and did not in any way in¯uence the polymerisation of MMA. The resulting PMMA also showed a broad molecular weight and was produced in low yield (entry 2, Table 1). The morphology of these materials was foamed, indicating no stabilisation. In order to test out the ROMP prepared stabiliser [poly((bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane)] the synthesised material, was then tested as a stabiliser for the free radical polymerisation of MMA. Initially, the polymerisation was performed in the presence of 2 wt.% poly((bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane). The reaction conditions are given in entry 3, Table 1. The PMMA produced was a ¯uy white solid collected in a 79% yield. The SEM images (Fig. 3) show that although particles appear to have formed initially, they have
Fig. 3. SEM image showing the material formed by 2% stabiliser 1.
subsequently aggregated to formed strings and other structural motifs. This type of behaviour has been seen with other stabiliser systems and has been attributed to there being insucient stabiliser present in the polymerisation [14,15]. The polymerisation was repeated under identical conditions, with the addition of 4 wt.% of stabiliser compared to monomer. Under these conditions the polymerisation is almost quantitative and high molecular weight material is produced (entry 4, Table 1). The molecular weight of the polymer has increased to Mn 82,000 and the molecular weight distribution remains constant. An SEM image of the resultant material (Fig. 4) shows discrete spherical particles with a
Table 1 Polymerisation of MMA Entry
% AIBNa
% Surfactanta
Average temperature (°C)
Average pressure (psi)
Yield (%)
Mn b
Mw b
PDIb
Particle sizec
Appearanced
1
1
±
66
3100
52
35 800
124 200
3.5
na
2e
1
±
66
2900
45
40 000
123 600
3.1
na
Oil/white solid Oil/white solid
3
1
2
67
3150
79
69600
182100
2.6
5.2
4
1
4
68
3200
97
81 700
219 800
2.7
2.7
5f
1
4
66
3100
91
37 000
146 100
3.9
na
a
Percentage weight/weight relative to monomer. Determined by gel permeation chromatography. c As determined by SEM. d Appearance of the material directly after venting. e On the addition of 4 wt.% monomer of (bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane. f On the addition of 4% with respect to monomer of extracted poly((bicyclo[2.2.1]hept-5-en-2-yl)triethoxysilane). b
Fluy white solid Fluy white solid Fluy white solid
1350
M.R. Giles, S.M. Howdle / European Polymer Journal 37 (2001) 1347±1351
Fig. 4. SEM image showing particles formed by 4% stabiliser 2.
diameter of 2.7 lm. The particle size distribution is also narrow as shown in Fig. 5, where data has been collected for 100 particles. A decrease in particle size has also been observed with increasing stabiliser concentration, an observation which is consistent with other stabiliser systems in scCO2 and also aqueous and non-aqueous media [4]. The ROMP of metathesis of (bicyclo[2.2.1]hept-5-en2-yl)triethoxysilane using WCl6 and Bui2 AlCl3 has been previously reported by Makovetsky et al. [16]. The material they produced was highly cross-linked as indicated by its low solubility in conventional solvents. Our experiments similarly indicate that there is substantial cross-linking in the ROMP prepared polymeric material. In fact, only a small proportion of the material was soluble in scCO2 . When the gel was exposed to scCO2 , in a microscale infrared spectroscopic cell [17] new features
appeared in the FTIR spectrum which clearly demonstrated the presence of a soluble species, but the majority of the material remained insoluble. The soluble species is most likely the soluble fraction observed in the GPC measurements described in the experimental section. Exhaustive extraction of the gel with chloroform for 24 h and then with scCO2 at 270 bar for 4 h revealed that 13% of the material could be extracted. Despite several attempts, we were unable to isolate this material directly due to small quantities involved. However, we were able to test the stabiliser activity of the remainder of the exhaustively extracted gel by running a polymerisation of MMA (entry 5, Table 1). These results clearly show that the stabilisation is substantially reduced; the molecular weights and polydispersity are closer to those obtained without any stabiliser. The SEM of the PMMA obtained shows that aggregated particles were formed, con®rming that the extracted material is much less eective as a stabiliser. We believe that it is only the soluble fraction of the material which shows activity as a stabiliser in scCO2: Clearly, the soluble material is very eective at even very low concentrations; only 13 wt.% of the 4 wt.% added (entry 4, Table 1) is the soluble portion. No amount of exhaustive extraction can fully remove all of the soluble material from the gel, hence the high residual yield obtained (entry 5, Table 1).
4. Conclusions The soluble portion of poly((bicyclo[2.2.1]hept-5-en2-yl)triethoxysilane) has been demonstrated to act as an eective stabiliser at levels approaching only 0.4 wt.% with respect to monomer in the free radical polymeri-
Fig. 5. Distribution of particle sizes for 100 particles.
M.R. Giles, S.M. Howdle / European Polymer Journal 37 (2001) 1347±1351
sation of MMA. The stabiliser contains a ``CO2 philic'' pendant siloxane group and a ``CO2 phobic'' unsaturated backbone. This we believe is the only example of such a stabiliser architecture and indicates another area for the development of new stabilisers. Work is currently under way to devise methods for extraction and collection of the soluble fraction for further detailed analysis. We will also investigate the eect of varying the molecular weight of the stabiliser and of changing reaction pressure/temperature and hence density on the PMMA obtained from the dispersion polymerisation.
Acknowledgements We gratefully acknowledge the EPSRC for support (MRG GR/K76023). We also acknowledge the Royal Society for a University Research Fellowship (SMH). We thank also Dr. K.H. Pickel (NWA GmbH) and Mr. K. Stanley for technical assistance, Miss R.M.T. Grif®ths, Prof. M. Poliako and Dr. M.W. George for their help and advice.
References [1] Kendall JL, Canelas DA, Young JL, DeSimone JM. Chem Rev 1999;99:543±63. [2] McHugh MA, Krukonis VJ. Supercritical ¯uid extraction: principles and practice. Boston: Butterworth; 1994.
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