1 Acta Chim. Slov. 1999, 46(1), pp. 1-13 SYNTHESIS OF BLOCK COPOLYMERS WITH TETRAPHENYL BIPHOSPHINE INIFERTER Ida Poljanšek*, Vid Margon & >Anton Šebenik Faculty of Chemistry and Chemical Technology, University of Ljubljana, Slovenia ABSTRACT In this study PS/PMMA block copolymers were synthesized from polystyrene – (PS-) with active aromatic phosphorous end groups as macroinitiator using UV light as the energy source. The yield of block copolymerization of styrene and methyl methacrylate (MMA) with PS-macroinitiator in bulk was determined in relation to the wavelength of UV light and on the ratio of initiator and monomer. The molecular weights of block copolymers increased with the increase of the reaction time. It was found out that block copolymerization of styrene and methyl methacrylate with PS-macroinitiator gives higher yields at the wavelength of 302 nm than at 366 nm. The macroinitiators decomposed into phosphorous radical and macroradical. The macromolecules were terminated by primary Ph2P· radicals and by the combination of two equal or different macroradicals as observed by NMR spectroscopy. The quantities of homopolymers and block copolymer in our products were determined by solid-fluid extraction. INTRODUCTION Modern materials research is oriented towards living polymer systems which allow for more control over the mechanism of polymerization. This mechanism also gives narrow molecular weight distributions of polymer products which has a direct influence on the mechanical properties of materials. The mechanism of living polymerization was first described by Szwarc [1] for anionic polymerization and afterwards for cationic polymerization. There has been a lot of research focussing on radical living polymerization which allows for a wide selection of monomers, solvents and higher _____________________________________________________________________________ > Dedicated to the memory of Prof.Dr. Anton Šebenik 2 temperatures [2]. Otsu [3] first described living radical mechanism, suggesting the important role of the iniferter in the polymerization process (the term iniferter is a combination of initiator, transfer agent and terminator). Iniferters containing bonds with low decomposition energy, such as S-S or P-P bonds, can be used. Pseudoliving radical polymerization was defined by Harwood [4] as a reaction between monomers and initiators in which polymers with active end groups are formed. These polymers can further be used as macroinitiators for the second step of polymerization with the same, or other monomers, to form block copolymers. Our work focused on the controled synthesis of polystyrene macroinitiators (PSMI) in bulk using tetraphenyl biphosphine (Ph4P2) iniferter. PSMI were further used in block copolymerization with methyl methacrylate to form PMMA-PS-PMMA block copolymers. The active end group bonding and an increase in molecular weight and polydispersity were determined. EXPERIMENTAL Materials: Tetraphenyl biphosphine (Ph4P2) (product of Aldrich) was used as the initiator. Substances methyl methacrylate (MMA) and styrene (S) were products of Rohm and Haas. Monomeres were washed with a 10% solution of sodium carbonate and distilled water, dried over night with nonaqueous sodium sulphate, and distilled twice under reduced pressure. Monomers were used immediately after the purification. All solvents were dried, purified and distilled twice in dry argon atmosphere to remove all oxygen. Polymerization: Polymerization of substances was carried out in two steps in a dry box with argon atmosphere with the content of oxygen below 1 ppm. In the first step, the PS-macroinitiators were synthesized. The monomer was mixed in a quartz reaction vessel with the selected quantity of a Ph4P2 initiator (Table 1). The initiator 3 concentration was between 0.035 and 1.050 mol/dm (molar ratio monomer to initiator 1:0.0001 to 1:0.003). The energy sources for polymerizations were UV lamps of 302 2 and 366 nm wavelength with the intensity of 4.5 mW/cm at 2.5 cm distance. The o reaction temperature was 25 C and the polymerization time between 1 and 4 h. At individual reaction time intervals, the samples for determining the molecular weight and 3 yield of the polymerization were withdrawn from the reaction vessel. After the polymerization, the PS macroinitiators were purified by precipitating a THF solution by methanol (ratio 1:10) at room temperature in the dry box. Precipitation was repeated three times to remove the unreacted initiator and monomer. Following this, the products were dried in argon atmpsphere. To evaluate the degree of photopolymerization, two control experiments without Ph4P2 under the same reaction conditions were carried out. The control experiments for thermal polymerization with Ph4P2 and without UV irradiation were carried out as well. Block copolymerization: In the second step of polymerization, some selected purified polymers were used as macroinitiators for the synthesis of block copolymers. The macroinitiator was dissolved in MMA as a 1 wt % to 10 wt% solution, and then irradiated. The reaction conditions were the same as for the synthesis of macroinitiators. After the polymerization was quenched, the block copolymers were precipitated and . dried. [2,5] Measurements: The structure, configuration of chain end groups, purity and the presence of unreacted initiator were followed by NMR spectroscopy of the polymers. 1 13 31 The H, C and P spectra were measured with Bruker AVANCE DPX-300 MHz NMR spectrometer in one and two dimensional techniques with homo and hetero correlation. The samples were dissolved in CDCl , and NMR tubes fused in a dry box 3 with argon atmosphere to prevent oxidation of phosphine end groups. All signals were quoted on TMS as internal standard and H PO was used as an external standard. The 34 molecular weights were measured by GPC using PL-gel columns with pore sizes of 50 nm, 100 nm and 1000 nm and THF as the eluent (1ml/min). PS standards were used for column calibration for PSMI samples; similarly, PMMA standards were used for block copolymers (such as for PS/PMMA where the shorter PS block is surrounded by longer PMMA blocks at both sides). RESULTS AND DISCUSSION Yield of polymerization: As shown in Table 1 the yield of polymerization is higher at 366 nm wavelength than at 302 nm in polymerization of styrene with Ph4P2. On the the other hand, block copolymerization of the polystyrene macroinitiator and methyl 4 methacrylate gives much higher yields at 302 nm wavelength than at 366 nm in spite of shorter polymerization time (Table 2). The UV spectra of the initiator and macroinitiators show that these absorb light at higher wavelengths than styrene. At the wavelength of 302 nm part of the energy is absorbed by styrene which leads to the formation of a reduced number of primary radicals [6] and thus results in a smaller conversion. Table 1: Bulk polymerization conditions of styrene with Ph4P2 Experiment Quantity of Ph4P2/g Ŕ/nm Polymerization time/h Quantity of O2/ppm Yield of polymerization/% 1 0.037 366 4 0.4 11.1 2 0.071 366 4 0.3 9.9 3 0.109 366 4 0.3 9.1 4 0.204 366 4 0.4 7.1 5 0.218 366 4 0.6 6.9 6 1.095 366 4 1 6.8 7 0.364 366 4 0.6 11.8 8 0.110 366 7 1 14.0 9 0.216 366 7 1 15.9 10 0.353 366 7 0.3 7.9 11 0.358 302 4 1.5 5.5 12 0.357 302 4 0.1 6.3 13 0.374 302 4 1 4.9 14 0.364 302 4 0.9 4.2 Considering the literature data [7] and our data obtained by calculation using the molecular orbital theory [8], we found the C-P bond dissociation energy to be up to 152 kJ/mol with bond lengths above 0.175 nm. The P-P bonds are longer (0.23 nm) and have smaller dissociation energy compared to C-P bonds. In the synthesis of block-copolymers the wavelength 366 nm has insuficient energy for the dissociation of Calif.-P bonds. Therefore the wavelength of 302 nm is more appropriate. This explains why the 5 yield of the synthesis of PSMI is higher at wavelength 366 nm while higher yields of block copolymerization of PSMI and MMA occur at 302 nm. Table 2: Bulk polymerization conditions of methyl methacrylate with PSMI Experime nt Quantity of PSMI/g Ŕ/nm Polymerization time/h Quantity of O2/ppm Yield of polymerization /% 1 0.697 366 4 0.6 12.6 2 0.204 366 4 0.8 7.07 3 0.341 366 4 2.5 12.5 4 1.055 366 4 2 17.6 5 0.648 366 4 2 20.9 6 1.050 366 4 2 22.8 7 0.111 366 4 1 10.7 8 0.450 366 4 1.2 12.2 9 0.108 366 4 0.5 7.6 10 0.200 366 4 1.2 13.7 11 0.263 302 2 0.1 53.2 12 0.226 302 2,5 1.6 100 13 0.221 302 2 1.4 64.9 14 0.233 302 2 1.6 42.3 Quantities of homopolymers and block copolymers in products: One evidence of the proofs of living polymerization is the ability of the macroinitiator to initiate the second step of polymerization. For this reason we used the PS macromolecule with two active end groups as a macroinitiator synthesized during the first step of polymerization for the synthesis of block copolymers. In this case, UV irradiation decomposed the C -P alif. bond into a macroradical and a primary radical. The reactivity of macroinitiator was confirmed by an increase in molecular weight, from 9800 g/mol (the first step) to 250000 g/mol (the second step) at wavelength 366 nm and from 5000 g/mol to 155000 g/mol at 302 nm (Table 3). 6 Table 3: Number-average molecular weights for PSMI (Mn(1)) and block copolymers (Mn(2)), the results of solid-fluid extraction and quantities of PS/PMMA block copolymers and homopolymers after the second step Exp. No. Mn (1) Mn (2) Mw/Mn PS/PMMA Ŕ/nm Reaction time /h PS /% PMMA and PS/PMMA /% 9 7000 140000 1.75 366 4 11.39 83.92 7 9800 250000 1.71 366 4 11.45 88.29 14 4500 138000 1.54 302 2 0.02 96.33 4 5000 155000 1.51 302 2 0.31 96.88 To determine whether a homopolymer was formed during the second step of polymerization, the block copolymers were successively extracted with cyclohexane (for PS) and with acetonitrile (for PMMA and for PS/PMMA block copolymers). All extractions were carried out in Soxhlet [9] aparatus for 10 hours. The results of the extraction for samples of block copolymers PS/PMMA, prepared with different quantities of the initiators in the first step and different quantities of PS-macoinitiators in the second step, are given in Table 3. It can be seen that the primary radicals formed from the macoinitiators terminated the block copolymers chain growth and also initiated homopolymerization of MMA. From the extraction data (Table 3), the share of unreacted PS-macroinitiators was found to be between 0.3% at wavelength 302 nm and 11% at wavelength 366nm. We believe that 96% of the product at wavelength 302 nm and 88% of the product at wavelength 366 nm is a mixure of PMMA homopolymer and PS/PMMA block copolymer, since PS/PMMA is also soluble in acetonitrile, because of a very small short PS block in comparison with the PMMA block. Chain end-groups: The determination of the polymer chain end-groups is a useful way of studing polymerization mechanism. The most appropriate method is high resolution 1H and 31P NMR spectroscopy. NMR spectra interpretation [10,11,12] is given for the sample PSMI No. 14, which was further used for block copolymerization with MMA. In 31P NMR spectra of PSMI (Figure 1) peaks close to -20 ppm represent polystyrene with diphenyl phosphine (Ph2P) end groups in head and tail forms. The two dimensional NMR spectrum (Figure 2) shows bonding of polystyrene with diphenyl phosphine end group in the tail form at 7,2 ppm and in the head form at 7,0 ppm on the proton scale. 7 Figure 1: 31P NMR spectra of PSMI a) from 50 to -50 ppm and b) from 18,5 to -22 ppm y \ rtf ^ ű . <Ç. ^ m % §^ ~i------------' r n--------' r 31 P ppm 7.8 7.6 7.4 7.2 7.0 6.8 . 6.6 1H Figure 2: 2D HMBC NMR spectrum of polystyrene macroinitiator PSMI 8 Figure 3: 31P NMR spectra of block copolymer a) from 80 to -80 ppm and b) from -18 to -25 ppm 31 P 1H Figure 4: 2D HMBC NMR spectrum of block copolymer PS-PMMA 9 The peak at -14 ppm on phosphorus scale represents Ph4P2 which is an impurity in this case. 31 In 31P spectrum of block copolymer PS/MMA (Figure 3) peaks close to -20 ppm belong to the polystyrene with (Ph2P) end-groups in head and tail forms. Peaks at –23.6 ppm belong to the end group of polymethyl methacrylate in the tail form and peaks at –21.2 ppm belong to the end group in the head form of polymethyl methacrylate. The two dimensional 31P-1H NMR spectrum (Figure 4) of the block copolymer has cross-peaks at 7.2 and 7.0 ppm on the proton scale representing tail and head bonding of end-groups to polystyrene. Peaks between 7.2 and 7.4 ppm on the proton scale represent tail and head bonding of end groups to polymethyl methacrylate. Reaction mechanism: Considering the results of all measurements, the following polymerization mechanisms were predicted [10]: Preparation of PSMI (Scheme 1); Initator Ph4P2 (A) dissociates under the influence of UV light into two radicals Ph2P·, (B) and react with styrene to form styrene free-radicals (C), which start the propagation -- Ph P . Ph P -- — Ph 2P' — Ph P — — — . Ph P hV Ph P CH C Ph P Ph P Ph P CH C P P h + Ph P Scheme 1: Mechanism of the synthesis of polystyrene macroinitiator (PSMI) 10 — . Ph P — . — . Ph P CH . C CH CH2 C . Scheme 2: Mechanism of the copolymerization of polystyrene macroinitiator (PSMI) and methyl methacrylate (MMA) CH C -• • I CH C CH C . . -. • • ..... CH C _ Ph P Ph P CH C Ph P • m H Ph P Ph P CH C Ph P Ph P CH C Ph P Ph P CH C ." • Scheme 3a: Termination mechanism of a macroradical G 11 . •••• " — • • . . -. CH2 CH — • • . . -. . . —. • — • • C CH CH CH • • • • • * * * L j t « « L Scheme 3b: Termination mechanism of a macroradical H of styrene monomer units. At the end of the polymerization the macroradical (D) terminates in three possible ways to form PSMI products (E) and (F) which can be stored in an inert atmosphere for later use. Synthesis of block copolymers (Scheme 2); Polystyrene macroinitiator (E) dissociates under the influence of UV light into two radicals which react with MMA to form intermediates: the polymethyl methacrylate free radical (G) and the polystyrene-polymethyl methacrylate free radical (H). Each of these two terminates in three possible ways. Intermediate (G) gives polymethyl methacrylate homopolymers (Scheme 3a) (I), (J), (K) and polystyrene homopolymers (L), whereas intermediate (H) terminates (Scheme 3b) into various polystyrene-polymethyl methacrylate block copolymers (M), (N) and (O). Ph P CH C CH Ph P CH Ph P ..... ..... Ph P CH C CH ..... Ph P CH C CH C 12 CONCLUSION The initiator tetraphenyl biphosphine (Ph4P2) is highly oxygen – sensitive. Consequently, it requires special reaction conditions. When exposed to UV light it decomposes into two radicals Ph2P· which react with styrene to form styrene - free radicals capable of starting propagation. These radicals can terminate in various ways. The synthesized polymer with Ph2P end groups acts as a macroinitiator in the second step of the polymerization in which block copolymers are formed. We found out that in the second step the pseudoliving mechanism is not predominant. ACKNOWLEDGEMENT This work was financed by the Ministry of Science and Technology of the Republic of Slovenia. The financial support of the Ministry is fully acknowledged. REFERENCES [1] M. Szwarz, M.Levy, R. Milkovich, J. Am. Chem. Soc. 1956, 78, 2656. [2] A. Šebenik, Prog. Polym. Sci. 1998, 23, 875. [3] T. Otsu, M. Yoshida, Makromol. Chem., Rapid commun. 1982, 3, 133. [4] H. J. Harwood, Encycl. Polym. Sci. Eng., Suppl. Vol.; Wiley, New York, 1990, pp 429-437. [5] M. Opresnik, A. Šebenik, Polym. Int., 1995, 36, 13. [6] I. Poljanšek, T. Kozamernik, A. Šebenik, KZLTET., 1997, 31, 81. [7] R. Pentiaud, Q. Tho Pham, Spectres RMN des polymeres 1H - 13C, Volume 1; Editions SCM, Paris, 1980, pp 186-193. [8] AMPAC Manual, a General MO Package, QCPE No.506. [9] Organikum, Praktikum iz organske hemije, Prosveta, Beograd, 1972, pp 59-61. [10] D. G. Gorenstein, Phosphorus-31 NMR Principles and applications; Academic Press Inc., Orlando, 1984, pp 551-562. [11] Osnovni eksperimenti za določanje kemijske strukture in konformacije molekul v tekočini z metodami NMR spektroskopije; FKKT - Univerza v Ljubljani, Ljubljana, 1996, pp 119-129. [12] V. Margon, Diplomsko delo, FKKT, Ljubljana, 1998, FKKT Ljubljana, pp 36-40. POVZETEK PS/PMMA blok kopolimere smo sintetizirali iz polistirenskih- (PS-) makroinicatorjev z aktivnimi fosforjevimi aromatskimi končnimi skupinami z uporabo UV svetlobe. Proučevali smo vpliv valovne dolžine sevane svetlobe in vpliv razmerja iniciator-monomer na stopnjo konverzije polimerizacije makroiniciatorja in blok-kopolimerizacije stirena in metilmetakrilata. Molska masa 13 blok kopolimerov narašča s polimerizacijskim časom. Za blok kopolimerizacijo stirena in metilmetakrilata s PS-makroiniciatorjem je primernejša svetloba z valovno dolžino 302 nm kot svetloba z valovno dolžino 366 nm. Makroiniciatorji razpadejo na primarni fosforjev radikal in makroradikal. Makromolekule terminirajo s primarnim radikalom, s kombinacijo dveh enakih ali različnih makroradikalov, kar smo opazovali z NMR spektroskopijo. Posamezne deleže homopolimerov in blok kopolimera v produktu smo določili z ekstrakcijo trdno-tekoče.