e-polymers 2003, no. 007. http://www.e-polymers.org ISSN 1618-7229 Controlled grafting of poly(styrene-ran-n-butyl methacrylate) to isotactic polypropylene with nitroxidemediated polymerization Yusuke Sugino 1, Katsuhiro Yamamoto 1 *, Youhei Miwa 1, Masato Sakaguchi 2, Shigetaka Shimada 1 1 Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan; Fax +81-52-735-5294; katuhiro@mse.nitech.ac.jp 2 Ichimura Gakuen College, Nagoya Keizai University, 61-1 Uchikubo, Inuyama, 484-8503, Japan (Received: January 21, 2003; published: February 26, 2003) Abstract: A nitroxide (2,2,6,6-tetramethylpiperidin-1-oxyl, TEMPO) mediated polymerization method was applied to the graft polymerization of styrene (ST)/ n-butyl methacrylate (BMA) to isotactic polypropylene (PP). PP peroxides produced by γ-irradiation in air were used as macroinitiator for grafting. The molecular characteristics of grafted and free poly(st-r-bma) chains were analysed by gel permeation chromatography, NMR and IR measurements. The molecular weight of grafted poly(st-r-bma) cleaved from PP was slightly higher than that of free poly(st-r-bma) generated simultaneously in the reaction system. Polydispersities of these polymers were narrow, indicating that grafting proceeded in a living fashion. The incorporated amount of BMA into grafted poly(st-r-bma) was smaller than that of free poly(st-r-bma). It is considered that the local concentration of BMA around propagating ends of graft poly(st-r-bma) was lower than that around free poly(st-r-bma) because PP cannot be dissolved in BMA monomer but in ST monomer at the polymerization temperature. Introduction Living radical polymerization (LRP) techniques have been very useful and widely investigated in the field of precision polymerization. LRP is based on the reversible activation of dormant species. These dormant species have potentially active covalent bonds that can be activated by heat, light, and catalysts to give propagating radicals. LPR includes nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP) and radical addition-fragmentation chain-transfer polymerization. Various random, block, and graft copolymerizations as well as homopolymerizations have been achieved by LRP techniques. Chemical modification of polyolefins has long been a scientific challenge. This is an important research field for industry because it serves as a route to expand polyolefins applications into polymer blends and composites. A radiation-induced graft polymerization to polyolefins was extensively used for chemical modification. However, molecular weight, molecular weight distribution, grafting ratio, and graft 1
density of grafted polymer chains could not be controlled. Since controlled radical polymerizations were developed, some controlled graftings to polyolefins were studied and reported. Chung has developed an interesting approach to polyolefin/ poly(methyl methacrylate) graft copolymers by careful introduction of oxygen to a borane-functionalized polyolefin [1-4]. Hawker reported the synthesis of PP-graft-PS by a combination of metallocene and nitroxide-mediated polymerization (NMP) [5]. Mülhaupt et al. synthesized highly branched polyethylene graft copolymers prepared by TEMPO-mediated graft polymerization [6]. Ying reported the synthesis of ethylene-propylene-diene terpoymer/poly(methyl methacrylate) graft copolymer by ATRP [7]. We previously reported some controlled graftings to polyolefins; a grafting of styrene (ST) to isotactic polypropylene (PP) [8,9] and a grafting of ST to high density polyethylene (PE) [10]; a grafting of methyl methacrylate to PE with ATRP and reverse ATRP [11]. This paper reports the controlled grafting of ST and BMA to a comercially available PP by combination of a radiation-induced polymerization with the NMP method. Experimental part Materials Isotactic PP (M v = 400k) was obtained from Mitsubishi Chemical Co., Ltd., dissolved in boiling toluene, cooled to room temperature, filtered, and dried in a vacuum oven. This procedure was repeated three times. ST (reagent, Nacalai Tesque Co., Ltd.) and BMA (extra pure reagent, Tokyo Chemical Co., Ltd.) were distilled under reduced pressure. Benzoyl peroxide (BPO) (reagent, Nacalai) was dissolved in chloroform, precipitated into methanol, and recrystallized. 2,2,6,6-Tetramethylpiperidin-1-oxyl (TEMPO, 99%) and trifluoroacetic acid (99%) were purchased from Aldrich Chemical Co., Ltd, and used as received. Tetrahydrofuran (THF), methanol, toluene, and chloroform were obtained from Nacalai Tesque Co., Ltd. and used as received. Measurements M n and polydispersity of the grafted and free polymers were determined by gel permeation chromatography (GPC) in THF (1 ml/min) at 313 K on four polystyrene gel columns - Tosoh TSK gel GMH, G4000H, G2000H, and G1000H - that were connected to an online degasser (SD-8022 Tosoh), a Tosoh CCPE pump, and a ERC-7522 RI refractive index detector (ERMA Inc.). The columns were calibrated against standard polystyrene (Tosoh) samples. NMR was performed on a Bruker AVANCE 200 spectrometer using deuterated chloroform at 25 C with tetramethylsilane as an internal reference. The grafting ratio was defined as follows: grafting ratio, GR (in wt.-%) = 100 x (W g - W 0 ) / W 0 Here W g is the weight of the sample after grafting, and W 0 is the initial weight of the irradiated PP. The grafting ratio was estimated by the FT-IR (IMPACT400D, Nicolet Instrument Co.; KBr) method because of a large experimental error involved in measuring the weight increase after grafting. Grafting ratios were calculated from the ratio of the IR absorbance of polystyrene (PS) (700 cm -1 ) and poly(n-butyl methacrylate) (PBMA) (1735 cm -1 ) to that of PP (810 cm -1 ), where the absorption at 700 cm -1 is assigned to the CH bending vibration of the PS benzene ring, that at 1735 cm -1 2
to the C=O stretching vibration of the PBMA carbonyl group, and that at 810 cm -1 is assigned to the CH 2 and CH rocking vibrations of PP [11]. In advance, some mixtures of poly(st-r-bma) and PP were prepared to make a calibration curve of the grafting ratio (weight ratio of the mixture) against the absorbance ratio. Living graft polymerization of styrene/n-butyl methacrylate to PP with TEMPO The reaction scheme is shown in Fig. 1. The PP powder was 60 Co γ-irradiated in air (the total doses were about 7.2 and 20.7 kgy at a dose rate of 0.123 kgy/h). The irradiated PP carries peroxides that were used as initiator of grafting. A typical grafting reaction proceeds as follows: The γ-irradiated PP (0.2 g), ST (16.0 mmol, 1.67 g), BMA (4.00 mmol, 0.57 g), TEMPO (50.0 10-2 mmol, 7.81 mg), BPO (38.5 10-2 mmol, 9.32 mg) were placed in a glass tube. The free initiator is necessary in order to control the polymerization (including grafting) itself. Without free initiators, the grafting cannot be well controlled due to the considerably low concentration of peroxides. After degassing by a freeze-pump-thaw method, the tube was sealed in vacuum. The sealed ampoule was heated at 368 K for 5 h, and then the polymerization was carried out at 398 K. During polymerization, the PP powder became completely soluble in monomer; the solution became transparent and colorless, and its viscosity increased substantially. The reactions were terminated by quenching to liquid nitrogen temperature. Total monomer conversions were calculated from the weight of the reaction mixture which included PP-g-(ST-r-BMA) and free poly(st-r- BMA) after residual monomers were completely evacuated in vacuum at 353 K for 24 h. After that, the sample was immersed into THF to extract poly(st-r-bma) from the grafted poly(st-r-bma). The free poly(st-r-bma) was precipitated from THF solution into methanol and dried in vacuum at 343 K for 24 h. The monomer conversion was 28.6%, and the grafting ratio of PP-g-PS was 3.6 wt.-% after polymerizing for 3 h. The sample of the poly(st-r-bma) grafted to PP was washed by Soxhlet extraction with THF for 24 h and dried in vacuum at 343 K for 24 h. Although a further Soxhlet extraction was carried out for 24 h, there was no change in the grafting ratio before and after the second extraction. Fig. 1. Reaction scheme of the graft polymerization of styrene and n-butyl methacrylate to PP. The most likely end structures R are benzoyl and hydroxyl groups generated by decomposition of BPO and PP peroxide, respectively 3
Cleavage of the polystyrene grafted to polypropylene The grafted poly(st-r-bma) was cleaved from the PP main chain by reaction with trifluoroacetic acid. As a typical reaction, a sample of poly(st-r-bma) grafted to PP (approx. 0.2 g) in trifluoroacetic acid (1.5 ml) and toluene (1.5 ml) mixture was allowed to stand at room temperature for a couple of days and then was evaporated in vacuum. The dried sample was immersed into THF, and the grafted poly(st-r- BMA) was extracted; thus, the grafted copolymer and the PP main chain were separated. The extracted grafted copolymer was precipitated in methanol, filtered, and dried in vacuum. The yield of copolymer was c. 1 mg. Results and discussion Living radical graft polymerization of poly(styrene-ran-n-butyl methacrylate) to PP The number of monomers whose homopolymerizations have been successfully controlled by NMP is limited to date. Acrylonitrile, methyl and ethyl acrylates, 9- vinylcarbazole, and BMA gave no polymer (or poorly defined polymer) under the conditions used successfully to polymerize ST with benzoyl peroxide initiator and TEMPO [13]. However, the random copolymerization of ST with an NMP-uncontrollable monomer proceeded in a living fashion [14-16]. The ability to form well-defined random copolymers from simple monomer mixtures is one of the advantages of living free radical procedures when compared to other living polymerizations such as anionic and cationic processes [17]. This feature can be exploited in the design of random graft copolymer chains. We performed a controlled random graft polymerization of ST with BMA to PP. Fig. 2. M n and M w /M n of free poly(st-r-bma) prepared with irradiated PP (20.7 kgy, 0.2 g), TEMPO (50.0 10-2 mmol ), BPO (38.5 10-2 mmol) and monomer (20 mmol). The solid and broken lines mean the theoretical M n for 100% BMA and ST. Solid and open symbols indicate M n and M w /M n. ST monomer content in feed is 80% (circles), 60% (triangles), 50% (squares), and 40% (butterflies) 4
Fig. 2 shows the molecular weight of free poly(st-r-bma) (non-grafted), produced simultaneously during grafting, against conversion with various initial monomer compositions. The results indicate that M n of free poly(st-r-bma) increased linearly with conversion and the polydispersity of the free copolymer was small. The polymerization in the range of these monomer compositions proceeded in a living fashion. The grafting ratio was plotted against M n of free poly(st-r-bma) as shown in Fig. 3. This figure shows a linear relation of the grafting ratio with M n, which suggests that the graft polymerization was also controlled. This is a similar trend as in the grafting of styrene to PP and PE reported in our previous papers [8-10]. Fig. 3. Grafting ratio vs. M n of the cleaved poly(st-r-bma) prepared with irradiated PP (20.7 kgy, 0.2 g), TEMPO (50.0 10-2 mmol ), BPO (38.5 10-2 mmol), ST (16 mmol) and BMA (4.0 mmol). The slope relates to the concentration of grafts along the PP main chain To further know the characteristics of the grafted poly(st-r-bma), the grafted chains were cleaved with trifluoroacetic acid from PP-g-poly(ST-r-BMA) at their point of attachment. M n of the cleaved copolymer increased linearly with monomer conversion, and the copolymer had a narrow polydispersity as shown in Fig. 4. This indicates that the graft polymerization had proceeded without chain transfer. As an example, GPC traces of cleaved and free poly(st-r-bma) are shown in the right side of the figure for 85% ST fraction at a conversion of 52%. GPC peaks of the cleaved graft copolymer of all samples showed a slightly higher molecular weight in comparison to the free copolymer. The same result has been observed for ST grafting to PP or PE [7-9]. The characteristics of the free poly(st-r-bma) can be a good measure of that of the grafted poly(st-r-bma). Taking into account that M n of grafted poly(st-r- BMA) was about 10% larger than that of free poly(st-r-bma), as indicated in present (Fig. 4, right) and previous papers, the concentration of grafts along the PP chain can be obtained from the slope of Fig. 3. The slope was 1.54 10-5 mol/g. The average concentration of grafts along the PP chain was calculated to be approx. 1.40 10-5 5
mol/g if M n of the grafted copolymer was assumed to be c. 10% larger than that of free copolymer. The concentration of PP peroxides produced by γ-ray irradiation with a dose of 20.7 kgy was measured to be about 2.3 10-5 mol/g PP. The initiation efficiency of the PP peroxides was estimated to be c. 60% corresponding to the reported value [8-10]. Fig. 4. M n, M w /M n, and GPC curves of the cleaved graft poly(st-r-bma). M n and M w /M n are indicated with filled and open symbols, respectively. Conditions: irradiated PP (7.2 kgy, 0.4 g), ST (27.7 mmol), BMA (5.06 mmol), BPO (8.38 10-2 mmol), and TEMPO (0.11 mmol). Dot-dashed lines represent the theoretical molecular weight. GPC curves indicate graft (solid line) and free (dotted line) copolymer Composition analysis of graft poly(styrene-ran-n-butyl methacrylate) The composition of free poly(st-r-bma) was characterized by NMR. The amount of cleaved poly(st-r-bma) was too small to analyze its composition by NMR. From FT- IR spectra of PP-graft-poly(ST-r-BMA) and free poly(st-r-bma), the ST content in the grafted poly(st-r-bma) was determined. The results are plotted in Fig. 5. Solid and open symbols indicate ST contents of grafted (cleaved) and free poly(st-r-bma), which were obtained for various monomer conversions. Star symbols mean the average composition for all samples at respective ST monomer concentration in the feed. (Here, the mole ratio of free poly(st-r-bma) to the grafted one was 11.6. This is the reason why the average values are close to the values of free poly(st-r-bma)). The curve was fitted with the Mayo-Lewis equation (Eq. (1)) using r ST = 0.20 and r BMA = 1.04, including an error of 0.17 < r ST < 0.22 and 0.97 < r BMA < 1.08, while 0.19 < r ST < 0.30 and 0.98 < r BMA < 1.22 were obtained from an interception method (Eq. (2)). The Fineman-Ross method (Eq. (3)) also gave r ST = 0.20 and r BMA = 1.04. d[st] d[bma] = [ST] [BMA] rst [ST] + [BMA] [ST] + rbma [BMA] (1) 6
r [ST] d[bma] [ST] = 1 + rst 1 [BMA] d[st] [BMA] 2 F f 1 = rst r (3) f BMA (2) F f ( ) BMA Here, [ST] and [BMA] are ST and BMA compositions in monomer feed, d[st] and d[bma] are their compositions in copolymer, respectively, F = [ST]/[BMA] and f = d[st]/d[bma]. From the monomer reactivity ratios and the product (0.16 < r ST r BMA < 0.24), it can be safely said that the ST sequence in poly(st-r-bma) is randomly distributed as expected. In this article, the monomer reactivity ratios are not discussed deeply, but we consider the composition. It is interesting that, as shown in the figure, the ST content in grafted poly(st-r-bma) was slightly larger than that in the free one for all samples. The probable reason is as follows: isotactic PP is not dissolved in BMA but in ST monomer at high temperature (polymerization temperature). It is reasonable to assume that BMA monomer cannot access the PP rich phase. Therefore, it is likely that the local concentration of BMA around the grafted polymer chains was lower than that around the free polymer chains, resulting in different compositions for both copolymers. Fig. 5. Composition curve of grafted (solid) and free (open) poly(st-r-bma)s for all samples in the present system. Star marks represent average compositions at respective ST concentrations. The curve was fitted with the Mayo-Lewis equation (Eq. (1)) using r ST = 0.20 and r BMA = 1.04 Conclusion We investigated a nitroxide (TEMPO) mediated graft polymerization of ST/BMA to isotactic PP. The PP peroxides produced by γ-irradiation in air were used as macroinitiator for grafting. The molecular characteristics of grafted and free poly(st-r- BMA) chains were analysed by GPC, NMR, and IR measurements. The molecular 7
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