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Complexes for Catalytic Chain Transfer

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Complexes for Catalytic Chain Transfer 中国科技论文在线 http://www.paper.edu.cn High Spin Co(II) Complexes for Catalytic Chain Transfer Polymerization of Methyl Methacrylate Luo Xiongxiong, Xu Shansheng, Wang Baiquan (College of Chemistry, Nankai University, TianJin 300071) Abstract: In this paper the first high spin cobalt (II) complexes system Co(PPh3)X2/AIBN for the chain transfer polymerization of MMA were developed. The chain transfer constant CT is low (1), and polymers with high molecular weight (up to 100000) and narrow polydispersities (ca 1.40) can be obtained. The molecular weight of polymers increases linearly with the ratio of monomer to initiator. So the molecular weight of polymer can be easily designed and achieved by adjusting the ratio of monomer to initiator. Keywords:Catalytic chain transfer polymerization; Cobalt; Methyl Methacrylate 0 Introduction Chain transfer catalysis in free radical polymerization has been established more than 30 years, since it was firstly discovered.[1,2] This type of polymerization is an efficient method to prepare narrow molecular weight distributions macromonomers, which can produce a wide variety of new materials such as block copolymers,[3] graft,[4] star,[5] telechelic polymers,[6] even hyperbranched polymers[4] for the automotive, paper, and coating industry.[4,7-9] The widely accepted mechanism of cobalt (II) mediated catalytic chain transfer is shown in Scheme 1.[4] Firstly, the cobalt (II) metalloradical abstracts a hydrogen atom from α-methyl group of the propagating polymeric radical, a polymer with vinyl end group and a cobalt (III) hydride are formed. In the second step, the cobalt (III) hydride transfers the hydrogen to the monomer to reinitiate a new chain radical, and a new polymer begins to grow. During the past three decades, a number of chain transfer catalysts were discovered.[10-17] Most of them are cobalt (II) complexes, which are all low spin complexes. Generally, their structures are planar macrocycles, which are used to stabilize the metalloradical, just like cobalt (II) porphyrins. However, these chain transfer catalysts are poor stable, poor soluble in polar medium, highly colored and difficult to synthesize. Although some efforts were made, the cost is still high. Scheme 1. Pn + Co(II) Pn + Co(III) H Co(III) H +M HM+ Co(II) HM+ (m-1)M Pm Recently, Matsumoto et al.[18] reported that the living radical polymerization of MMA through ATRP can be achieved by tri(triphenylphosphine) cobalt (I) halides. But when we used di(triphenylphosphine) cobalt(II) bromide for the reverse ATRP, we found it was a catalytic chain transfer polymerization. The single crystal X-ray diffraction showed that Co(PPh3)2Br2 adopts a distorted tetrahedron configuration,[19] and is a high-spin complex. In this paper, we will report the catalytic chain transfer polymerization of MMA catalyzed by the first high-spin Co(PPh3)Br2/AIBN system. Foundations: NSFC (20474031) Brief author introduction:雒雄雄，（1979-），男，博士生，金属有机化学 Correspondance author: 王佰全，（1968-），男，教授，金属有机化学与均相催化. E-mail: [email protected] - 1 - 中国科技论文在线 http://www.paper.edu.cn 1 Results and Discussion [20] Co(PPh3)2Br2 was easily prepared according to the literature. The homogeneous poly merization of MMA catalyzed by Co(PPh3)Br2/AIBN was conducted in toluene at 80 °C under argon with [MMA]0:[AIBN]0:[Co(PPh3)2Br2]0 = 400:1:0.2 (Table 1, entry 1). The plots of -1 Ln([M0/[M) vs. time are linear with a first order rate constant (kobs) of 0.32236 h , indicating the radical concentration is constant through the polymerization (Figure 1). The molecular weight (Mn) is constant of 52000, which has no relationship with the reaction time or conversion, indicating the chain transfer mechanism. The polydispersity is narrow (Mw/Mn = 1.31). Increasing the catalyst concentration from 0.002 M to 0.02 M (entry 2), it was also first order kinetically, the -1 polymerization became faster with the rate constant (kobs) of 0.40739 h , the molecular weight decreased from 52000 to 45000. This indicates that increasing the concentration of the cobalt complex can effectively increase propagating chain termination constant. Increasing the ratio of monomer to initiator (entry 1, 3, 4, 5), the polymerizations were also first order kinetically, the reaction rates became a little faster, the molecular weight of polymers obtained increased linearly with the ratio of monomer to initiator (Figure 2), while the polydispersities were still narrow (1.27-1.37). This means that the molecular weight of polymer can be easily designed and achieved by adjusting the ratio of monomer to initiator. Generally, radical polymerization in a polar solvent is much faster but the polydispersity is much broader. Indeed, when the polar solvent DMF was used, the conversion was up to 85% in 2 hours, the reaction rate was much faster than that in toluene, but the PDI was still as narrow as 1.32 while the molecular weight increased to 56000 (entry 6). So, DMF can be used to synthesize higher molecular weight polymers instead of toluene. The effect of temperature was also examined (entry 1, 7, 8). Decreasing temperature, the polymerization became much slower, but the molecular weight increased from 52000 (80 °C) to 77000 (70 °C), even up to 100000 (60 °C). This is due to that the half-life of AIBN is protracted, and the effective ratio of monomer to initiator is increased. And with the increase of the molecular weight, the polydispersities vary a little, from 1.31 (80 °C) to 1.34 (70 °C), and to 1.40 (60 °C). Table 1. Results of Polymerization of MMA catalyzed by Co(PPh3)2X2/AIBN [Cat] temperature time Yield Entry X [MMA] /[AIBN] Mn PDI (M) 0 0 °C (h) (%) GPC 1 Br 0.002 400 80 3 60 52000 1.31 2 Br 0.02 400 80 2.5 69 45000 1.34 3 Br 0.002 100 80 4 58 25000 1.27 4 Br 0.002 200 80 4 71 33000 1.33 5 Br 0.002 800 80 3 72 79000 1.37 6a Br 0.02 400 80 2 85 56000 1.32 7 Br 0.002 400 70 7 67 77000 1.34 8 Br 0.002 400 60 12 56 100000 1.40 9 Cl 0.02 400 80 3 67 45000 1.32 10 I 0.02 400 80 3 74 42000 1.35 Experimental conditions: AIBN = 0.01 M, in toluene. a in DMF. - 2 - 中国科技论文在线 http://www.paper.edu.cn 1.4 1.2 1.0 0.8 /[M]) 0 0.6 0.4 Ln([M] 0.2 0.0 0123 Time/h Figure 1. Time dependence of Ln([M0/[M]]) for the polymerization of MMA in toluene at 80 °C. [MMA]0 = 4.00 M, AIBN = 0.01 M, [Co(PPh3)2Br2] = 0.02 M (■), 0.002 M (▲). 80000 70000 60000 50000 GPC Mn 40000 30000 20000 0 100 200 300 400 500 600 700 800 [MMA]0/[AIBN]0 Figure 2. Plot of MnGPC vs. [MMA]0/[AIBN]0, R = 0.99691. Chain transfer constant is used to evaluate the catalyst activity, which was determined by Mayo equation (eq 1). PN is the number-average degree of polymerization; PN0 is the number-average degree of polymerization in the absence of chain transfer agent (CTA); ktr is the rate constant for chain transfer by a propagating polymer chain; kp is the rate constant for addition of an additional monomer to a propagating polymer. 1 1 ktr [Cat]0 = + (1) PN PN0 kp [MMA]0 The polymerization at 80 °C was too fast to get low conversion (<10%) to determine CT, so o 60 C was selected for measurement. Representative Mayo type plots of 1/PN vs. [Co(II)]0/[MMA]0 were given in Figure 3. Low conversions (ca 0.5%) were obtained, varying the -3 -3 catalyst concentration from 10 M to 4×10 M. The chain transfer constant, CT, is only 1, which is 3 [4] much smaller than those cobalt (II) porphyrin type catalysts (CT>10 ). So polymers with high molecular weight (up to 100000) and low PDI (ca 1.40) can be easily achieved. - 3 - 中国科技论文在线 http://www.paper.edu.cn 2.0 1.8 3 1.6 N x10 1/P 1.4 1.2 246810 4 [Co(PPh3)2Br2]/[MMA]0x10 Figure 3. Mayo plot for Co(PPh3)2Br2 and MMA ([MMA]0 = 4.0 M) in toluene at 60 °C with 0.5% conversion. R= 0.99317 According to the Mayo equation, molecular weight should decrease with the conversion. However, only one study on CCTP of MMA has got this trend.[21] The molecular weight can be invariant or increase with conversion.[22-24] In order to test the CCT mechanism, the polymer end group was characterized by 1H NMR (sample of entry 3 in Table 1), Figure 4 demonstrated the end group was carbon-carbon double bond. However, the polydispersities of obtained polymer were around 1.3, which showed the characterization of living polymerization. Wayland reported (TMP)Co−R could catalyze the living radical polymerization of MA through the reversible cobalt-carbon bond.[25] Because the α-methyl hindered the cobalt-carbon formation, the living radical polymerization of MMA was not reported yet. Wayland investigated the formation of organocobalt porphyrin by cobalt(II) porphyrin and dialkylcyanomethyl radical, and proved the cobalt porphyrin hadried by 1H NMR.[26] Then we tried to proved whether the cobalt-carbon bond was formatted, because the Co(PPh3)2Br2 is a distorted tetrahedron configuration and can reduce the hinder of cobalt-carbon bond formation. The reaction between Co(PPh3)2Br2 and AIBN was 1 carried out with the ratio of entry 1 in Table 1 at 60 °C in CDCl3, 1 h later, H NMR spectrum was recorded (Figure 5).
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