Atom transfer radical polymerizations of styrene and butadiene as ...

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46 J. Hua et al. 8 6 4 2 0 ppm Fig. 10 1 H NMR spectrum of poly(butadiene-styrene) Fig. 8 FTIR spectra of PSt (a), St/Bd copolymer (Bd/St=1/4) (b), St/ Bd copolymer (Bd/St=1/3) (c), St/Bd copolymer (Bd/St=1/2) (d), St/ Bd copolymer (Bd/St=1/1) (e), PBd (f) absorption peaks of the styrene units became stronger and those of the butadiene structure unit became weaker as the molar ratio of styrene in the copolymer increased. At the same time, the characteristic peaks of the 1,2-butadiene structural units increased while those of the cis-1,4- and trans-1,4-butadiene structural units decreased. Figure 9 shows 13 C NMR spectra of the styrene and butadiene copolymer. The peaks between 40.9 and 45.5 ppm (40.9, 42.6, 42.8, 43.0, 43.2, 45.5 ppm) in Fig. 9 correspond to the carbons of the S*V, S*c, V*S, and S*t chain structures (here, S, V, c and t denote styrene and 1,2-, cis-1,4- and trans-1,4-butadiene structural units, respectively); the peak at 145.15 ppm corresponds to the carbon of the S unit between c and t [25]. There is no absorption peak over 6.800 in the 1 HNMR spectrum of the copolymer (Fig. 10), which shows that there is no block styrene chain segmental structure in this copolymer [26]. This verifies that the copolymer is a random copolymer. Figure 11 shows the single glass transition temperature in the DSC curve of the styrene and butadiene copolymer. The T g of the copolymer dropped from 16.2°C to −16.6°C as the molar ratio of Bd/St increased from 1/3 to 1/2. These results further prove that the copolymer is a random copolymer. Analysis of the polymerization mechanism Figure 12 shows UV-vis spectra of toluene solutions of MoCl 3 (OC 8 H 17 ) 2 , MoCl 3 (OC 8 H 17 ) 2 / P(Ph) 3 , MoCl 3 (OC 8 H 17 ) 2 / P(Ph) 3 / C 6 H 5·CH 2 Cl, and St / MoCl 3 (OC 8 H 17 ) 2 / P(Ph) 3 / C 6 H 5·CH 2 Cl. In the UV-vis spectrum of MoCl 3 (OC 8 H 17 ) 2 , characteristic absorption bands were observed for Mo 5+ near 430 nm and 730 nm and for Mo 4+ near 510 nm [27]. Mo 4+ was probably present because the reduction of MoCl 3 (OC 8 H 17 ) 2 took place to a small degree when creating the 1-octanol-substituted MoCl 5 solution. The absorption peak at 730 nm was caused by a d–d transition in the Mo atom in MoCl 3 (OC 8 H 17 ) 2 .After Fig. 9 13 C NMR spectra of the styrene and butadiene copolymer. The molar monomer ratio of St/Bd was 3/1 Fig. 11 DSC curves for styrene and butadiene copolymers with different monomer ratios
Atom transfer radical polymerizations of styrene and butadiene as well as their copolymerization 47 Abs 3 2 1 4 0 200 400 600 800 1000 wavelength(nm) Fig. 12 UV-vis spectra of MoCl 3 (OC 8 H 17 ) 2 (1), MoCl 3 (OC 8 H 17 ) 2 /P (Ph) 3 (2), MoCl 3 (OC 8 H 17 ) 2 / P(Ph) 3 / C 6 H 5·CH 2 Cl (3) and St / MoCl 3 (OC 8 H 17 ) 2 / P(Ph) 3 /C 6 H 5·CH 2 Cl (4) in toluene 2 1 3 PPh 3 was added, the absorption at 730 nm became weaker, perhaps because of competition between the PPh 3 and 1- octanol ligands to coordinate with the Mo atom. At the same time, the absorption at 430 nm increased, which indicated an increase in the Mo 4+ content. After the solution of St / MoCl 3 (OC 8 H 17 ) 2 / P(Ph) 3 / C 6 H 5·CH 2 Cl in toluene was placed in a oil bath at 90°C for 5 h, the absorption at 730 nm became smooth, and that at 430∼460 nm became weaker. At the same time, a yellow deposit appeared in the solution, which accounted for the increased content of Mo 6+ . These results reveal that Mo 4+ , Mo 5+ and Mo 6+ were present together in this reaction system. The valence state of molybdenum changed during the reaction. All of these results imply that the oxidation and reduction reaction of MoCl 3 (OC 8 H 17 ) 2 occurred according to the ATRP mechanism. A similar rule also applies to the Bd and St/Bd copolymerization systems. Therefore, we can deduce that the mechanism for the polymerization of styrene or butadiene or their copolymerization using a C 6 H 5·CH 2 Cl / MoCl 3 (OC 8 H 17 ) 2 / PPh 3 initiation system is that shown in Scheme 1. When MoCl 3 (OC 8 H 17 ) 2 reacts with C 6 H 5·CH 2 Cl, an electron transfer reaction can occur, where the Mo(V) or Mo(IV) is oxidized to Mo(VI) or Mo(V), respectively, with the concomitant formation of a benzyl radical. This reaction is typical of the initiation step of an ATRP. The benzyl radicals formed react with the alkene (diolefin) to produce polymer chain radicals. Both of the radicals can then react with Mo n+1 Cl n+1–2 (OC 8 H 17 ) 2 / PPh 3 to give Mo n Cl n−2 (OC 8 H 17 ) 2 / PPh 3 and the corresponding alkyl halide (dormant species, Scheme 1). The occurrence of more than one kind of reversible oxidation and reduction reaction in this reaction system may be another reason for the rather wide molecular weight polydispersity index. Conclusions The atom transfer radical polymerizations of butadiene and styrene and their copolymerization were successfully initiated by C 6 H 5·CH 2 Cl / MoCl 3 (OC 8 H 17 ) 2 / PPh 3 system. The almost linear kinetics plot, suggesting a first-order rate, and the linear increase in the number average molecular weight with conversion demonstrate that it has “living” radical polymerization character. However, the rather wide molecular weight distributions observed indicate that control over this polymerization system needs to be improved. PBd was an atactic polymer, as detected by IR. The microstructure of the butadiene and styrene copolymer, as characterized by IR, 13 C NMR, 1 H NMR and DSC, indicates that it is a random copolymer. The primary reaction mechanism was shown to proceed according to the ATRP mechanism using UV-vis spectroscopy. Scheme 1 The mechanism for the polymerization of styrene or butadiene or their copolymerization using the C 6 H 5·CH 2 Cl / MoCl 3 (OC 8 H 17 ) 2 / PPh 3 initiation system Initiation Ka CH-Cl n CH . n + 1 2 + Cl (OC8H17 + Mo Cl (OC8H17 ) n-2 ) / 2 PPh3 2 Mo Kd n +1-2 2 / PPh3 monomer Propagation 1 R 1 Ka R 1 R n n + 1 CH- ( CH C ) 2 Cl + + Cl (OC8H17 p Cl (OC8H17 n-2 ) CH- ( CH C ) / PPh3 2 Mo 2 / PPh3 2 ) Kd p CH2 C Mo 2 n +1-2 2 R2 R2 R2 Kp 1 R monomer n = 4, 5 ; CH2 C was monomer = styrene or butadiene R2
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