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

More documents

Recommendations

Info

42 J. Hua et al. the authors found only one work [17] in the literature referring to the ATRP of butadiene using MoO 2 Cl 2 / triphenyl phosphine as the catalyst and organic halide compounds such as methyl 2-chloropropionate, CCl 4 , 1,4- dichloromethyl benzene, 1-phenylethyl chloride and benzyl chloride as initiators. Several kinds of polystyrene copolymer have been synthesized by ATRP. Syntheses of forced gradients of styrene and acrylonitrile as well as spontaneous gradients of styrene and n-butyl acrylate by ATRP have been reported [18]. Block copolymers were synthesized by ATRP with a yield of 30% and low polydispersity from the polystyrene with Br end-group macroinitiators and the azo monomer, catalyzed by copper(II) chloride [19]. The preparation of polystyrene-grafted magnesium hydroxide nanoparticles via surface-initiated atom transfer radical polymerization in a toluene system with 2,2′-bipyridine and Cu(I)Br catalysts has also been reported [20]. In this work, 1-octanol-substituted MoCl 5 [i.e., MoCl 3 (OC 8 H 17 ) 2 ] and triphenyl phosphine (PPh 3 ) were used as catalyst and ligand, respectively, in the ATRPs of styrene and butadiene and in their copolymerization. This is an easily accessible catalyst system with good solubility in the organic solvent. We are aware that there are already many methods for synthesizing polybutadiene (PB) and the copolymer of butadiene (Bd) and styrene (St), such as radical polymerization and anionic polymerization, but the development of other new and effective techniques for synthesizing these polymers is still very important. Therefore, the primary aim of this study was to investigate whether the ATRPs of butadiene and its styrene copolymer can be realized using the system mentioned above. The effects of the reaction parameters on the monomer yields, molecular weights and polydispersity indices of the polymerized butadiene, styrene and their copolymer are investigated, so that the structural unit compositions and yields of the copolymers can be effectively controlled. It is expected that polybutadiene and the copolymer of St and Bd will have suitable molecular weights and copolymer compositions. Experimental Materials Butadiene was distilled using a 4 Å molecular sieve. Styrene (AR, Shanghai Chemical Reagent Co.) was distilled under reduced pressure from calcium hydride powder before use. Toluene (AR, Shanghai Chemical Reagent Co.) was purified by refluxing over sodium metal under a nitrogen atmosphere. Triphenyl phosphine (AR, Shanghai Chemical Reagent Co.) was recrystallized from ethanol. Benzyl chloride (AR, Beijing Chemical Industrial Co.) was purified by distillation under reduced pressure. 1-Octanol-substituted MoCl 5 , MoCl 3 (OC 8 H 17 ) 2 , was prepared by reacting MoCl 5 (AR, Beijing Chemical Industrial Co.) with 1-octanol (AR, Shanghai Chemical Reagent Co.) at 30°C under nitrogen. The molar ratio of MoCl 5 to 1-octanol was 1:4; the hydrochloric acid produced by the reaction was removed under vacuum, and the number of chlorines replaced by 1-octanol in MoCl 5 was detected by titration. General procedure for polymerization The flask was sealed with a rubber septum and cycled between vacuum and nitrogen at 120°C for 2 h using a high-purity nitrogen gas (99.99%). After that, the solution containing the monomer, initiator, ligand and solvent was degassed by purging with nitrogen for 15 min before it was injected into the reaction flask using a syringe. The reaction flask was then placed in a preheated oil bath at a desired temperature. At a given time, a given volume of the reaction solution was removed by syringe. The polymer was precipitated into a large amount of ethanol. The dried product was then characterized by gravimetry and GPC. Measurements and analysis Molecular weights and their distributions were measured using GPC with a Shimadzu system consisting of a set of KF-1, KF-2, KF-3, KF-4, and KF-6 microstyragel columns in tetrahydrofuran with polystyrene standard calibration. Theoretical molecular weight was calculated using the formula: M th =([monomer]/[initiator])×molecular weight of polymer repeating unit×monomer conversion. FT-IR spectra were recorded on a Nicolet FT-IR Magna 750 spectrophotometer using KBr pellets. The content of each type of polybutadiene structural unit was calculated by the following equations [21]: Content of cis 1; 4PB¼ 17667D 738 =A; Content of trans 1; 4PB¼ 4741:4D 967 =A; Content of 1; 2PB ¼ 3673:8D 911 =A; where A=17667D 738 +4741.4D 967 +3673.8D 911 , D=logI 0 /I, I 0 is the intensity of the incident light and I is the intensity of the transmitted light at that wavelength. 13 C NMR spectra were measured in CDCl 3 with a Bruker MSL-300 NMR spectrometer using 13 C as probe and deuterated acetone as reference standard. 1 H NMR spectra were measured in CDCl 3 using tetramethylsilane as internal reference. The UV-vis spectra were obtained with a Tu-1800PC spectrophotometer. Thermal properties were measured using a Netzsch 204 DSC (differential scanning
Atom transfer radical polymerizations of styrene and butadiene as well as their copolymerization 43 the rate of reversible oxidation and reduction of MoCl 3 (OC 8 H 17 ) 2 and the rate constant of polymerization with polymerization temperature. Figure 3 shows the 1 H NMR spectrum of the PSt obtained with this catalyst system. The signal at δ 4.56 ppm arises from the terminal proton adjacent to the chlorine atom in the ω-end group, i.e., –CH 2 C–(C 6 H 5 )H–Cl. This further verified that the mechanism of the synthesis of the polystyrene had ATRP character. A chloride atom was transferred between the polymer radical and the dormant species. Butadiene polymerization Fig. 1 Plot of Ln([M] 0 /[M]) vs polymerization time for the ATRP of styrene in toluene. [C 6 H 5·CH 2 Cl] 0 :[MoCl 3 (OC 8 H 17 ) 2 ]:[P(Ph) 3 ]:[St] 0 = 1:1:3:135; [St] 0 =1. 4 M; T=90°C calorimeter) under a nitrogen atmosphere. The heating rate was 10°C/min. Results and discussion Styrene polymerization A novel initiator system, C 6 H 5·CH 2 Cl /MoCl 3 (OC 8 H 17 ) 2 / PPh 3 , was used for the ATRP of styrene. Figure 1 presents the kinetics of the polymerization of St in toluene at 90°C. As shown, ln([M 0 ]/[M]) increased continuously with time, and a semilogarithmic plot of the kinetics could be approximated by a line, thus indicating that there was an approximately constant concentration of the growing radical. Coordination between the benzene ring and the Mo catalyst may be one of the reasons for the induction period of styrene ATRP. Figure 2 shows a linear dependence of M n on conversion and a decrease in M w /M n with conversion. The results shown in Figs. 1 and 2 together demonstrate a “living” radical polymerization process. The rather wide polydispersities (M w /M n ≈1.9) observed may be attributed to many factors, such as the different chemical structures of benzyl chloride and styrene and the low rate of reversible oxidation and reduction of MoCl 3 (OC 8 H 17 ) 2 . Table 1 shows the conversion, molecular weight, and polydispersity index of the polymerized styrene as a function of reaction temperature and reaction time. The polymerization rate increased and the polydispersity index of the product narrowed with increasing polymerization temperature. This can be attributed to the increases in Figure 4 presents plots of the kinetics of butadiene polymerization when using C 6 H 5·CH 2 Cl / MoCl 3 (OC 8 H 17 ) 2 / PPh 3 as initiator. The curve of ln([M 0 ]/[M]) versus time was almost linear, suggesting that it is first order in the rate. Figure 5 shows a linear dependence of M n on conversion, while the polydispersity index (M w /M n ≈1.7) was rather wide and remained almost constant during the polymerization. The reaction time had an important influence on the monomer conversion and the molecular weights. Both factors increased with reaction time. As can be seen, the measured molecular weights were much greater than those of the theoretical value M th at a low monomer conversion. This fact is due to the period needed to reach the equilibrium between the active propagation species and the dormant species and thus the high concentration of the active propagation species at the beginning of polymerization. The efficiency of the benzyl chloride initiator was about 20%, a little higher than that of a lithium molybdate complex catalytic system [4]. The polydispersity index of the polymer was about 1.7, which MnX10 -3 14 12 10 8 6 4 2 0 1.5 0 20 40 60 80 100 Conversion (%) Fig. 2 Dependences of M n,GPC and M w /M n on the conversion during the polymerization of styrene. The conditions were the same as in Fig. 1. Squares, M n ; circles, M w /M n 2.0 1.9 1.8 1.7 1.6 Mw/Mn
Page 1: J Polym Res (2011) 18:41-48 DOI 10.
Page 5 and 6: Atom transfer radical polymerizatio
Page 7 and 8: Atom transfer radical polymerizatio

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?