ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ, Серия Б, 2012, том 54, № 4, с. 649-660
КОМПОЗИТЫ
УДК 541(64+515):547(538.141+39)
EFFECT OF MONOMER/NANOCLAY INTERACTION ON THE KINETICS OF ATOM TRANSFER RADICAL HOMO- AND COPOLYMERIZATION OF STYRENE AND METHYL ACRYLATE1
© 2012 г. Mahdi Abdollahi and Mohammad Ali Semsarzadeh
Polymer Engineering Group, Faculty of Chemical Engineering, Tarbiat Modares University, 14115-143 Tehran, Iran
e-mail: semsarzadeh@modares.ac.ir
Received August 17, 2011 Revised Manuscript Received October 23, 2011
Abstract — Atom transfer radical homo- and copolymerization of styrene and methyl acrylate initiated with CCl3-terminated poly(vinyl acetate) macroinitiator were performed at 90oC in the presence of nanoclay (Cloisite 30B). Controlled molecular weight characteristics of the reaction products were confirmed by GPC. It was shown that nanoclay slightly decreased the rate of styrene polymerization, while it significantly enhanced the rate of methyl acrylate polymerization and its copolymerization with styrene. The reactivity ratios of the monomers in the presence and in the absence of nanoclay were calculated (rSt = 1.002 ± 0.044, rMA = 0.161 ± 0.018 by extended Kelen-Tudos method and rSt = 1.001 ± 0.038, rMA = 0.163 ± 0.016 by Mao-Hug-lin method), confirming that the presence of nanoclay has no influence on monomer reactivity. The enhancement in the homopolymerization rate of methyl acrylate as well as its copolymerization rate with styrene was attributed to nanoclay effect on the dynamic equilibrium between active (macro)radicals and dormant species. Dipole moments of the monomers were successfully used to predict structure of the polymer/clay nano-composites prepared via in situ polymerization.
INTRODUCTION
Development of controlled/living radical polymerization (CLRP) for synthesis of polymers with controlled architecture, molecular weight, and narrow polydispersity is among the most significant accomplishments in polymer chemistry [1-3]. Among the known types of CLRP methods, atom transfer radical polymerization (ATRP) is one of the most successful techniques of obtaining polymers based on styrenes, methacrylates, acrylates and a variety of other monomers in a controlled fashion [1-3].
Dynamic equilibrium between active propagating (macro)radicals and dormant species in the controlled/living radical polymerization systems such as ATRP may result in the decreased concentration of the propagating (macro)radicals and thereby decreased polymerization rate. It has been reported that solvents such as acetonitrile [4], ethylene carbonate [5] or water [6, 7] and additives such as carboxylic acid [8] or phenol [9] can act as accelerator in ATRP.
Recently, researches have been focused on development of tailor-made polymer/clay nanocomposites [10, 11]. In situ ATRP technique has been used to produce tailor-made polymer/layered silicate nanocomposites [11-16] with considerable improvements of numerous polymer properties. Although the use of in situ ATRP by intercalating initiator within the silicate
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layers has been reported for production of poly-mer/organoclay (organically modified clay) nanocomposites, however, effect of organoclay on the kinetics ofATRP reactions have not been investigated in detail.
Recently, two works have been reported about the unusual role of nanoclay in the ATRP of ethyl acrylate (EA) [16] and methyl methacrylate (MMA) [17]. Results showed that use of nanoclay in the ATRP of the mentioned monomers led to a significant increase in the homopolymerization rate. Performing atom transfer radical copolymerization of any pair comonomers in the absence and presence of nanoclay seems to be as an interesting approach to investigate effect of nano-clay on the kinetics of ATRP reactions, from which possible interactions between the monomers and nan-oclay can be revealed. In the previous studies [17, 18], we have studied effect of nanoclay (Cloisite 30B) and macroinitiator on the kinetics of atom transfer radical homo- and copolymerization of styrene (St) and methyl methacrylate (MMA) initiated with trichlorome-thyl (CCl3)-terminated poly(vinyl acetate) (PVAc) macroinitiator. It was observed that homopolymeriza-tion rate and reactivity ratios of St and MMA can be affected by presence of nanoclay in the reaction medium. These observations were attributed to activated conjugated C=C bond of MMA monomer via interaction/coordination between the carbonyl group of MMA monomer and hydroxyl moiety (Al—O—H) of
Table 1. Atom transfer radical homo- and copolymeriza-tion recipes of St and MA initiated with CCl3-terminated PVAc macroinitiator in the presence of nanoclay
Exp. No. [St]0, mol l-1 /St
MS1.0b 8.59 1.0
MS0.9 7.90 0.9
MS0.7 6.43 0.7
MS0.5 4.81 0.5
MS0.3 3.03 0.3
MS0.1 1.06 0.1
MS0.0 0 (10.92)c 0.0
Notes: a [CCl3-PVAc]0 : [CuCl]0 : [PMDETA]0 : ([St]0 + [MA]0) = 1 : 1 : 2 : 300. Polymerization was carried out at 90(±0.1)°C. Cloisite 30B was used as nanoclay with 2 wt % relative to the total weight of monomer(s) in all experiments.
b Numbers given in the symbols indicate the mole fraction of styrene in the initial comonomer mixture (/gt = [St]0/([St]0 + [MA]0)).
c Value given inside the parenthesis shows the molar concentration of MA.
nanoclay and/or to effect of nanoclay on the dynamic equilibrium between active (macro)radicals and dormant species. Effect of nanoclay on the activity of conjugated C=C bond of MMA was certainly verified by considering effect of nanoclay on the reactivity ratios of St and MMA.
Kinetics of atom transfer radical homo- and copo-lymerization of St and MA initiated with CCl3-termi-nated PVAc macroinitiator have been studied [19]. In the present study, atom transfer radical homo- and co-polymerization of St and MA initiated with CCl3-ter-minated PVAc macroinitiator were performed in the presence of nanoclay (Cloisite 30B) at 90oC, and the structure of polymer/clay nanocomposites was investigated.
EXPERIMENTAL
Materials
St (Merck, >99%) and MA (Merck, 99%) were distilled over calcium hydride under reduced pressure before use. CuCl (Merck, 97%) was washed three times by glacial acetic acid, absolute ethanol and diethyl ether in turn and then dried under vacuum. N,N,N',N",N"-pentamethyldiethylenetriamine (PMDETA) (Merck, 99.8%) as a ligand and THF as a solvent were used without further purification. Cloisite 30B as an organically modified nanoclay (modifier: (bishydroxyethyl)methyl tallow quaternary ammonium cation) with cation exchange capacity of 90 mil-liequivalents per 100 g clay was purchased from Southern Clay Products Inc. CCl3-terminated PVAc telom-
er with 91.2% end functionality (i.e., telomer percentage), Mn = 1370 g mol-1 (calculated from 1H-NMR) and polydispersity index of1.85 (measured by GPC) was synthesized by telomerization of VAc monomer in the presence of chloroform at 60°C for 5 h [17-22] and used as a macroinitiator in the atom transfer radical homo- and copolymerization of St and MA (Table 1) [19].
Atom Transfer Radical Homo- and Copolymerization of St and MA
Reaction conditions are similar to those reported in previous work [19] with exception of using nanoclay in the present study. Nanoclay was dispersed in the monomer solution with continuous stirring for 3 h at 25°C (Table 1). To investigate possible penetration of macroinitiator into the nanoclay galleries, two additional solutions containing nanoclay, macroinitiator and monomer (one in the presence of pure St and another in the presence of pure MA) were prepared in the absence of ligand and CuCl. These two samples were dried and then analyzed by X-ray diffraction (XRD). In fact, these solutions have conditions same as those used in the atom transfer radical homopolymeriza-tions of St and MA at beginning of the reaction. To the rest of glass tubes, the ligand (PMDETA) and finally CuCl were added in the sequential order. Glass tubes then were sealed with rubber septums and were cycled between the vacuum and nitrogen three times. Molar ratio of ingredients [St+MA]0/[PMDE-TA] 0/[CuCl]0/[PVAc-CCl3]0 was kept constant in all reactions (300/2/1/1) (Table 1). Weight amount of nanoclay was also kept constant in all reactions (two parts per hundred monomers). Molar ratio of comonomers was only variable in the reaction mixture. The sealed and degassed tubes were then immersed in a pre-heated oil bath at a desired temperature (90 ± 0.1°C). At various time intervals tubes were removed from oil bath and reaction mixture was immediately diluted with THF under cooling and dried under vacuum. Mass conversion was then determined gravimetrically. Amounts of CuCl, nanoclay and macroinitiator were considered in the calculation of mass conversion. The dried polymer was purified (see the next section) and then used in 1H-NMR and GPC analyses. Samples were also taken from glass tubes in the case of homopolymerization reactions and then analyzed directly by 1H-NMR to measure unreacted monomers as well as monomer conversion. There was a good agreement between the conversions obtained from gravimetric and 1H-NMR analyses (see Table 2).
Characterization
Overall mass conversion of comonomers (X) was calculated gravimetrically. X-ray diffraction (XRD) patterns were obtained using a Philips Analytical X-ray
Table 2. Data obtained for the atom transfer radical homo- and copolymerization of St and MA initiated with CCl3-PVAc macroinitiator in the presence of nanoclay at 90°C
Exp. No. X, %a — b . Mn, theor. , g mol 1 Mn, GPC, g mol 1 PDI
Telomer 0 1370 1380 1.85
MS1.0 19.60 (19.72)c 7496 7680 1.51
MS0.9 29.08 10298 10560 1.63
MS0.7 27.91 9638 9271 1.48
MS0.5 29.46 9775 10109 1.53
MS0.3 25.44 9284 9489 1.28
MS0.1 25.35 8055 8180 1.35
MS0.0 13.27 (13.43)c 4796 5168 1.57
Notes: a Overall mass conversion was obtained gravimetrically.
Mn, theor ^
[Mi ]0
-—-i xMn M
[ CCl3 - PVAc] 1 i Mi
+ M,
n, macroinitiator
in which M,
n, macroin
i
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