Abstract
Ruthenium benzylidene complexes were evaluated as catalysts in atom-transfer radical polymerization (ATRP) of methyl methacrylate (MMA) under different reaction conditions. The mechanism by which Grubbs 1st and 2nd generation catalysts mediate olefin metathesis has been studied, little is known regarding the mechanism of ATRP reaction promoted by these complexes. Conversion and semilogarithmic kinetic plots as a function of time were correlated to the different catalysts and reaction conditions; especially in the presence of Al(OiPr)3 as additive. Molecular weight (Mn) and polydispersity index (PDI) values changed with different catalysts in the presence or absence of Al(OiPr)3. Kinetic studies by 1H NMR revealed that two complexes in the presence of Al(OiPr)3 are converted into ATRP-active with the dissociation of PCy3, but with the benzylidene group preserved. More controlled polymerizations were achieved using Grubbs 1st and, the presence of Al(OiPr)3 improved the control levels for both catalysts.
Highlights
Conversion was determined from the concentration of residual monomer measured by gas chromatography (GC) using a Shimadzu GC-2010 gas chromatograph equipped with flame ionization detector and a 30 m (0.53 mm i.d., 0.5 μm film thickness) SPB-1 Supelco fused silica capillary column
0.4 a[monomer]/[initiator]/[Ru] = 200/2/1 molar ratio, T = 85 °C, reaction time = 16 h; bdetermined from the concentration of residual monomer measured by gas chromatography (GC); cdetermined with size exclusion chromatography (SEC) with polystyrene calibration; dinitiation efficiency f = M /M n,theor n,exp with Mn,theor = ([monomer]/[initiator]) × Mw,(monomer) × conversion: e[Al(OiPr)3]/[Ru] = 4 molar ratio; f[monomer]/[initiator]/[Ru] = 800/2/1 molar ratio, T = 85 °C, reaction time = 16 h.22 was higher than 1 (f = 1.5), indicating the generation of additional polymer chains through transfer reactions (Table 1)
Studies show that the polymerization of vinyl monomer via atom-transfer radical polymerization (ATRP) mediated by Ru catalysts in the presence of metal alkoxides, for instance Al(OiPr)[3], in some cases, increases the polymerization rate and affords polymers of controlled molecular weights by interaction with the ruthenium complex and thereby stabilizes the higher oxidation state RuIII species to facilitate radical generation from a dormant species.[29]
Summary
Atom-transfer radical polymerization (ATRP) is a catalyst-based process which the growing radicals can be reversibly activated or deactivated via a dynamic equilibrium with a transition metal complex with an exchange of halogen atom between the chain and metal complex.[1,2,3,4,5,6,7,8,9,10,11,12,13] A variety of metal complexes used as catalysts in the reaction provides the control of the molecular weight distribution, which in turn can enable the facile synthesis of well-defined polymers.[14,15,16,17]Ruthenium-based metathesis catalysts have shown an excellent application profile in obtaining such polymers using the ATRP protocol.[18,19,20,21] This area of research has attracted widespread interest, since the first report of ruthenium-based catalysts, this system itself was not effective in methyl methacrylate (MMA) polymerization, but the addition of an aluminum alkoxides such as Al(OiPr)[3] accelerated the reaction and produced polymers with narrow with narrowThe present study is aimed to optimize the reaction conditions for the controlled polymerization of MMA by ATRP using 1 or 2. The linear dependence of ln ([M]0/[M]) on time, with kobs = 1.25 × 10−5 s−1 for 1 and kobs = 1.24 × 10−5 s−1 for 2, and the linear increase of molecular weight with conversion coupled with lower PDIs (Figures 2 and 3), illustrate an improvement of the control that 1 or 2 exert over MMA polymerization.
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