An Experimental and Computational Study on the Effect of Al(OiPr)₃ on Atom‐Transfer Radical Polymerization and on the Catalyst—Dormant‐Chain Halogen Exchange

Abstract

Compound Al(OiPr)3 is shown to catalyze the halide-exchange process leading from [Mo(Cp)Cl2(iPrN=CH-CH=NiPr)] and CH3CH(X)COOEt (X=Br, I) to the mixed-halide complexes [Mo(Cp)ClX(iPrN=CH-CH=NiPr)]. On the other hand, no significant acceleration is observed for the related exchange between [MoX3(PMe3)3] (X=Cl, I) and PhCH(Br)CH3, by analogy to a previous report dealing with the Ru(II) complex [RuCl2(PPh3)3]. A DFT computation study, carried out on the model complexes [Mo(Cp)Cl2(PH3)2], [MoCl3(PH3)3], and [RuCl2(PH3)3], and on the model initiators CH3CH(Cl)COOCH3, CH3Cl, and CH3Br, reveals that the 16-electron Ru(II) complex is able to coordinate the organic halide RX in a slightly exothermic process to yield saturated, diamagnetic [RuCl2(PH3)3(RX)] adducts. The 15-electron [MoCl3(PH3)3] complex is equally capable of forming an adduct, that is, the 17-electron [MoCl3(PH3)3(CH3Cl)] complex with a spin doublet configuration, although the process is endothermic, because it requires an energetically costly electron-pairing process. The interaction between the 17-electron [Mo(Cp)Cl2(PH3)2] complex and CH3Cl, on the other hand, is repulsive and does not lead to a stable 19-electron adduct. The [RuCl2(PH3)3(CH3X)] system leads to an isomeric complex [RuClX(PH3)3(CH3Cl)] by internal nucleophilic substitution at the carbon atom. The transition state of this process for X=Cl (degenerate exchange) is located at lower energy than the transition state required for halogen-atom transfer leading to [RuCl3(PH3)3] and the free radical CH3. On the basis of these results, the uncatalyzed halide exchange is interpreted as the result of a competitive S(N)i process, whose feasibility depends on the electronic configuration of the transition-metal complex. The catalytic action of Al(OiPr)3 on atom-transfer radical polymerization (and on halide exchange for the 17-electron half-sandwich Mo(III) complex) results from a more favorable Lewis acid-base interaction with the oxidized metal complex, in which the transferred halogen atom is bound to a more electropositive element. This conclusion derives from DFT studies of the model [Al(OCH3)3]n (n=1,2,3,4) compounds, and on the interaction of Al(OCH3)3 with CH3Cl and with the [Mo(Cp)Cl3(PH3)2] and [RuCl3(PH3)3] complexes.

Synthesis and Characterization of MoOI2(PMe3)3 and Use of MoOX2(PMe3)3 (X = Cl, I) in Controlled Radical Polymerization

“…In dichloromethane, this exchange is much slower, as no significant amount of mixed halide species (A) is formed after 20 h of stirring at room temperature. This phenomenon could either take place by an internal nucleophilic substitution assisted by solvent coordination to the metal center, as we have recently pointed out for other transition-metal complexes, [16] or by reversible atom transfer. In the latter case, the phenomenon would suggest that this system could serve as a good catalyst in atom transfer radical polymerization.…”

“…[14] In fact, whereas the Mo IV 3 , has revealed two possible mechanisms for this exchange process. [15,16] The results of DFT calculations have shown that the Mo-X and Mo-R bond strengths decrease upon raising the metal oxidation state, the computed values suggesting that the Mo III/IV redox couple is ideally placed to insure appropriate equilibrium positions for both ATRP and SFRP processes. Indeed, no radical polymerization occurs in the presence of the complex CpMo(CH 2 SiMe 3 ) 2 (η 4 -C 4 H 6 ), showing that this particular Mo III -R bond is too strong.…”

“…[13] The proposed mode of action of this cocatalyst consists of a shift of the atom transfer equilibrium, caused by the stronger interaction of the Al Lewis acid with the halogen lone pairs in the more polarized Mo-X bond, relative to the C-X bond of the initiator/dormant species. [16] Table 3 shows that Al(OiPr) 3 is also effective in conjunction with the MoOX 2 (PMe 3 ) 3 catalysts, although by a smaller margin. In each case, the apparent polymerization rate approximately doubles when 1 equiv.…”

Frontal Polymerization Synthesis of Starch‐Grafted Hydrogels: Effect of Temperature and Tube Size on Propagating Front and Properties of Hydrogels

“…The newly formed free radicals may recombine either with similar radicals or monomers to form random crosslinking networks. [25,26] This random linking of polymer chains tends to reduce sol content and increase gel content.…”

“…Figure 2 shows the dependence of front velocity on the tube size x at three initial temperatures (25,30, and 35 8C). For three sets the front velocity was found to follow a relationship of higher order with the tube size (linear fitting correlation coefficients, r 2 > 0.98), as in Equation (1):…”