Lewis acids with ATRP

Lewis acids (dicthylaluminum chloride, ethyl aluminum scsquichloridc) have been used in conjunction with ATRP to provide greater alternating tendency in S-MMA copolytnerization.519 However, poor control was obtained because of interaction between the catalyst (CuCI/dNbpy) and the Lewis acid. Better results were obtained by RAFT polymerization/10 Copper catalysts, in particular Cu(lI)Br/PMDETA, have been shown to coordinate monomer but this has negligible influence on the outcome of copolymerization/6 ... 

Copper based systems are generally distinguished by their superior reactivity and their excellent compatibility with functional groups, whereas ruthenium complexes turned out to be less active despite the beneficial effect of Lewis acid cocatalysts. In view of the preliminary observations indicating that styrene polymerised under conditions used for olefin metathesis and/or cyclopropanation, we were prompted to probe the performance of some ruthenium complexes under ATRP conditions. 

A special case of control over the structure of copolymers may include the first stereoblock copolymers made by CRP. By applying either RAFT or ATRP to polymerization of acrylamides in the presence of rare-earth triflates such as Y(OTf)3 and Yb(OTf)3, it was possible to enhance isotacticity of A(A/-dmiethylacrylamide (DMA) from 50% meso to 90% meso dyads (170). At the same time control of molecular weights and polydispersity was preserved. Similar results were obtained for RAFT of A(-ispropylacrylamide (171). The ATRP and RAFT of DMA was applied to the first one-pot stereoblock synthesis by radical mechanism. RAFT or ATRP of DMA were started without Lewis acid to produce the first atactic block. Subsequently, the complexing agent was added at the desired conversion to continue the chain growth with the preferential isotactic placement (170). 

While ATRP of methyl acrylate was reported only for the copper catalyst system [290-292] methyl meth(acrylate) was also polymerized with copper [290,293 295], ruthenium/aluminum alkoxide [296,297], iron [298,299] and nickel [300 303] eatalyst systems (Table 9). Thereby, it must be noted that in principle, the ruthenium-based system proposed by Sawamoto et al. requires the addition of Lewis acids, e.g., Al(0- -Pr)3 [297]. Recent investigations showed, that the half-metallocene -type ruthenium(II) chloride Ru(Ind)Cl(PPh3)2 (Ind = indenyl) led to a fast and well controlled polymerization even without the addition of Al(0- -Pr)3, whereas in case of a polymerization with Ru(Cp)Cl(PPh3)2 (Cp = cyclopentadienyl), the addition of Al(0-z-Pr)3 is necessary. The activity of Ru(II)-catalysts decreases in the order Ru(Ind)Cl(PPh3)2 > RuCl2(PPh3)2 > Ru(Cp)Cl(PPh3)2 [304]. 

Poly(N,N-dimethylacrylamide) with controlled MW, low values for M /Mn, and a high proportion of meso dyads (approx. 85%) was prepared using ATRP (methyl 2-chloropro-pionate/CuCl/MeeTREN) and RAFT (with cumyl dithiobenzoate transfer agent) in the presence of Y(OTf)3. These systems were used for the first one-pot synthesis of stereoblock copolymers by RP. Well-defined stereoblock copolymers, atactic-l -isotactic poly(N,N-dimethylacrylamides), were obtained by adding Y(OTf)3 to either an ongoing RAFT or ATRP polymerization, started in the absence of the Lewis acid. "... 

In general, the compounds of the Group 4 metals, such as halides and alkoxides, are well known as Lewis acids to catalyze two-electron electrophilic reactions, and their metallocenes coupled with alkylation and/or reduction agents were effective catalysts for the coordination polymerization of olefins. For the transition metal-catalyzed radical polymerization, their alkoxides, such as Ti(Oi-Pr)4, have also been employed as an additive for a better control of the products. Contrary to the common belief that the Group 4 metals rarely undergo a one-electron redox reaction under mild conditions, there have been some reports on the controlled radical polymerization catalyzed or mediated by titanium complexes, although the conflict in the mechanism between the (reverse) ATRP and OMRP is also the case with the Group 4 metal complexes. 

The living cationic polymerizations of some vinyl monomers with an initiating system based on alkyl halides and Lewis acids always leads to halogen-terminated polymers [134-136], which can be used as macroinitiators in subsequent ATRP without any modificaton [136 138]. For example, chloride-terminated polystyrenes, such as CLI-20, were obtained by living cationic polymerization of St using the l-PhEtCl/SnCU initiating system in the presence of tetrabuty-lammonium chloride at —15 °C in methylene chloride as shown in Scheme 3.32. 

Jiang J, Lu X, Lu Y. Stereospecific preparation of polyacrylamide with low polydisper-sity by ATRP in the presence of Lewis acid. Polymer. 2008 49 1770-1776. 

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