Novel Initiating System for the Stereocontrolled Radical Polymerization of Acrylamides: Alkyl Bromide/Rare Earth Metal Triflate System

Abstract

ABSTRACT:A novel radical initiating system -rare-earth metal triflate [Y(OTf) 3 , Yb(OTf) 3 , Lu(OTf) 3 , Sc(OTf) 3 ] and alkyl halide containing -esters (ethyl 2-bromoisobutyrate and ethyl 2-bromopropionate) or -amide (N,N-dimethyl-2-bromo-2-methylpropanamide) was used for the stereocontrolled radical polymerization of N-isopropylacrylamide (NIPAM). The Lewis acids serve a dual role as the promoter of the radical initiation and the controller of the stereospecific polymerization. The effects of the Lewis acid type and concentration, the initiator type and concentration, solvents, and temperature on the polymerization of NIPAM have been reported. The chain-end structure of the resulting polymer was determined by 1 H NMR. We also extended this initiating system for the polymerization of 1 Recent progresses in controlled radical polymerization 2-5 have widened the scope of the radical initiators, one of which should be an initiating system consisting of an alkyl halide and a late transition metal complex of ruthenium, iron, copper, etc.3 In this system, the initiating radical is born from the alkyl halide due to the metal-mediated cleavage of the carbon-halogen bond. The organic halide is thus regarded as an initiator, which generates an initiating radical species, while the metal complex as a catalyst, which mediates the radical formation via the reversible one-electron redox reaction of the metal center. This transitionmetal catalyzed radical polymerization system originates from a similar metal-catalyzed radical addition reaction as already reported. 6 An alternative way for the generation of the radical species from an alkyl halide is to use a free radical initiator like AIBN and Et 3 B/O 2 together for the halogen abstraction.6 This combination is often employed for the organic synthesis of small and sometimes cyclic compounds in atom-transfer free radical addition. 6 Recent progress in this area concerns the addition of a Lewis acid such as metal triflates [M(OTf) n , M ¼ Sc, Yb, etc.] into the addition reaction between an -halo ester, nitrile, or amide and an olefin in the presence of Et 3 B/O 2 , where the added Lewis acid improves the reactivity of the slow atomtransfer reaction. [7][8][9][10][11][12] The improvement stems from the weakening of the C-Br bond due to the coordination of the Lewis acid to the carbonyl or the nitrile group of the -halo ester, nitrile, or amide compounds. 12-14This results in an increase in both the rate of the addition of the -carbonyl radical to the olefin and also the halogen abstraction by the resulting nucleophilic carbon radical.Quite recently, we have found that the metal triflates also dramatically accelerated the radical polymerization of acrylamides and other meth(acrylic) monomers with AIBN. [15][16][17][18][19][20][21][22] The Lewis acids further affected the stereochemistry of the polymers. Especially, for the acrylamides, highly isotactic polymers were obtained with rare-earth metal triflates. The increase in the rate and the stereoregularity is considered to...

Ligand- and solvent-free ATRP of MMA with FeBr3 and inorganic salts

“…Alternatively, the interaction between KBr and FeBr3, yielding K[FeBr4] may enhance the Lewis acidity of K + , resulting in the same homolytic C-Br bond labilisation. 39 The second experiment, using KOH, leads to the GC trace shown in Figure 9.2. In this case, the activation is very efficient, leaving no residual EBrPA after the 2 h heating period.…”

Study of theThermo-/pH-Sensitivity of Stereo-Controlled Poly(N-isopropylacrylamide-co-IAM) Copolymers via RAFT Polymerization

“…Controlled-living radical polymerization, especially reversible addition-fragmentation chain-transfer (RAFT) polymerization, provides the benefit of being able to readily synthesize polymers with predetermined molecular structures and narrow molecular weight distributions [28,29,30,31]. In addition, RAFT polymerization can be applied to bulk, solution, emulsion or suspension polymerizations.…”

“…Thiocarbonylthio compounds, including dithoester and trithiocarbonate, are commonly used as RAFT agents. The addition of Lewis acid during polymerization can affect the stereochemistry of the polymers [19,29,30,31].…”

Self-Templated Free-Radical Polymerization To Form Tactic Chains in Confined Environment

“…Living radical polymerization methods, such as atom transfer radical polymerization (ATRP) , and reversible addition–fragmentation chain transfer (RAFT), , are effective in controlling block copolymers, but have yet to show the capability in affecting the stereochemistry of the polymers. Stereoselective polymerization based on the chain polymerization mechanism has been attempted by using auxiliaries, such as metal triflates, to introduce stereoselectivity into the chain-propagating site. , Nevertheless, the use of strongly associative metal triflates calls for an addition purification step in the postpolymerization treatment. Recently, stereoregular polymers have been synthesized by utilizing hydrogen bonding rather than coordination to an organotransition metal complex to sterically control the reactive radical sites .…”