A DFT study of various model systems has addressed the interference of catalytic chain transfer (CCT) as a function of the R2 substituent in the atom‐transfer radical polymerization (ATRP) of styrene catalyzed by [FeCl2(R1N?C(R2)?C(R2)?NR1)] complexes. All model systems used R1=CH3 in place of the experimental Cy and tBu substituents and 1‐phenylethyl in place of the polystyrene (PS) chain. A mechanistic investigation of 1) ATRP activation, 2) radical trapping in organometallic‐mediated radical polymerization (OMRP), and 3) pathways to the hydride CCT intermediate was conducted with a simplified system with R2=H. This study suggests that CCT could occur by direct hydrogen‐atom transfer without any activation barrier. Further analysis of more realistic models with R2=p‐C6H4F or p‐C6H4NMe2 suggests that the electronic effect of the aryl para substituents significantly alters the ATRP activation barrier. Conversely, the hydrogen‐atom‐transfer barrier is essentially unaffected. Thus, the greater ATRP catalytic activity of the p‐NMe2 system makes the background CCT process less significant. The DFT study also compares the [FeCl2(R1N?C(R2)?C(R2)?NR1)] systems with a diaminobis(phenolato) derivative for which the CCT process shows even greater accessibility but has less incidence because of faster ATRP chain growth and interplay with a more efficient OMRP trapping. The difference between the two systems is attributed to destabilization of the FeII catalyst by the geometric constraints of the tetradentate diaminobis(phenolato) ligand. 相似文献
An unprecedented level of control for the radical polymerization of vinylidene fluoride (VDF), yielding well‐defined PVDF (at least up to 14 500 g mol?1) with low dispersity (≤1.32), was achieved using organometallic‐mediated radical polymerization (OMRP) with an organocobalt compound as initiator. The high chain‐end fidelity was demonstrated by the synthesis of PVDF‐ and PVAc‐containing di‐and triblock copolymers. DFT calculations rationalize the efficient reactivation of both head and tail chain end dormant species. 相似文献
Poly(vinyl acetate) by OMRP : Increasing the steric encumbrance of the β‐diketonate R substituents in vinyl acetate (VAc) polymerization mediator [Co{OC(R)CHC(R)O}2] from Me to tBu sufficiently weakens the CoIII? PVAc bond of the polymer chain to allow it to operate by both associative (degenerative transfer) and dissociative (organometallic radical polymerization, OMRP) mechanisms (see scheme). The CoIII? PVAc species also acts as a transfer agent in the absence of Lewis bases, whereas the CoII complex shows catalytic chain transfer (CCT) activity.
Controlled radical polymerization has come along in leaps and bounds following the development of efficient transition-metal catalysts for atom-transfer radical polymerization. Another type of controlled radical polymerization process, namely organometallic radical polymerization, uses the reversible formation of metal-carbon bonds. Metals are also implicated in catalytic chain transfer, a process that involves the abstraction of hydrogen atoms. This Minireview discusses the importance of one-electron transition-metal reactivity in metal-mediated controlled radical polymerization processes. 相似文献
A strategy that uses carbon monoxide (CO) as a molecular trigger to switch the polymerization mechanism of a cobalt Salen complex [salen=(R,R)‐N,N′‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐cyclohexanediamine] from ring‐opening copolymerization (ROCOP) of epoxides/anhydrides to organometallic mediated controlled radical polymerization (OMRP) of acrylates is described. The key phenomenon is a rapid and quantitative insertion of CO into the Co?O bond, allowing for in situ transformation of the ROCOP active species (Salen)CoIII‐OR into the OMRP photoinitiator (Salen)CoIII‐CO2R. The proposed mechanism, which involves CO coordination to (Salen)CoIII‐OR and subsequent intramolecular rearrangement via migratory insertion has been rationalized by DFT calculations. Regulated by both CO and visible light, on‐demand sequence control can be achieved for the one‐pot synthesis of polyester‐b‐polyacrylate diblock copolymers (?<1.15). 相似文献
Compound CpMoI2(iPr2dad) (iPr2dad = iPrNCHCHNiPr), obtained by halide exchange from CpMoCl2(iPr2dad) and NaI, has been isolated and characterized by EPR spectroscopy, cyclic voltammetry, and X-ray crystallography. Its action as a catalyst in atom transfer radical polymerization (ATRP) and as a spin trap in organometallic radical polymerization (OMRP) of styrene and methyl acrylate (MA) monomers has been investigated and compared with that of the dichloro analogue. Compound CpMoCl2(iPr2dad) catalyzes the ATRP of styrene and MA with low efficiency factors f (as low as 0.37 for MA and ethyl 2-chloropropionate as initiator), while it irreversibly traps the corresponding growing radical chains under OMRP conditions. On the other hand, compound CpMoI2(iPr2dad) has a greater ATRP catalytic activity than the dichloro analogue and yields f = 1 for MA and ethyl 2-iodopropionate as initiator. Under OMRP conditions, it does not irreversibly trap the growing radical chains. This comparison serves to illustrate the general principle that low initiator efficiency factors, sometimes observed in ATRP, may result from the interplay of the ATRP and OMRP mechanisms, when the latter ones involves an irreversible radical trapping process. 相似文献
Ambient temperature atom transfer radical polymerization (ATRP) of methyl acrylate (MA), methyl methacrylate (MMA) and styrene (Sty) in the presence of polar solvents (dimethyl sulfoxide: DMSO, dimethylformamide: DMF and acetonitrile: MeCN) with a mixed transition metal catalyst system (Fe(0) as initial activator and CuBr2/Me6TREN complex as deactivator) provides a rapid synthesis of polymers with very low polydispersity (PDI) values and predetermined molecular weights. The polymethylacrylate (PMA) prepared using this novel approach contains the Br-terminated chain ends (functionality ∼100%) and can be successfully used for block copolymer synthesis (as demonstrated on the chain extension experiment performed using the PMA–Br macroinitiator). The key elementary reactions involved in this novel ATRP system and some preliminary mechanistic aspects of the process are also discussed. 相似文献