Copolymerization of a series of cyclic acid anhydrides with several epoxides using (salen)CrCl/onium salt catalysts has afforded polyesters with high molecular weights and narrow molecular weight distributions. The (salen)CrCl catalyst in the presence of the onium salts with formula PPNX (X = Cl–, N3 –) for the copolymerization of the anhydrides, maleic (MA), succinic (SA), phthalic (PA), cyclohexene (CHE), and cyclohexane (CHA) with the epoxides, cyclohexene oxide (CHO), propylene oxide (PO), and styrene oxide (SO) resulted in completely alternating enchainment of monomers to provide pure polyesters. Temperature dependent studies of the ring-opening copolymerization of phthalic anhydride and cyclohexene oxide monomers in toluene solution have yielded activation parameters of ΔH ‡ = 67.5 kJ mol–1 and ΔS ‡ = −95.3 J mol–1, where the rate limiting step was ring-opening of the epoxide by the enchained anhydride. For the cyclic acid anhydride (CHA), the relative order of reactivity with epoxides decreased PO > CHO ≥ SO, and for the epoxide (CHO) the relative rate of copolymerization was CHA > PA > CHE. The (salen)CrCl/PPNN3 catalyst system was also shown to effectively terpolymerize CHO/phthalic anhydride/CO2 to afford diblock copolymers, thereby producing in a one pot synthesis poly(ester-co-carbonate). T g values of the synthesized polyesters displayed a temperature range over 130 °C, from +95 °C to −39 °C.

Chromium(III) derivatives of the tetradentate phenoxyamine ligand H 2 salan, where salan = the N,N 0 -dimethylated bis(aminophenoxide) ligand or the saturated version of the corresponding salen ligand, in the presence of 1 equiv of [PPN]N 3 (PPN=Ph 3 PdN þ dPPh 3 ) are shown to effectively catalyze the copolymerization of cyclohexene oxide and carbon dioxide. X-ray crystallographic analysis reveals the structure of the complex to be different from that of its salen analogue, with an all cis arrangement of the nitrogen and oxygen atoms. Although these catalysts are selective for copolymerizing propylene oxide and CO 2 at ambient temperature with a high degree of regioselectivity, the copolymerization of cyclohexene oxide and CO 2 requires higher temperatures (e.g., 60°C). Nevertheless, the random polymerization of cyclohexene oxide and propylene oxide with CO 2 at ambient temperature provides a terpolymer with a nearly statistical distribution of monomer units. In addition, these catalysts have been shown to be efficient at producing diblock copolymers of poly(propylene carbonate) and poly(cyclohexylene carbonate) as well as triblock copolymers of poly(propylene carbonate)/ poly(cyclohexylene carbonate)/poly(vinylcyclohexylene carbonate).

The selective transformation of CO2 and epoxides to afford completely alternating copolymers remains a topic of much interest for the potential utilization of carbon dioxide in chemical synthesis. The use of salicylaldimine (salen)‐metal complexes and their saturated (salan)‐metal versions have proven to be the most effective and robust single‐site catalyst for these processes. Herein, we report on mechanistic aspects of the copolymerization of alicyclic and aliphatic epoxides with CO2 in toluene solution and in neat epoxides in the presence of a (salan)CrCl/onium salt catalyst system. The activation barriers for both cyclohexene oxide(CHO)/CO2 and propylene oxide(PO)/CO2 were shown to be significantly higher in toluene solution than those previously reported for reactions carried out under solventless conditions. Terpolymerization of CHO/vinylcyclohexene oxide/CO2 was shown via Fineman‐Ross analysis at 60 °C to proceed with little monomer selectivity, for example, rCHO = 1.03 and rVCHO = 0.847. On the other hand, terpolymerization of CHO/PO/CO2 occurred at 25 °C with a propensity for incorporation of PO in the polymer. However, at 40 °C, Fineman‐Ross analysis revealed rCHO and rPO values of 0.869 and 1.49, thereby affording a terpolymer with a more equal distribution of monomers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

The scorpionate ligand, hydridotris(1-pyrazolyl)borate (Tp), is often considered as an equivalent of the more commonly used cyclopentadienyl (Cp) ligand in transition-metal-containing organometallic complexes. 1 Although both Tp and Cp are facially coordinating six-electron anionic donors, they have different steric and electronic characteristics. 2 With estimated cone angles of 262°for Tp and 150°for Cp, the former ligand is considerably more sterically demanding than Cp. However, the relative electron donor characteristics of the two ligands are not so straightforward and vary with the identity of the metal center, its oxidation state, and other coordinated ligands. 2 In some instances, steric factors have been judged to play a primary role in influencing the reactivity trends. 3 For example, a computational study of CÀH activation by Re complexes found that while oxidative addition of methane to CpRe(CO) 2 was exothermic, the same transformation was endothermic for TpRe(CO) 2 . 4 Steric differences between the Tp and Cp ligands were thought to play a primary role in the calculated reactivity difference. However, in some cases, differences in reactivity are the result of an electronic effect. For example, dissociation of the triflate ligand from the more sterically encumbered Tp Me 2 (PMe 3 )Ir-(Me)OTf (Tp Me 2 = hydridotris(3,5-dimethylpyrazolyl)borate) complex was found to be slower than from the analogous Cp* complex, pointing to an electronic rather than a steric effect imposed by the Tp Me 2 ligand. 5 Also, trends in the relative

The mechanism and energetics of the displacement of solvent from photolytically generated (eta(5)-DMP)Mn(CO)(2)(Solv) complexes has been studied [DMP = 2,5-dimethylpyrrole, Solv = solvent]. Rate enhancement relative to the eta(5)-cyclopentadienyl (Cp) system is not observed in the displacement of weakly bound solvents. The bond dissociation enthalpies obtained from the kinetic analysis are in good agreement with the values obtained by detailed density functional theory (DFT) calculations. The results indicate that for both the Cp and the DMP based systems the displacement of weakly bound solvents proceeds by a dissociative or I(d) mechanism. This is in sharp contrast to CO displacement from (eta(5)-DMP)Mn(CO)(3), which is known to proceed by an associative mechanism by way of an eta(3) ring slip intermediate. The associative substitution pathway only becomes competitive with the dissociative channel when the Mn-Solv bond dissociation enthalpy is more than 33 kcal/mol.