Mechanistic Studies of Redox-Switchable Copolymerization of Lactide and Cyclohexene Oxide by a Zirconium Complex

Abstract

Several aspects of the copolymerization of L-lactide (LA) and cyclohexene oxide (CHO) by a redox switchable zirconium catalyst, (salfan)Zr(O t Bu)2 (salfan = 1,1'-di(2-tert-butyl-6-N-methylmethylenephenoxy)ferrocene), were examined such as the mechanism of cyclohexene oxide polymerization, the reactivity of [(salfan)Zr(O t Bu)2][BAr F ] (BAr F = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) toward lactide, and co-monomer effects on polymerization rates. Experimental methods and DFT calculations indicate that the likely mechanism of CHO polymerization by [(salfan)Zr(O t Bu)2][BAr F ] is coordination insertion and not a cationic pathway, as employed by the majority of cationic catalysts. Furthermore, DFT calculations showed that the polymerization of LA by [(salfan)Zr(O t Bu)2][BAr F ] is not thermodynamically favored, in agreement with experimental results. Lastly, we found that the conversion times of CHO or LA from block to block correlate with the amount of monomer left from the previous block rather than other factors.

Coordination Ring-Opening Polymerization of Cyclic Esters: A Critical Overview of DFT Modeling and Visualization of the Reaction Mechanisms

“…As mentioned above, redox-switchable catalyst 35 (Figure 16) exhibits different activities in TMC ROP depending on an oxidation state of Fe in ferrocene moiety [110] (see Section 3). A closely related study of l- LA ROP was implemented by Diaconescu et al [137] for post-metallocene Zr complex 59 (Figure 19). This study included DFT modeling at the B3LYP/6-31G(d)-LANL2DZ [51,53,54,78] level that resulted in the paradoxical result of the formation of Zr/Fe (III) ring-opened chelate complex from initial chelate formed after first LA insertion with a total change in free energy of +24.7 kcal/mol.…”

“…Mononuclear metal complexes studied in DFT modeling of lactide ROP [50,75,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140]. …”

Exploring Oxidation State-Dependent Selectivity in Polymerization of Cyclic Esters and Carbonates with Zinc(II) Complexes

“…However, in the case of the reduced species, a direct zinc-phosphine interaction was not observed. Despite this finding, and because of our interest in redox-switchable catalytic processes ( Abubekerov et al., 2016 , Abubekerov et al., 2017 , Wang et al., 2014 , Wang et al., 2015 , Quan and Diaconescu, 2015 , Brosmer and Diaconescu, 2015 , Abubekerov and Diaconescu, 2015 , Upton et al., 2014 , Broderick et al., 2011a , Broderick et al., 2011b , Broderick et al., 2011c , Quan et al., 2016 , Quan et al., 2017 , Lowe et al., 2017 , Shepard and Diaconescu, 2016 ), we set out to investigate the influence of the redox state of fc P,B on the zinc-mediated ring-opening polymerization of cyclic esters and carbonates. In addition, an investigation into the redox and polymerization activity of a monomeric ferrocene-chelating heteroscorpionate zinc complex, (fc P,B )Zn(OPh), is reported.…”

Geometry Change in a Series of Zirconium Compounds during Lactide Ring-Opening Polymerization

“…Long's paper, the ring opening polymerization of L-LA with the reduced state catalyst was significantly slower than that with the oxidized state catalyst, which represented an opposite trend in catalytic activity as compared with most catalysts employed in redox switchable ring opening polymerization. [32][33][34][35][36][37][38][39][40][41][42] However, if the catalyst was oxidized in the presence of L-LA and then reduced back, the reduced species showed a dramatically increased polymerization rate. Once it was oxidized back in situ, the newly formed compound had no activity toward the ring opening polymerization of LA anymore.…”