Cationic ring-opening polymerization of cyclic carbonates and lactones by group 4 metallocenes: A theoretical study on mechanism and ring-strain effects

Abstract

Group 4 metallocene-mediated cationic ring-opening polymerizations of a series of lactones and cyclic carbonates, with different ring sizes ([Formula: see text]–8) have been theoretically studied. Using the “naked cation” approach in combination with density functional theory, the activated chain-end mechanism and the influence of transition metals, solvent and monomer ring size on the polymerizability were explored in detail. The results showed that the cationic metallocene–monomer complex, [catalyst][monomer][Formula: see text], is formed, generating cationic (carbocation ion) species responsible for polymer chain growth. We found that poor polymerizability of five-membered lactone and six-membered ring carbonate depends not only on the nature of the monomer ring size but also the relative stability of the complex, which was found to correlate well with the ring strain. Subsequently, several propagation steps take place through an SN2 reaction which involves ring opening of an active monomer, via alkyl–oxygen bond cleavage. Based on the computed activation energies of all metallocene systems, the first propagation was found to be the rate-determining step of the overall propagation and the hafnocene was found to be most active with the energy barrier of 17.6[Formula: see text]kcal/mol, followed by zirconocene (18.6[Formula: see text]kcal/mol) and titanocene (19.5[Formula: see text]kcal/mol), respectively. The mechanistic study may be applicable to the cationic ROP of lactides and other related monomers.

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

“…Note that the results of these works are important for other zirconocene-catalyzed processes such as α-olefin polymerization since the reactivity of cationic complexes formed after the first TMC insertion can be seen as a measure of the electrophilicity of zirconium cationic catalytic center. Finally, it can be mentioned that the results of DFT modeling of TMC ROP were reported in some publications mentioned above [70,77].…”

“…The mechanism of ROP catalyzed by 14 slightly differ from this common scheme by the participance of non-coordinated nucleophilic agent (ROH). However, metal-catalyzed ROP of lactones may occur by a far different living cationic mechanism (Figure 15) that was studied in silico by Jitonnom and Meelua for (η 5 -C 5 H 5 ) 2 MMe cations 32 [77] at the B3LYP/6-31G*-LANL2DZ [51,53,54,78] level. These calculations were performed for gas phase and using CPCM solvent model for THF, CHCl 3 , toluene and hexane.…”

“…Metal complexes studied in DFT modeling of εCL ROP [55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77]. …”

Data on electronic structures for the study of ligand effects on the zirconocene-mediated trimethylene carbonate polymerization

“…The effective core potential double-ζ basis set (LANL2DZ) [6] was used for Zr atom, while a double-ζ basis set (6–31G(d)) for all non-metal atoms (C, H, O, F, Si, and Ge). This DFT/mixed basis set method has been shown to reproduce the X-ray structure [7] and has been widely applied in the field of transition metal complexes [7] , [8] , [9] , [10] , [11] , [12] , [13] , [14] , [15] , [16] . Frequency calculations were performed on all optimized structures to confirm the nature of stationary points as minima or transition states.…”

DFT study of formation and properties of dinuclear zirconocene cations: Effects of ligand structure, solvent, and metal on the dimerization process

“…Cationic alkyl complexes of Group 4 metallocenes of the type [Cp2MR] + (M = Ti, Zr or Hf, Cp = C5H5) have been recognized as the catalytically active species in olefin polymerization [1,2] and cationic polymerization [3] of polar cyclic monomers such as lactones and cyclic carbonates [4][5][6][7][8]. There are relatively few reports for the latter reactions by these catalysts, despite the fact that earlier studies [5,[9][10][11] have shown that zirconocene complex catalyst can be effective for the cationic ring-opening polymerization (CROP) of cyclic ester monomers.…”