Ring-opening Polymerization Of ε-carprolactone Initiated By Yttrium Phenolate And Diol System: Experimental And DFT Computational Studies

Yttrium triphenolate [Y(OC₆H₅)₃] was studied in the ring-openingpolymerization (ROP) of e-carprolactone (CL). It was found that Y(OC₆H₅)₃ alonecould initiated the ROP of CL in 60 min at the temperature higher than 80℃. and thatit had good catalytic activity with the [CL]/[Y] ratio of 300. Although thepolymerization was not a living process, the molecular weight (M_n) of PCL increasedwith increasing the molar ratio of CL and Y(OC₆H₅)₃. The SEC curves weremonomodal and molecular weigth distribution (MWD) was moderate between 1.40and 1.74.When 1,2-propanediol was introduced, active species of yttrium alkoxides was insitu generated by the reaction of Y(OC₆H₅)₃ and 1,2-propanediol and thepolymerization exhibited a living characteristic. Polymerization of CL was carried outin toluene, achieving high yields (>95%) in 60 min at 100°C. When feed ratio[OH]/[Y] increasing from 8.7 to 28.9, the M,, values of the produced PCL calculatedby ¹H NMR analyses met the theoretical results quite well and the MWD data werereasonably narrow between 1.10 and 1.29. When the molar ratio of [OH]/[Y] wasfixed, the M_n value of obtained PCL was almost unchanged with the increasing ofmolar ratio of [OH]/[Y]. Therefore, M_n of PCL was controlled by adjusting the ratioof CL and 1,2-propanediol and independent with the concentration of Y(OC₆H₅)₃.Moreover, we found a living CL polymerization when the ratio of [OH]/[Y] reachedas high as 28.9 indicating an extremely fast exchange reaction between active chainend Y-OR and dormant hydroxyl end group H-OR.The structure of PCL initiated by 1,2-propanediol and Y(OC₆H₅)₃ was obtainedaccording to MALDI-TOF, ¹H NMR, ¹³C NMR, ¹H-¹H COSY and ¹H-(13)C HMQCanalyses. All proton and carbon atoms in the backbone, the end groups and theinitiator 1,2-propanediol residue of the obtained PCL were fully characterized. As theconclusion, both the primary and secondary hydroxyl groups of 1,2-propanediolmolecules initiated the ROP of CL simultaneously.The mechanism of polymerization of CL initiated by Y(OC₆H₅)₃ and 1,2-propanediol system was studied. The propagation mechanism of s-caprolactone(CL) insertion into Y-OCH₃ bond was investigated using density functional theory(DFT) calculations. The optimized geometries and corresponding Gibbs free energiesof the reactants, intermediates, transition states and products were obtained, whichconfirmed a four-step coordination-insertion mechanism.The coordination of CL ontoyttrium center led to a nucleophilic addition of the carbonyl group of CL, and then anintramolecular alkoxide ligand exchange reaction occured. A monomer insertion wascompleted by the CL ring opening via acyl-oxygen bond cleavage. The formation oftransition state TS, the five-coordinated yttrium complex, was found to be the ratedetermining step whose active Gibbs free energyâ–³G^≠at 298 K was +23.1 kcal/mol.The dipole moment values of intermediates 3 and 4 were smaller than those of reagent2 and product 5. Therefore, a solvent with low polarity stabilized 3 and 4 and a fastCL insertion reaction could be expected.â–³G^≠of intermolecular exchange reactionbetween yttrium alkoxide active center and free hydroxyl end group was +3.0kcal/mol. According to the transition state theory, intermolecular exchange reactionwas 4.25×10¹² times faster than chain propagation reaction. Therefore, yttriumalkoxide active center transfered extremely fast from one free hydroxyl end group toanother, which resulted in the living behavior of the CL ROP.During the CL polymerization initiated by Y(OC₆H₅)₃ and 1,2-propanediol, thehydroxyl groups involved in the exchange reaction with active Y-OR center includedthe primary hydroxyl groups of the dormant polymer chain ends and the primary andsecondary hydroxyl groups of the excess 1,2-propanediol molecules. Three kinds ofmodel reactions were calculated by DFT method and theirâ–³G^≠were 3.0 kcal/mol, 3.6kcal/mol and 18.5 kcal/mol at 373 K, respectively. Hydrogen exchange reaction rateswere respectively 4.25×10¹², 3.56×10¹² and 3.85×10³ times faster than chainpropagation reaction. Therefore, exchange reaction rates between different polymerchains or alcohols were much higher than CL propagation rate and thus the newcoming CL were not able to know which original hydroxyl group was the one it wasgoing to grow onto.The initiating system of Y(OC₆H₅)₃ with ethylene glycol and 1,3-propanediolwere also studied and the produced polymer were diol-embedded PCL. The primary hydrogen of ethylene glycol (or 1,3-propanediol) exhibited the same activity andinitiated the ROP of CL.In summary, experimental analyses and DFT calculations confirmed that bothprimary and secondary hydroxyl groups of diol molecules were able to initiate theROP of CL simultaneously in the presence of Y(OC₆H₅)₃ catalyst.