Elucidating a Unified Mechanistic Scheme for the DBU-Catalyzed Ring-Opening Polymerization of Lactide to Poly(lactic acid)

Abstract

The synthesis of poly(lactic acid), PLA, is facile in the presence of the cyclic, organic amidine catalyst 1,8-diazabicyclo[5.4.0]undec-7-ene, DBU. Since DBU’s catalytic capability was first reported by Lohmeijer and colleagues in 2006 for ring-opening polymerizations (ROP), there have been numerous studies conducted by a variety of groups on the catalytic functioning of DBU in the ROPs of cyclic esters resulting in a large body of un-unified material from a mechanistic standpoint. This lack of clarity will hamper engineering polymers with desired characteristics from cyclic ester and lactone monomers. The work outlined in this paper seeks to propose a unified picture of the mechanisms in the DBU catalyzed ROP of lactide. In providing this unified picture of the ROP our work encompassed: (i) proposing a detailed reaction network scheme, (ii) conducting syntheses of lactide and DBU over a range of initial concentrations, and (iii) kinetic modeling to further support the proposed reaction network. As a result, our work has produced: (i) kinetic data, (ii) a consistent, viable reaction scheme verified through kinetic modeling, (iii) deduced and quantified the interplay between polymerization routes facilitated by the presence of DBU, thus demonstrating the need for detailed kinetic studies to deconstruct complex reaction networks, (iv) the first experimental evidence in support of the combination of ketene aminal-ended chains with alcohol-ended chains, and (v) analyzed the robustness of the catalyst to acid contamination.

Dormant Polymers and Their Role in Living and Controlled Polymerizations; Influence on Polymer Chemistry, Particularly on the Ring Opening Polymerization

“…Recently, a comprehensive study of the lactide LA polymerization with DBU/alcohol has been published [49]. This study encompasses the earlier works and shows, in agreement with previous observations, a simultaneous coexistence of an unreacted DBU, DBU H-bonded with growing macromolecules and dormant HO-ended macromolecules.…”

TBD‐Catalyzed Ring‐Opening Polymerization of Alkyl‐Substituted Morpholine‐2,5‐Dione Derivatives

“…One could thus speculate that the activation of the carbonyl moiety by the catalyst is hampered due to steric hindrance of the iso-butyl substituent of M4 (compare structures depicted in Figure 2). Current state of the art describes the ROP of lactones using organobases to proceed via two pathways: [39,40] A) An activated alcohol pathway resulting in polymer chains carrying the according alcohol moiety as α-end group, and B) a nucleophilic attack of the catalyst at the monomer that would produce TBD initiated chains in our case. To investigate if the end groups of the PEAs PM3-PM5 could be controlled by the established ROP conditions, MALDI TOF MS measurements were performed from samples taken after 10 min using 2,5-dihydroxybenzoic acid (DHB) as a matrix (Figure 3).…”

Controllable synthesis of bio‐based polylactide diols using an organocatalyst in solvent‐free conditions

“…Interestingly, the calculated M n of diols was in good linear relation with [ M ] 0 /[CTA] 0 , and the linear fitting equations were given in Figure (a) with all the correlation coefficient ( R 2 ) of above 0.99, indicating the M n of PLA diols could be precisely controlled via simply altering the molar ratio of LA and CTA. Furthermore, the M n of diols is independent of the amount of catalyst, further demonstrating that every molecule of CTA participated in the initiation and propagation steps . As shown in Table , there is some discrepancy between the GPC‐measured M n and the absolute M n , which was attributed to employing polystyrene standards on GPC measurement .…”

“…Sherck et al described activated‐alcohol pathway mechanism for DBU‐catalyzed ROP of LA when the molar ratio of [ROH]:[DBU] is 1:1 or slightly greater, ensuring that conducted polymerizations favor “living” characteristics . However, in all the aforementioned synthesis of PLA diols, the molar ratio of [BDO]:[DBU] is (36–351):1, which is far larger than 1:1, indicating that BDO not only acted as an initiator to participate in the polymerization.…”