Monodisperse Cyclic Polymer Mechanochemistry: Scission Kinetics and the Dynamic Memory Effect with Ultrasonication and Ball-Mill Grinding

A series of monodisperse cyclic and linear poly(d,l-lactide)s (c-PLA and l-PLA, respectively) were prepared with various degrees of polymerization (DP) using an iterative convergent synthesis approach. The absence of a molecular weight distribution provided us a chance to study their mechanochemical reactivity without obstructions arising from the size distribution. Additionally, we prepared l- and c-PLAs with identical DPs, which enabled us to attribute differences in scission rates to the cyclic polymer architecture alone. The polymers were subjected to ultrasonication (US) and ball-mill grinding (BMG), and their degradation kinetics were explored. Up to 9.0 times larger scission rates were observed for l-PLA (compared to c-PLA) with US, but the difference was less than 1.9 times with BMG. Fragmentation requires two backbone scission events for c-PLA, and we were able to observe linear intermediates (formed after a single scission) for the first time. We also developed a new method of studying the dynamic memory effect in US by characterizing and comparing the daughter fragment molecular weight distributions of l- and c-PLAs. These results provide new insights into the influence of the cyclic polymer architecture on mechanochemical reactions as well as differences in reactivity observed with US and BMG.

Understanding the Structure-Polymerization Thermodynamics Relationships of Fused-Ring Cyclooctenes for Developing Chemically Recyclable Polymers

Polymers that can be chemically recycled to their constituent monomers offer a promising solution to address the challenges in plastics sustainability through a circular use of materials. The design and development of monomers for next-generation chemically recyclable polymers require an understanding of the relationships between the structure of the monomers/polymers and the thermodynamics of polymerization/depolymerization. Here we investigate the structure-polymerization thermodynamics relationships of a series of cyclooctene monomers that contain an additional ring fused at the 5,6-positions, including trans-cyclobutane, trans-cyclopentane, and trans-five-membered cyclic acetals. The four- and five-membered rings trans-fused to cyclooctene reduce the ring strain energies of the monomer, and the enthalpy changes of polymerizations are found to be in the range of -2.1 to -3.3 kcal mol^-1. Despite the narrow range of enthalpy changes, the ceiling temperatures at 1.0 M span from 330 to 680 °C, due to the low entropy changes, ranging from -2.7 to -5.0 cal mol^-1 K^-1. Importantly, geminal substituents on the trans-five-membered cyclic acetal fused cyclooctenes are found to reduce the ceiling temperature by ∼300 °C, although they are not directly attached to the cyclooctene. The remote gem-disubstituent effect demonstrated here can be leveraged to promote depolymerization of the corresponding polymers and to tune their thermomechanical properties.

A Cyclic Ruthenium Benzylidene Initiator Platform Enhances Reactivity for Ring-Expansion Metathesis Polymerization

Ring-expansion metathesis polymerization (REMP) has shown potential as an efficient strategy to access cyclic macromolecules. Current approaches that utilize cyclic olefin feedstocks suffer from poor functional group tolerance, low initiator stability, and slow reaction kinetics. Improvements to current initiators will address these issues in order to develop more versatile and user-friendly technologies. Herein, we report a reinvigorated tethered ruthenium-benzylidene initiator, CB6, that utilizes design features from ubiquitous Grubbs-type initiators that are regularly applied in linear polymerizations. We report the controlled synthesis of functionalized cyclic poly(norbornene)s and demonstrate that judicious ligand modifications not only greatly improve kinetics but also lead to enhanced initiator stability. Overall, CB6 is an adaptable platform for the study and application of cyclic macromolecules via REMP.

Magnesium bromide (MgBr2) as a catalyst for living cationic polymerization and ring-expansion cationic polymerization

Magnesium bromide (MgBr₂) was found to be an effective catalyst for the ring-expansion cationic polymerizations of isobutyl vinyl ether (IBVE) initiated by a “cyclic” hemiacetal ester (HAE) bond-based initiator leading to the syntheses of cyclic poly(IBVE)s.

Hemiacetal Ester Exchanges, Study of Reaction Conditions and Mechanistic Pathway

A range of hemiacetal esters were synthesized by the reaction between carboxylic acids and butyl vinyl ether using n-dodecyl dihydrogen phosphate as catalyst. Specifically, nonanoic, propionic, acrylic, sebacic, and fumaric acids were used as substrates to prepare the corresponding hemiacetal esters. These compounds were used as model molecules to demonstrate the ability of hemiacetal ester functional groups to undergo the exchange reaction in the presence of weak carboxylic acids without any catalyst. Kinetics studies examined the eect of the carboxylic acid concentration on the exchange rate, and revealed that the exchange reaction proceeds through an associative mechanism.