Bis(Thio)ureas catalysts for Ring Opening Polymerization

Over the past two decades, significant progress has been made in organocatalysis. While widely studied in organic synthesis, their application in polymerization has received less attention. Organocatalysis offers advantages over metal catalysis, such as lower cost, stability against protic impurities, and high functional group tolerance. However, organocatalysts are less reactive, require higher loadings, and often fail under industrial conditions.[1][2] Most of the published research on organocatalysed polymerization focuses on the cyclic esters ring opening polymerization (ROP) for the synthesis of aliphatic polyesters. The two-component systems formed by a hydrogen bond donor and a hydrogen bond acceptor are the most abundant.[3] The stereoselective organocatalysed polymerization, remains unexplored; stereocontrol in certain polymerization reactions is crucial as the tacticity of the resulting polymer drastically influences the physical, chemical and mechanical properties of the final material. An emblematic example concerns Takemoto thiourea in chiral form.[4] Based on their premises, this work presents two-component catalysts composed of bisureas and bisthioureas with a chiral binaphthyl bridge as hydrogen bond donor and a base as hydrogen bond acceptor. These catalysts were investigated to understand the stereochemical control mechanisms in the ROP of lactide. Polymerization tests were conducted using 1,8-diazabiciclo[5.4.0]undec-7-ene (DBU) as hydrogen bond acceptor of the two-component system, methanol as initiator alcohol and dichloromethane as solvent. Microstructural analysis of the polymer and comparison with Bernoullian and non-Bernoullian statistics, show that both stereocontrol mechanisms, enantiomorphic site control (ESC) and chain-end control (CEC), are operative, with ESC being the predominant one. The study of these catalysts was continued by investigating how each enantiomer of the catalyst preferentially interacts with a specific enantiomer of the monomer. The reaction conditions for these catalytic systems were studied to assess their effect on stereocontrol, lowering the temperature to -10 °C and -78 °C. Additional tests were conducted to evaluate how other reaction parameters, such as the choice of solvent or the use of a different base as a hydrogen bond donor, could influence the progress of the reaction to investigate these effects, polymerization tests were performed using toluene or tetrahydrofuran as solvents and DBU was replaced with alternative bases such as DMAP or (-)-nicotine.