First Use of Thiosquaramides as Polymerization Catalysts: Controlled ROP of Lactide Implicating Key Secondary Interactions for Optimal Performance

We herein report the first use of thiosquaramides as polymerization catalysts, which are shown to be effective for the controlled ROP of lactide in the presence of an alcohol source and NEt3. Comparison of their catalytic performances with the less acidic squaramides are also discussed. The observed catalytic activity of variously N-substituted thiosquaramides suggest that a balanced NH Bronsted acidity is required for optimal performance. Most interestingly and rather unexpectedly, DFT-supported calculations on thiosquaramide-mediated lactide ROP catalysis suggest that secondary interactions between the thiosquaramide N-substituents and the incoming lactide (presently of type C-H & sdot;& sdot;& sdot;pi-arene) are crucial for catalytic activity. Though this type of interactions is quite common in organo-catalysis, it has rarely been evidenced to play a key role in the area of organo-catalyzed polymerizations. Such catalyst substituents/substrate interactions may well play a significant role in the catalytic performances of various other systems. The use of thiosquaramides as polymerization catalysts is shown to be effective for the controlled ROP of lactide in the presence of an alcohol source and NEt3. Comparison of their catalytic performances with less acidic squaramides is also discussed. DFT-supported calculations on thiosquaramide-mediated lactide ROP catalysis suggest that secondary interactions between the thiosquaramide N-substituents and the incoming lactide (presently of type C-H & sdot;& sdot;& sdot;pi-arene) are crucial for catalytic activity.image

Automated summary

We report the first use of thiosquaramides as polymerization catalysts, showing their effectiveness for controlled ring-opening polymerization of lactide in the presence of an alcohol source and NEt3. Comparison with less acidic squaramides reveals the importance of a balanced NH Brönsted acidity for optimal catalytic performance. Surprisingly, DFT-supported calculations demonstrate that secondary interactions between thiosquaramide N-substituents and the incoming lactide are crucial for catalytic activity, which is rarely observed in the field of organo-catalyzed polymerizations. Such findings suggest the significance of catalyst substituent/substrate interactions in various other systems.