Stereoselective Ring-Opening Polymerization of rac-Lactide Catalyzed by Squaramide Derived Organocatalysts at Room Temperature†

Main observation and conclusion Substantial progresses have been made toward the development of metal-free catalysts for stereoselective ring-opening polymerization (ROP) of rac-lactide. Yet the discovery of organic catalysts effective at ambient temperature remains a major challenge. Here, the bifunctional H-bonding catalyst SQ-1 containing a basic tertiary amine and squaramide motif proved to be good candidate for the stereoselective ROP of rac-lactide at room temperature, yielding stereoregular polylactide with controlled molecular weights (up to 21.1 kg/mol) and high tacticity (P-i(ESC) up to 0.88). Furthermore, binary H-bonding catalytic system consisting of squaramides (SQ-2 to SQ-6) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) could efficiently promote the ROP of rac-lactide at room temperature within short reaction time. Among them, the most bulky squaramide SQ-2 exhibited the best steroselectivity towards the ROP of rac-lactide without transesterification side reactions during the polymerization process. The resulting polylactides were proved to have controlled molecular weights as high as 22.2 kg/mol and narrow distributions (1.10-1.24).

Automated summary

A bifunctional H-bonding catalyst SQ-1 and the binary H-bonding catalytic system consisting of squaramides (SQ-2 to SQ-6) and DBU were discovered to efficiently promote the stereoselective ring-opening polymerization (ROP) of rac-lactide at room temperature. The resulting polylactides exhibited controlled molecular weights and high tacticity.