The ring opening polymerization (ROP) of lactide has been widely reported.2 However, most of the reactions use solvents and metal-catalysts, which are non-green elements. Using ball-mill, we could achieve more environmentally friendly polymerizations.3 The first example of mechanochemical ring opening polymerization of lactide is developed by ball-mill grinding. Mechanical energy from the collision between the balls and the mill beaker effectively promotes an organic based mediated metal- and solvent-free solid state polymerization. Importantly, we have found that the chains degradation, the inherent problem of polymer mechanochemistry, could be solved with liquid assisted grinding (LAG). Liquid assisted grinding accelerates the reaction rate and allows to obtain high molecular weight poly(lactic acid) (> 1 X 105 g/mol) by preventing polymer chain degradation from high energy of ball collision. We also examined frequency and ball effects and it is found that the strength of mechanical energy truly promoted the polymerization. References 1 N. Ohn, J. Shin, S. S. Kim, J. G. Kim, ChemSusChem DOI: 10.1002/cssc.201700873 2 Hu, Y., Daoud, W. A., Cheuk, K. K. L., Lin, C. S. K., Materials 2016, 9, 133. 3James, S. L.; Friščić, T. Chem. Soc. Rev. 2013, 42, 7494-7496.