An unanticipated mechanochemical behavior was identified for polynorbornenes with pendant pentafluorophenyl activated esters. Ring-opening metathesis polymerization of exo-2-hexyl-3-(pentafluorophenyl)norbornene-2,3-dicarboxylate was performed to obtain poly[exo-2,3-(pentafluorophenyl)norbornene-2,3-dicarboxylate], using Grubbs' first-generation catalyst to afford polymers with number-average molecular weights (Mn) from 20 to 70 kg/mol. The different polymers were then irradiated with high intensity ultrasound in acetonitrile at 6 to 9 �C for 2 h, and a distinct molecular weight dependence was found. Polymers with Mn > Mlim exhibited a significant reduction in molecular weight by gel permeation chromatography, while polymers with Mn below Mlim = 50 kg/mol displayed no apparent change. After ultrasonication, new signals corresponding to free pentafluorophenol were observed by 19F-NMR spectroscopy only for the high Mnpolymers. The appearance of these signals suggests that the reduction in molecular weight is due to the cleavage of fluorinated side-groups by ultrasound-induced shear forces. These unexpected results indicate that the mechanical activation can be extended to remote active groups and could be used to obtain diverse functionalities in polymers.

Mechanochemistry of pendant pentafluorophenyl ester

248th National Meeting (2014)

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