Metal-Free Ionic ROP

DBU was also shown to be a produce well ontrolled polymers (PDl 1.09-1.15) when ROP was performed in bulk at 65 °C. 

During the 1980s, Kricheldorf and coworkers also demonstrated the cationic ROP of lactide [48, 49]. In more recent study, Bourissou et al. showed that the application of trifluoromethanesulfonic acid (triflic acid) was able to efficiently catalyze the ROP of lactide at room temperature in the presence of a protic initiator [50]. The synthesis of PLAs with molecular weights up to 20000gmol and with relatively narrow polydispersities ( 1.5) was demonstrated, with the well-controlled character of the polymerizahons being proven by the linear correlation of molecular weight and monomer conversion throughout the polymerization. 

The ROP of lactones can also be achieved by the application of much milder organic acids. Several reports have detailed the ROP of e-caprolactone and 8-valerolactone where the polymerization has been catalyzed by additional lactic add [52, 53], fumaric acid [44], maleic add [44], tartaric acid [52], dtric add [52] and a range of amino acids [52, 54]. Tartaric acid was shown to the most active catalyst for the ROP of e-caproladone in a comparative study performed by Cordova and colleagues [52], The application of this methodology, in which the polymerizations are carried out in neat lactone at either 120 or 160 °C, results in the synthesis of polymers with molecular weights that agreed weU with the initial monomer initiator ratio, and with polydispersities ranging from 1.2 to 1.9. Extension to the synthesis of dendrimers such as star poly(ester)s [55] and the polymerization of e-caprolactone from carbohydrates [56] has also been reported. 

Organocatalytic cationic polymerization has also been reported for the ROP of TMC and DTC [57-59]. These methods displayed relatively poor control over the polymerization, and were characterized by the incomplete conversion of monomer and broad polydispersities that resulted from extensive back-biting of the polymer chains and decarboxylation. It is worth noting here that the ROP of oxazolines is usually performed by the application of metal-free cations (for an extensive reviewed of this subject, see Chapter 6). 

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