Frank Wiesbrock

The π-electron delocalization in 2-oxazolines revisited: Quantification and comparison with its analogue in esters

The single crystal X-ray analysis of the ester-functionalized 2-oxazoline, methyl 3-(4,5-dihydrooxazol-2-yl)propanoate, revealed π-electron delocalization along the N–C–O segment in the 2-oxazoline pentacycle to significant extent, which is comparable to its counterpart along the O–C–O segment in the ester. Quantum chemical calculations based on the experimental X-ray geometry of the molecule supported the conjecture that the N–C–O segment has a delocalized electronic structure similar to an ester group. The calculated bond orders were 1.97 and 1.10 for the N=C and C–O bonds, and the computed partial charges for the nitrogen and oxygen atoms of −0.43 and −0.44 were almost identical. In the ester group, the bond orders were 1.94 and 1.18 for the C–O bonds, while the partial charges of the oxygen atom are −0.49 and −0.41, which demonstrates the similar electronic structure of the N–C–O and O–C–O segments. In 2-oxazolines, despite the higher electronegativity of the oxygen atom (compared to the nitrogen atom), the charges of the hetero atoms oxygen and nitrogen are equalized due to the delocalization, and it also means that a cationic attack on the nitrogen is possible, enabling regioselectivity during the initiation of the cationic ring-opening polymerization of 2-oxazoline monomers, which is a prerequisite for the synthesis of materials with well-defined structures.

Microwave-Assisted Synthesis of Polyesters and Polyamides by Ring-Opening Polymerization

Microwave-assisted heating has been described as an efficient heating technique that can enhance the reaction rate for many reactions. Consequently, it is a key strategy for ring-opening polymerizations, which are often limited by low polymerization rates. This review summarizes recent efforts in the field of microwave-assisted polyester and polyamide syntheses from cyclic monomers and dimers, with a broad focus on poly(lactic acid)s and poly(e-caprolactone)s. Homo- and copolymerizations as well as graft polymerizations are discussed. Both the polymerizations themselves as well as the preparation of composites/materials are addressed. Special attention is directed towards the discussion of non-thermal microwave effects.

Living cationic ring-opening polymerization of 2-ethyl-2-oxazoline following sustainable concepts: Microwave-assisted and droplet-based millifluidic processes in an ionic liquid medium

The polymerization of 2-ethyl-2-oxazoline in an ionic liquid has been studied using batch and continuous processes, and different heating sources. The living radical polymerization has been performed in batch and in a droplet-based millifluidic device under conventional or microwave-assisted heating, from 80 to 140 °C. The use of a droplet-based fluidic process leads to an enhanced polymerization rate as demonstrated by the determination of the activation energy (Ea = 61 kJ mol−1) while keeping polymerization control and taking advantage of safer, versatile and cost-effective conditions.