Biomedical subjects
Publications and source records attributed to Anja R A Palmans.
Racemic omega-substituted caprolactones can be completely converted into chiral polyesters of remarkable MW and high ee by combining lipase-catalyzed ring-opening polymerization with Ru-catalyzed racemization.
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Iterative tandem catalysis is presented as a flexible tool for obtaining chiral macromolecules from racemic or prochiral monomers. Here, we combine lipase-catalyzed ring-opening of omega-substituted lactones with ruthenium-catalyzed racemization. In a two-pot system, enantioenriched oligomers of 6-methyl-epsilon-caprolactone were synthesized, which could not have been obtained by enzymatic ring-opening alone. A one-pot experiment proved highly promising in developing a novel route toward enantiopure polyesters.
C(3)-symmetrical disks 1, preorganized by acylated 2,2'-bipyridine-3,3'-diamine moieties and decorated with nine identical chiral, lipophilic tails, aggregate into a dynamic helix in apolar solvents. The aggregates, previously shown to be governed by the "sergeants-and-soldiers" principle when mixed with achiral analogues, are now also revealed to obey the "majority-rules" effect, a phenomenon not earlier observed in nonpolymers. Our experimental circular dichroism data can be accurately described with a recently developed theory. A fit of the theory to the experimental results shows that the mismatch penalty, i.e., the free energy of a monomer present in a helix of its nonpreferred screw sense (0.94 kJ/mol), is about 8 times lower than the penalty for a helix reversal (7.8 kJ/mol). This corresponds well to our vision of the supramolecular architecture of the disks.
The enantioselective polymerization of methyl-substituted epsilon-caprolactones using Novozym 435 as the catalyst was investigated. All substituted monomers could be polymerized except 6-methyl-epsilon-caprolactone (6-MeCL), which failed to propagate after ring opening. Interestingly, an odd-even effect in the enantiopreference of differently substituted monomers was observed. The combination of 4-methyl-epsilon-caprolactone with Novozym 435 showed good enantioselectivity also in bulk polymerization and resulted in enantiomerically enriched P((S)-4-MeCL) (eep up to 0.88). Subsequently, a novel initiator combining a primary alcohol to initiate the ring opening polymerization and a tertiary bromide to initiate atom transfer controlled radical polymerization (ATRP) was synthesized, and showed high initiator efficiencies (> 90%) in the ring opening polymerization of 4-methyl-epsilon-caprolactone in bulk. In addition, the enantioselectivity was retained (E = 11). By using Ni(PPh3)2Br2 as the ATRP catalyst, Novozym 435 could be effectively inhibited at the desired conversion of 4-methyl-epsilon-caprolactone, thus ensuring a high enantiomeric excess in the polymer backbone. At the same time, Ni(PPh3)2Br2 catalyzed the ATRP of methyl methacrylate resulting in the formation of P((S)-4-MeCL-b-MMA) block copolymers. By this combination of two inherently different polymerization reactions, chiral P((S)-4-MeCL-b-MMA) block copolymers can be conveniently obtained in one pot without intermediate workup.