No abstract

Higher ring-opening metathesis propagation rates of exo-norbornene derivatives over endo derivatives are well established in the literature. Here, we report for the first time that endo-isomers of oxanorbornene derivatives show...

Polycondensation polymers typically follow step growth kinetics assuming all functional groups are equally likely to react with one another. If the reaction rates with the chain end can be selectively accelerated, living polymers can be obtained. Here, we report on two chlorophosphonium iodide reagents that have been synthesized from triphenylphosphine and tri(o-methoxyphenyl)phosphine. The former activates aromatic carboxylic acids as acid chlorides in the presence of secondary aromatic amines and the latter even in the presence of primary aromatic amines. These reagents allow p-aminobenzoic acid derivatives to form solution stable activated monomers that polymerize in a living fashion in the presence of amine initiators. Other aryl amino acids and even dimers of aryl amino acids can be polymerized in a living fashion when slowly added to the phosphonium salt in the presence of an amine initiator. Diblock copolymers as well as a triblock terpolymer of aryl amino acids could be prepared even in the presence of electrophilic functional groups. preventing classical step-growth kinetics. The polymers obtained by this elegant method showed narrow dispersity and the possibility to form block copolymers. Poly(aromatic amides) are amongst the best investigated polymers in this context. However, low temperatures, the use of strong bases and several known side reactions 14 limit this technique as far as the average molar mass of the polymers is concerned (Mn=22 kDa). 15 As strong nucleophiles are required throughout the polymerization, the method is limited to monomers devoid of electrophilic functional groups. In particular, this technique could not yet be applied to the growing field of aromatic amide foldamers which aims at mimicking the function of biological macromolecules with similarly sized synthetic oligo and polyamides.

We describe a protocol to synthesize alternating telechelic ROMP copolymers of 7-oxa-norbornene derivatives and cycloalkenes under catalytic conditions. These copolymers were synthesized using Grubbs' second-generation catalyst. The sterically less hindered backbone double bonds of the resulting alternating copolymers facilitate the chain transfer (secondary metathesis) reactions. In the presence of symmetrical chain transfer agents (CTA), alternating copolymers could be synthesized catalytically. This procedure allows the synthesis of telechelic polymers based on potentially functional 7-oxanorbornene derivatives under thermodynamic equilibrium conditions. The molar mass of the alternating copolymer was controlled by the monomer to CTA ratio. The end group of the copolymers synthesized in the catalytic manner was addressed by the CTA functionality, yielding telechelic copolymers in excellent yields. 1 H NMR spectroscopy, MALDI-ToF mass spectrometry, and SEC analysis confirmed the chemical identity of the alternating telechelic copolymers with excellent control over the molar mass.

Well-defined telechelic poly(benzamide)s were synthesized by chain-growth polycondensation of phenyl-4-amino benzoate and pentafluorophenyl-4-amino benzoate derivatives with a bifunctional initiator in the presence of LiTMP as base. The polymerization was carried out at −70°C to prevent self-initiated polymerization. To confirm the control over molecular weight, different defined molecular weight polymers were synthesized and analyzed by GPC. Taking advantage of the labile ester end groups of these poly(benzamide)s, we carried out postpolymerization modifications to introduce different end functional groups such as alkyne, amine, alcohol, alkyl halide, and olefin suitable for different types of postpolymerization reactions. Successful end group modification was confirmed by 1 H NMR spectroscopy and isotopically resolved MALDI-ToF mass spectrometry.

For many decades, it has been challenging to synthesize auxetic materials at the molecular level. Auxetic materials exhibit counterintuitive behavior; they expand perpendicularly to the direction in which they are stretched. An aromatic macrocycle containing a sequence of N-substituted and N-unsubstituted amides was designed to resemble the re-entrant structure found in macromolecular auxetic materials. Upon application of tensile force, bent cis amides change to linear trans amides. This was anticipated to trigger the expansion of the macrocycle perpendicular to the direction of the applied force. To investigate the proposed configurational change by atom force microscopy (AFM), we designed and prepared a cis–trans aramide motif incorporated into an end-functionalized polymer which ensured covalent attachment to the AFM tip. At large extensions, polymer chains were envisioned to unfold and induce cis–trans isomerization.