Native Chemical Ligation: Ultrafast Synthesis of Block Copolymers

Orthogonal Covalent Entrapment of Cargo into Biodegradable Polymeric Micelles via Native Chemical Ligation

Polymeric micelles (PMs) are promising platforms for enhanced tissue targeting of entrapped therapeutic agents. Strategies to circumvent premature release of entrapped drugs include cross-linking of the micellar core as well as covalent attachment of the drug cargo. The chemistry employed to obtain cross-linked micelles needs to be mild to also allow entrapment of fragile molecules, such as certain peptides, proteins, oligonucleotides, and fluorescent dyes. Native chemical ligation (NCL) is a mild bio-orthogonal reaction between a N-terminal cysteine residue and a thioester that proceeds under physiological conditions. Here, we designed a trifunctional cross-linker containing two cysteine residues for the micelle core-cross-linking reaction and an azide residue for ring-strained alkyne conjugation of fluorescent dyes. We applied this approach to thermosensitive methoxypolyethylene glycol-b-N-(2-hydroxypropyl)methacrylamide-lactate (mPEG-b-HPMAmLacn) based block copolymers of a core-cross-linked polymeric micelle (CCPM) system by attaching thioester residues (using ethyl thioglycolate-succinic anhydride, ETSA) for NCL cross-linking with the trifunctional cross-linker under physiological conditions. By use of mild copper-free click chemistry, we coupled fluorescent dyes, Sulfo.Cy5 and BODIPY, to the core via the azide residue present on the cross-linker by triazole ring formation. In addition, we employed a recently developed cycloheptyne strain promoted click reagent (TMTHSI, CliCr) in comparison to the frequently employed cyclooctyne derivative (DBCO), both achieving successful dye entrapment. The size of the resulting CCPMs could be tuned between 50 and 100 nm by varying the molecular weight of the thermosensitive block and ETSA content. In vitro cell experiments showed successful internalization of the dye entrapped CCPMs, which did not affect cell viability up to a polymer concentration of 2 mg/mL in PC3 cells. These fluorescent dye entrapped CCPMs can be applied in diagnostic imaging and the chemistry developed in this study serves as a steppingstone toward covalently entrapped fragile drug compounds with tunable release in CCPMs.

Catalytic living ROMP: block copolymers from macro-chain transfer agents

Vinyl ether based macro-chain transfer agents (m-CTAs) are used to produce different di or tri-block copolymers under catalytic living ROMP conditions. Polystyrene (PS) vinyl ether m-CTA and polycaprolactone (PCL) or polylactide vinyl ether (PLA) m-CTAs are synthesized straightforwardly via ATRP and ROP respectively. Regioselectivity as well as the high metathesis activity of these m-CTAs enabled us to synthesise a range of metathesis-based A-B diblock copolymers with controlled dispersities (Đ < 1.4). In this manner, PS-ROMP (here, ROMP refers to a poly(MNI-co-DHF) block), PCL-ROMP and PLA-ROMP were synthesized using substoichiometric amounts of ruthenium complex in a living fashion. Also, a more complex PEG-PCL-ROMP tri-block terpolymer was obtained catalytically. All block copolymers were characterized by SEC and DOSY NMR spectroscopy. We believe that this methodology of using macro-chain transfer agents to prepare degradable ROMP polymers under catalytic living ROMP conditions will find applications in biomedicine.

Chain transfer agents for the catalytic ring opening metathesis polymerization of norbornenes

Here, we present a detailed study of the metathesis activity of conjugated 1,3 diene derivatives in ring opening metathesis polymerization (ROMP) using Grubbs' 3rd generation catalyst (G3). A comprehensive screening of those derivatives revealed that monosubstituted 1,3 dienes show similar reactivities towards G3-alkylidenes as norbornene derivatives. Therefore, they represent perfect candidates for chain transfer agents in a kinetically controlled catalytic ROMP. This unprecedented reactivity allowed us to catalytically synthesize mono-end-functional poly(norborneneimide)s on the gram scale. Much more complex architectures such as star-shaped polymers could also be synthesized catalytically for the very first time via ROMP. This inexpensive and greener route to produce telechelic ROMP polymers was further utilized to synthesize ROMP block copolymers using bifunctional ROMP and ATRP/NCL initiators. Finally, the regioselective reaction of G3 with 1,3 diene derivatives was also exploited in the synthesis of a ROMP-PEG diblock copolymer initiated from a PEG macroinitiator.