Nontoxic N-Heterocyclic Olefin Catalyst Systems for Well-Defined Polymerization of Biocompatible Aliphatic Polycarbonates

Herein, N-heterocyclic olefins (NHOs) are utilized as catalysts for the ring-opening polymerization (ROP) of functional aliphatic carbonates. This emerging class of catalysts provides high reactivity and rapid conversion. Aiming for the polymerization of monomers with high side chain functionality, six-membered carbonates derived from 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) served as model compounds. Tuning the reactivity of NHO from predominant side chain transesterification at room temperature toward ring-opening at lowered temperatures (-40 °C) enables controlled ROP. These refined conditions give narrowly distributed polymers of the hydrophobic carbonate 5-methyl-5-benzyloxycarbonyl-1,3-dioxan-2-one (MTC-OBn) (Đ < 1.30) at (pseudo)first-order kinetic polymerization progression. End group definition of these polymers demonstrated by mass spectrometry underlines the absence of side reactions. For the active ester monomer 5-methyl-5-pentafluorophenyloxycarbonyl-1,3-dioxane-2-one (MTC-PFP) with elevated side chain reactivity, a cocatalysis system consisting of NHO and the Lewis acid magnesium iodide is required to retune the reactivity from side chains toward controlled ROP. Excellent definition of the products (Đ < 1.30) and mass spectrometry data demonstrate the feasibility of this cocatalyst approach, since MTC-PFP has thus far only been polymerized successfully using acidic catalysts with moderate control. The broad feasibility of our findings was further demonstrated by the synthesis of block copolymers for bioapplications and their successful nanoparticular assembly. High tolerability of NHO in vitro with concentrations ranging up to 400 μM (equivalent to 0.056 mg/mL) further emphasize the suitability as a catalyst for the synthesis of bioapplicable materials. The polycarbonate block copolymer mPEG₄₄-b-poly(MTC-OBn) enables physical entrapment of hydrophobic dyes in sub-20 nm micelles, whereas the active ester block copolymer mPEG₄₄-b-poly(MTC-PFP) is postfunctionalizable by covalent dye attachment. Both block copolymers thereby serve as platforms for physical or covalent modification of nanocarriers for drug delivery.

Chiral Diboranes as Catalysts for the Stereoselective Organopolymerization of Epoxides

It is demonstrated that stereoselective polymerization of epoxides, long a domain of metal-based compounds, can also be achieved via the application of organocatalysts. A simple two-step synthesis starting from widely available 1,1′-bi-2-naphthol (BINOL) backbones yields diboranes which, in tandem with organobases, deliver isotactic-enriched (it) polyethers from the homopolymerization of racemic propylene oxide (PO) and other epoxides. Thereby, isotactic diad contents of up to 88% can be achieved, resulting in well-defined (1.1 < Đ_M < 1.3) polyethers with high molar masses (M_n > 100 000 g mol⁻¹). Notably, it is also possible to grow it-enriched sequences of PPO on aliphatic polyester-type initiators, thus enabling the incorporation of stereocontrolled polyether blocks in more complex polymer architectures. It is expected that this ability will greatly benefit the preparation of polyether-containing additives. The BINOL-type diboranes can be readily modified, suggesting further potential as a platform from which optimized catalysts can be developed.

Chiral diborane catalysts deliver well-defined isotactic-enriched polyether, whereby also polyester-type macroinitiators can be employed.

Self-Association in EO-BO-EO Triblock Copolymers as a Nanocarrier Template for Sustainable Release of Anticancer Drugs

Ethylene oxide (EO)-butylene oxide (BO)-ethylene oxide (EO)-based triblock copolymers with varying hydrophilic-hydrophobic ratios in arrangement, generally referred to as EBE, were scrutinized in an aqueous environment. Various self-associative (micellization) physicochemical properties of these EBEs were examined at different temperatures unified with a quantum chemical study. The salting-out effect on 5%w/v EBE was examined by observing their aqueous solution behavior where the clear transparent solution/turbidity suggested the probable presence of spherical or ellipsoidal micelles, which was confirmed from the scattering outline. The hydrodynamic radius (D_h) of the formed micellar geometry as a function of temperature and electrolyte (2 M NaCl) was inspected from dynamic light scattering and further supported by small-angle neutron scattering, where the Q-range prototype and scattering parameters were evaluated by the best fitting of the structure factor. Furthermore, these micelles were employed as potential nanocarriers for anticancer (curcumin and quercetin) drugs, where its release profile at a particular time interval was estimated using UV-vis spectroscopy. Different kinetic models were employed to fit the release profile data that enabled this study to act as an ideal platform for drug delivery. Also, the plausible interactions between EO-BO-EO blocks and the anticancer drugs were inferred from the evaluated computational descriptors.