Water-soluble polyphosphonate-based bottlebrush copolymers via aqueous ring-opening metathesis polymerization

Ring-opening metathesis polymerization (ROMP) is a versatile method for synthesizing complex macromolecules from various functional monomers. In this work, we report the synthesis of water-soluble and degradable bottlebrush polymers, based on polyphosphoesters (PPEs) via ROMP. First, PPE-macromonomers were synthesized via organocatalytic anionic ring-opening polymerization of 2-ethyl-2-oxo-1,3,2-dioxaphospholane using N-(hydroxyethyl)-cis-5-norbornene-exo-2,3-dicarboximide as the initiator and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the catalyst. The resulting norbornene-based macromonomers had degrees of polymerization (DPn) ranging from 25 to 243 and narrow molar mass dispersity (Đ ≤ 1.10). Subsequently, these macromonomers were used in ROMP with the Grubbs 3rd-generation bispyridyl complex (Ru-G3) to produce a library of well-defined bottlebrush polymers. The ROMP was carried out either in dioxane or in aqueous conditions, resulting in well-defined and water-soluble bottlebrush PPEs. Furthermore, a two-step protocol was employed to synthesize double hydrophilic diblock bottlebrush copolymers via ROMP in water at neutral pH-values. This general protocol enabled the direct combination of PPEs with ROMP to synthesize well-defined bottlebrush polymers and block copolymers in water. Degradation of the PPE side chains was proven resulting in low molar mass degradation products only. The biocompatible and biodegradable nature of PPEs makes this pathway promising for designing novel biomedical drug carriers or viscosity modifiers, as well as many other potential applications.


Size Exclusion Chromatography (SEC)
SEC measurements were performed in DMF (0.1 M LiCl) at 60°C with an Agilent Technologies 1260 Infinity II, PSS SECcurity system at a flow rate of 1 mL min -1 .Each sample injection volume was 50 µL, performed by Agilent 1260-ALS autosampler.2 GRAM columns (PSS) in series with dimensions of 8 × 300 mm 2 , 10 µm particle size, and pore sizes of resp.1000 and 30 Å, including a 10 µm GRAM guard column were employed.Calibration was carried out using polystyrene standards supplied by Polymer Standards Service.The SEC data were plotted using the software OriginPro 8G from OriginLab Corporation.

Atomic Force Microscopy (AFM )
The bottle brushes were deposited from a solution (0.2 mg mL -1 ) on silicon wafers by spin-coating (2000 rpm, 60 s, room temperature).Silicon wafers were cleaned by ultrasonication in acetone (10 min) and Piranha solution treatment (20 min).AFM height and peakforce error images were obtained in air and at room temperature using a MultiMode 8 AFM instrument with a NanoScope V controller (Bruker) operated in the PeakForce Tapping mode.The ScanAsyst setting was set to "on" in order to apply optimized scanning parameters, particularly the feedback loop and the applied load (hundreds of pN), for imaging bottle brushes.The AFM data was collected following a sine-wave sample-tip trajectory with a frequency of 2 kHz and utilizing a peak-force amplitude value of 20-50 nm.Soft AFM cantilevers were chosen with a nominal spring constant of 0.4 N/m and a tip with a nominal radius of 2 nm (Bruker, ScanAsyst-Air).The AFM optical sensitivity (deflection sensitivity) was calculated based on the thermal tune method based on the nominal spring constant.S4

Scanning electron microscopy (SEM)
SEM measurements were performed on a cryo-field emission SEM equipped with an energy-selective detector for 16-bit image series acquisition with up to 40,000×50,000-pixel resolution and in lens, chamber.Samples for SEM measurements were prepared by putting one drop of an approximately 0.1 g L -1 sample dispersion on a mica wafer and dried for at least 6 h.The sputtering was prepared with a Quorum PP3010T-Cryo chamber with integrated Q150T-Es high-end sputter coater and an Au-Cd target layer of 4 nm was sputtered on the samples.

Dynamic light scattering (DLS)
The DLS measurements were carried out using an ALV/CGS-3 (ALV-LSE-5004 correlator) goniometer system with a He-Ne laser light source emitting at a wavelength (λ) of 633 nm, providing a power of 35 mW.The temperature of the sample was precisely maintained at 293.1 ± 0.2 K, and a minimum equilibration time of 15 minutes was observed before each measurement to minimize convection effects.
Toluene was utilized as the matching bath, and the temperature was monitored using the built-in sensor of the goniometer system.For the angle-dependent measurements, observation angles (θ) ranging from 40° to 120° were employed.Particle size distributions were determined at an observation angle (θ) of 90°.To ensure accuracy and reliability, all measurements were performed in triplicate.The obtained correlation functions were subjected to analysis using the Cumulant and CONTIN methods, which were implemented using the ALV-Correlator Software (Version 3.0.5.9).S1), the DP n was determined by end-group analysis by 1 H NMR spectroscopy.
Ethyl ethylene phosphonate (1g, 7.35 mmol, 25 eq) and N-(Hydroxylethyl)-cis-5-norbornene-exo-2,3dicarboximide (61 mg, 0.29 mmol, 1 eq) were dissolved in anhydrous CH 2 Cl 2 (1.83 mL) in an ovendried 4 mL vial equipped with a magnetic stirring bar.The reaction mixture was homogenized by stirring at 20 °C followed by the addition of DBU (132 L, 134 mg, 0.88 mmol, 3eq) and the solution was stirred at room temperature for 2 h before the reaction mixture was quenched by the rapid addition of an excess of formic acid solution in CH 2 Cl 2 (20 mg mL -1 ).The crude product was purified by precipitation into cold diethyl ether (-28 °C) three times, and drying in vacuo to yield PetPn 26 exo-norbornene macromonomer as a colourless viscous liquid (1.02 g, 95 %).The molar mass was determined by endgroup analysis 1 H NMR spectroscopy by comparing the integral of the -CH 2 signal of the CTA agent (6.26 ppm) with the backbone signal (4.25 ppm), similar to as a previous report.S5

Kinetics of the polymerization of ethyl phosphonate to the exo-norbornene PEtn macromonomer.
Ethyl ethylene phosphonate (400 mg, 2.94 mmol, 50 eq) and N-(Hydroxyethyl)-cis-5-norbornene-exo-2,3-dicarboximide (12 mg, 0.059 mmol, 1 eq) were dissolved in anhydrous CH 2 Cl 2 (0.74 mL) in an oven-dried 4 mL vial equipped with a magnetic stirring bar.The reaction mixture was homogenized by stirring at 20 °C followed by the addition of DBU (26 L, 27 mg, 0.18 mmol, 3eq), and the solution was stirred at room temperature.The sampling involved periodic extraction of aliquots from the reaction mixture (terminated by the addition of one drop of formic acid solution in CH 2 Cl 2 (20 mg mL -1 )), followed by dissolution in CDCl 3 , which is a good solvent for both ethyl ethylene phosphonate and the corresponding polymer.Each sample was analyzed by 1 H and 31 P NMR spectroscopy (the chemical shift of the cyclic monomer (52.5 ppm) to the corresponding linear phosphonic acid ester (35.1 ppm) was observed as previously reported S2 and SEC in DMF (0.1 M LiCl) was conducted at 60 °C. Entry
Scheme S3.Synthesis of bottle brush polymers by ring-opening metathesis polymerization of exonorbornene polyphosphonates macroinitiator using the 3 rd generation Grubbs catalyst (Ru-G3) in aqueous media.NOTE: During the course of this work different samples of bottle brush polymers were prepared with
Scheme S4.Synthesis of double hydrophilic block copolymers by ring-opening metathesis polymerization of exo-norbornene polyphosphonates via of a second macroinitiator using the 3 rd generation Grubbs catalyst in aqueous conditions.

exo-norbornene PEtn macromonomers were prepared
by varying the [initiator / monomer] ratio and different DP n values were targeted (see Table
Conversion of NB-PEtPn 33 to brush polymer is determined by integration of the peak areas of brush polymer and residual NB-PEtPn 33 from SEC measurement of the crude product.b Determined from integration of the SEC signals of the high molar mass fraction (measured in DMF (0.1 M LiCl) at 60 °C vs. polystyrene standards using RI detector by conventional SEC). a