Synthesis of Monodisperse Cylindrical Nanoparticles via Crystallization-driven Self-assembly of Biodegradable Block Copolymers

Our protocol focuses on both ring-opening and RAFT polymerizations to produce block copolymer cylinders of a controlled length using living crystallization-driven self-assembly. The combination of both ring-opening and RAFT polymerizations allows the production of polymers with both semi-crystalline and functional regions. This opens the field of CDSA to other potential applications.

The success of the protocol resolves around the dryness of the starting reagents. Make sure the system never comes into contact with the atmosphere once the drying procedure has begun. The method requires knowledge in air-sensitive chemistry techniques.

We believe that committing these practices to video form will give researchers freedom to introduce new ideas without hindrance from a lack of practical experience. To begin, transfer all glassware and stir bars, dried in a 150 degrees Celsius oven overnight, to the bench. Clamp the two-neck 250-milliliter round-bottom flask, equipped with a stir bar, on a stand over a stir plate, and secure an air-tight tap on the small neck.

Add 100 milliliters of epsilon-caprolactone to the flask. Connect the round-bottom flask to a nitrogen line through the tap on one neck. Under a steady flow of nitrogen at three psi, add one gram of calcium hydride into the flask through the primary neck.

Fit the second neck with a glass stopper, and stir overnight at room temperature under a nitrogen atmosphere. Attack the flask to the Schlenk line, and open to a steady flow of nitrogen. Assemble the vacuum distillation equipment on two stands, by clamping the epsilon-caprolactone round-bottom flask to one stand and another round-bottom flask to the other stand.

Maintain a steady flow of nitrogen through the system to prevent water from entering the system. Insert a thermometer into the still head, and seal in place. Attach the adapter to the Schlenk line.

Turn off the nitrogen flow, and open the system to vacuum through this new connection to the Schlenk line. Heat the epsilon-caprolactone in a 60-to-80-degrees heating mantle, collecting the first five milliliters in the small round-bottom flasks and the rest in the two-neck round-bottom flask. Place the flasks in liquid nitrogen to condense the caprolactone effectively.

To speed up the process, wrap the distillation equipment in cotton wool and foil. Attach the Schlenk line to the two-neck round-bottom flask. Turn the line to vacuum for 120 seconds and to nitrogen for 10 seconds, a total of three times, to purge the line.

Open the line and the system to nitrogen. Then add one gram of calcium hydride to the collection flask fit with a stopper, and stir under a nitrogen atmosphere overnight. Meanwhile, dispose of the excess calcium hydride by dropwise addition of 10 milliliters of isopropanol, followed by five milliliters of methanol.

Once bubbling ceases, add an excess of water. Rinse the distillation equipment with acetone, and place in the oven overnight. In the morning, repeat the vacuum distillation again without adding calcium hydride to the monomer once finished.

Then open the flask to nitrogen to start transferring the caprolactone, via cannula, into an ampule. Transfer the ampule to a glove box. In the glove box, prepare stock solutions.

Into three separate vials, add 0.011 grams of hydroxyl chain transfer agent, as initiator, 0.01 grams of diphenyl phosphate, as catalyst, and 0.25 grams of the dried monomer, caprolactone. Add 0.5 milliliters of toluene to each of the initiator and catalyst vials, and gently agitate until the reagents are dissolved. Then mix the initiator and diphenyl phosphate stock solutions into one vial, and add a stir bar.

Fix the vial on a stirrer, adjust the speed to moderate stirring, and add the monomer solution into the initiator-catalyst vial. Fit the vial with a lid, and stir for eight hours at room temperature. Then remove the vial from the glove box, and immediately use a Pasteur pipette to add the mixture, dropwise, into an excess of cold diethyl ether, to precipitate.

Through a Buchner funnel, filter the white solid, dry in the ambient atmosphere, and dissolve it in one milliliter of tetrahydrofuran. Precipitate in diethyl ether twice more, and dry thoroughly. First prepare several basic alumina plugs in three-milliliter syringes.

Filter the dioxane and MMA into separate vials to remove stabilizers. Weigh 0.5 grams of the previously synthesized PCL, 0.424 grams of MMA, measure two milliliters of dioxane into a vial, and allow to dissolve. Prepare a pure AIBN stock solution of 10 milligrams per milliliter, and pipette 139 microliters into the reaction mixture.

Transfer the mixture into an ampule equipped with a stir bar, and seal. Next attach the ampule to the Schlenk line, and place into liquid nitrogen. Once the solution is frozen, introduce the ampule to vacuum until the vacuum gauge reads at most one times 10 to the negative one millibar.

Close the ampule, and allow to thaw completely. Repeat this freeze-pump-thaw cycle two additional times. Upon completion of the last cycle, backfill the ampule with nitrogen, and place it into a preheated oil bath at 65 degrees Celsius for four hours.

To monitor conversion, obtain the ampule from the oil bath, switch the cap for super seal under a flow of nitrogen, extract two drops, mix with 600 microliters of deuterated chloroform in a vial, and transfer to an NMR tube. Transfer the sample to the autosampler of an NMR instrument to run a proton spectrum. Replace the cap whilst under a steady flow of nitrogen, and place back into the oil bath.

Now, place the ampule that has been heated in the oil bath into liquid nitrogen until frozen, and then in a fume hood, open the ampule to air to quench the polymerization for one minute. Next, dropwise add the mixture into a vast excess of cold diethyl ether to precipitate. Isolate by Buchner filtration, and dry.

Dissolve the polymer and tetrahydrofuran, and precipitate twice more. Dry the polymer thoroughly for proton NMR spectroscopy and SEC analysis. Repeat this procedure with 0.5 grams of the generated PCL-PMMA, 1.406 grams of DMA, two milliliters of dioxane, and 111 microliters of ten milligrams per milliliter AIBN and dioxane.

Heat the polymerization at 70 degrees Celsius for one hour, and precipitate the reaction mixture in cold diethyl ether three times. Place five milligrams of the generated triblock copolymer into a vial, and add one milliliter of ethanol. Seal the vial with lid and parafilm.

Heat at 70 degrees Celsius and a heating mantle for three hours. After that, take the vial out to cool slowly to room temperature. Leave the solution to age at room temperature for two weeks.

The solution turns cloudy and forms a distinct layer at the bottom when fully assembled. Dilute the dispersion to one milligram per milliliter with ethanol in a sonication-proof tube. Place the tube in an ice bath.

Insert the tip of the sonication probe into the middle area of the dispersion. Sonicate the solution for 15 cycles of two minutes at the lowest intensity, allowing to cool for 15 minutes between cycles. Take an aliquot of the sonicated seed dispersion and dilute to 0.18 milligrams per milliliter with ethanol in a tube.

Then, prepare a solution of unimer and tetrahydrofuran to the concentration of 25 milligrams per milliliter. Add 32.8 microliters of the unimer solution into the diluted seed dispersion and gently shake to allow full dissolution. Leave the dispersion to age for three days with the lid slightly ajar, so the tetrahydrofuran can evaporate.

This will produce 90 nanometer starting seeds to 500 nanometer cylinders. Ring-opening polymerization of epsilon-caprolactone resulted in a degree of polymerization of 50, determined by the proton NMR spectrum with resonances of the end-group ethyl protons at 3.36 PPM, and the end-chain ester alpha protons at 4.08 PPM. GPC trace shows a typical molecular weight distribution with a single peak, having a dispersity value of 1.07 and a number average molecular weight of 10, 800 grams per mole.

By comparison, polymerization that was reacted for 12 hours gave a high molecular weight shoulder at 15, 500 grams per mole. Using reagents not correctly dried yielded a product mixture with a low molecular weight tail. The successive RAFT polymerizations achieved a degree of polymerization of 10 for the PMMA block with a unimodal peak in the GPC trace.

However, when deliberately taken to too high conversions greater than 70 percent, a broadening of molecular weight and a high molecular weight shoulder was observed. The degree of polymerization of the final block of PDMA was 200, upon comparison of the PCL end-chain ether protons at 4.08 PPM and the DMA side-chain methyl protons at 2.93 PPM. The GPC trace was narrow and unimodal.

Upon repetition of the chain extension using impure PCL-PMMA, a low molecular weight shoulder appeared. Remember to take your time on drying the system, as any presence of water can cause the polymerization of caprolactone to fail. Make sure everything is dry.

A buffer region between the core and corona of the cylinder could allow other CDSA construct to be stabilized in water, opening new avenues of research and collaboration.