Self-assembling Morphologies Obtained from Helical Polycarbodiimide Copolymers and Their Triazole Derivatives

Summary

Here, we present a protocol to prepare and visualize secondary structures (e.g., fibers, toroidal architectures, and nano-spheres) derived from helical polycarbodiimides. The morphology characterized by both atomic force microscopy (AFM) and scanning electron microscopy (SEM) was shown to depend on molecular structure, concentration, and the solvent of choice.

Abstract

A facile method for the preparation of polycarbodiimide-based secondary structures (e.g., nano-rings, "craters," fibers, looped fibers, fibrous networks, ribbons, worm-like aggregates, toroidal structures, and spherical particles) is described. These aggregates are morphologically influenced by extensive hydrophobic side chain-side chain interactions of the singular polycarbodiimide strands, as inferred by atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques. Polycarbodiimide-g-polystyrene copolymers (PS-PCDs) were prepared by a combination of synthetic methods, including coordination-insertion polymerization, copper(I)-catalyzed azide alkyne cycloaddition (CuAAC) "click" chemistry, and atom transfer radical polymerization (ATRP). PS-PCDs were found to form specific toroidal architectures at low concentrations in CHCl₃. To determine the influence of a more polar solvent medium (i.e., THF and THF/EtOH) on polymer aggregation behavior, a number of representative PS-PCD composites have been tested to show discrete concentration-dependent spherical particles. These fundamental studies are of practical interest to the development of experimental procedures for desirable architectures by directed self-assembly in thin film. These architectures may be exploited as drug carriers, whereas other morphological findings represent certain interest in the area of novel functional materials.

Introduction

The helix is a ubiquitous chiral motif observed in nature. Complex biological systems and their components, such as proteins, polypeptides, and DNA, all utilize the helical structure as a means of performing complex tasks for applications like information storage, tissue molecular transportation support, and localized chemical transformations.

Helical polymeric macromolecules¹ have been a target for the design of functional materials and composites possessing interesting properties, which enabled their practical use in many areas^2,3,4,5. So far, numerous helical scaffolds^6,7,8,9, as well as their secondary structure motifs, have been successfully exploited to achieve promising results, both in the field of physical engineering^10,11,12 and in biological applications^13,14. Current studies represent a logical extension of our earlier efforts to synthesize optically active alkyne polycarbodiimides bearing one or two modifiable alkyne moieties per repeat unit^15,16,17.

Recently, we reported²² the homo- and co-polymerization of carbodiimide monomers leading to chiral helical macromolecules — a family of (R)- and (S)-polycarbodiimides with modifiable pendant groups that offer further functionalization through a CuAAC "click" protocol. Br-terminated polycarbodiimides obtained from their respective ethynyl precursors were shown to act as ATRP macroinitiators in graft-polymerization with styrene²³.

The specific aim of this manuscript is to provide a practical guide for morphological characterizations (AFM measurements and SEM inspection) of the secondary structures formed from PS-PCDs synthesized from their corresponding ethynyl precursors by using a well-known click protocol²¹. In particular, experimental details, such as the solvent of choice, the temperature, the deposition method, the substrate chosen for deposition, and the polymer structure, were shown to be highly important to obtain specific morphologies (e.g., fibers, including right- and left-handed helical senses; nano-spheres; and nano-rings). They may also be of use for the development of materials with tunable properties based on polycarbodiimides with precisely controlled chiral architecture.

Protocol

NOTE: All reactions were performed in a glove box (or fume hood, when noted) using standard scintillation vials.

1. Synthesis of the (R)- and (S)-series of Ethynylpolycarbodiimides

1. Place 1.0 g (0.00442 mol) of N-(3-ethynylphenyl)-N'-hexylcarbodiimide monomer (ET) and 0.894 g (0.00442 mol) of N-phenyl-N'-hexylcarbodiimide monomer (Ph) as transparent, viscous liquids in a clean scintillation vial (20 mL) with a magnetic stirring bar (glove box) to obtain a representative R-50-ET-50-Ph composition.
   NOTE: Use only one monomer to generate the respective homopolymers. Mixing two monomer precursors at different ratios afforded a library of random copolymers²².
2. Weigh out 0.018 g (0.00004 mol) of (R)- BINOL Ti(IV) diisopropoxide catalyst as a red (sometimes orange), fine powder material in a glove box (monomer-to-catalyst molar ratio is 250:1) and add it to the scintillation vial.
3. Add ~3-5 mL of anhydrous CHCl₃ to dissolve both the monomer and the catalyst. Gentle stirring may be required at this step to dissolve the catalyst, which may otherwise form chunks of material. Perform all manipulations with reagents under an inert atmosphere (glove box) at 25 °C.
4. Cap the scintillation vial containing all the reagents and allow the reaction mixture to stir overnight at 25 °C in a glove box.
5. Remove the magnetic bar and add ~5 mL of additional CHCl₃ to re-dissolve the dark red, viscous material (outside the glove box).
6. Inject the solution obtained in the previous step into cold MeOH (250 mL) containing 0.5 mL of 1,8-diazabicyclo[2.5.0]undec-7-ene (DBU) to precipitate the polymer material as yellowish fibers.
7. Collect the polymer formed by filtration (fritted funnel, 15 mL, 4-8 µm) and wash it with MeOH (~10 mL, 3x).
8. Re-dissolve the material obtained from the previous step in CHCl₃ and re-precipitate it in MeOH to remove the residual Ti(IV)-BINOL catalyst. Dry the precipitate under high vacuum (200 mTorr) for 24 h to remove the MeOH. Repeat this procedure once to ensure the purity of the resulting polymer.

2. Synthesis of the (R)- and (S)-series of Triazole Polycarbodiimides under a "Click" Protocol

1. Add 5 mL of anhydrous THF (glove box) and a magnetic stirring bar to the scintillation vial (20 mL) containing 0.25 g (0.00117 mol) of R-50-ET-50Ph to synthesize a representative R-50-TRZ-50-Ph composition.
2. Weigh out 0.146 g (0.00059 mol) of N-(3-azidopropyl)-2-bromo-2-methylpropane amide²² in the glove box and add it to the scintillation vial.
3. Weigh out 0.022 g (0.00012 mol) of Cu(I) iodide catalyst in the glove box and load it into the scintillation vial. Let the solution stir for 2 min to form a homogeneous suspension.
4. Charge the same vial with 0.713 g (0.00468 mol) of DBU, cap the vial, and allow it to stir for 2 h in the glove box at 25 °C (avoid a longer reaction time to prevent hard gel formation).
5. Remove the magnetic bar and inject the reaction mixture (greenish gel-like solution) obtained in step 2.4 into cold MeOH (250 mL) containing 0.5 mL of DBU (outside the glove box).
6. Collect the formed triazole polymer by filtration (fritted funnel, 15 mL, 10 µm) and wash it with MeOH.
7. Repeat purification step 2.6 (i.e., dissolution in THF and precipitation from MeOH) one more time to remove the residual catalyst.
8. Dry the product of the "click" reaction under a high vacuum (200 mTorr) for 24 h to remove traces of MeOH.

3. Synthesis of the (R)- and (S)-series of Polycarbodiimide-g-polystyrene Copolymers

1. Mix 0.029 g (0.00029 mol) of Cu(I) chloride catalyst with 0.1 g (0.00029 mol) of R-50-TRZ-50Ph macroinitiator in the scintillation vial (20 mL) containing 0.101 g (0.00058 mol) of N,N,N',N',N''-pentamethylenediethylenetriamine (PMDETA). Place a magnetic stirring bar into the vial (glove box) to obtain a representative R-50-TRZ-50Ph-graft-polystyrene copolymer.
2. Charge a vial from step 3.1 with 1.510 g (0.0145 mol) of freshly distilled styrene.
3. Add ~12 mL of anhydrous toluene (or DMF)²³ into the vial from step 3.2 to dissolve the reagents; seal the vial tightly before taking it out of the glove box.
4. Within a fume hood, immerse the sealed vial in an oil bath and increase the temperature. Once the temperature reaches the desired value (temperature may vary from 57 to 100 °C, depending on the particular copolymer)²³, maintain it for 12 h (actual reaction time may range from 6 h to 4 days, depending on the experiment).
5. Remove the vial from the hot plate and cool the white, viscous material down to 25 °C.
6. Take the reaction vessel with resulting solid out of the glove box.
7. Unscrew the vial, remove the stirring bar, and pour the reaction mixture into 250 mL of cold MeOH containing 0.5 mL of DBU.
8. Collect the formed flakes of PS-PCDs by filtration (fritted funnel, 15 mL, 4-8 µm) and wash the material with cold MeOH (discard the supernatant left after filtration).
9. Repeat purification step 3.8 (i.e., dissolution in DMF and precipitation from MeOH) one more time to remove the residual catalyst.
10. Dry the material (white powder) under a high vacuum (200 mTorr) for 24 h to remove the MeOH.

4. Thin-film Preparation for Tapping Mode Atomic Force Microscopy (TMAFM) Measurements

1. Weigh 10 mg of polymeric material and place it in a 5-mL vial.
2. Add 1 mL of the solvent of choice (e.g., CHCl₃ or THF) into the vial and vortex the polymer suspension to dissolve the material.
   NOTE: Some polymer compositions require an extended period of standing time (~6 h) to completely dissolve the polymer.
3. Perform a successive dilution (i.e., using more dilute solutions at each step as the "stock") to prepare a series of stocks of 5.0, 2.5, 1.25, 0.625, 0.313, and 0.156 mg/mL concentrations.
4. Filter the stock solution through a 0.45-µm PTFE syringe filter prior to deposition on the silicon wafer (200 µL) with the following specifications (diam.: 25.4 ± 0.5 mm; orientation: 100 ± 0.5°; thickness: 250-300 µm; surface: single-side polished; type: N/Phos).
   NOTE: The deposited solution must cover the entire area of a silicon wafer.
5. Use a spin-coating machine immediately after depositing the sample (1 min, 1,000 rpm) to cover the entire wafer surface with a uniform polymeric film).
6. Acquire AFM images at 25 °C by using silicon cantilevers with nominal spring constants of 42 N/m, nominal resonance frequencies of 320 kHz, and standard silicon OTESPA or OTESPA-R3 tips (e.g., OTESPA-R3 material: 0.01-0.02 ohm-cm silicon, cantilever: T: 3.7 µm, f_o: 300 kHz, L: 160 µm, k: 26 N/m, W: 40 µm). Vary the amplitude set-point values from 425 to 273 mV, with scan rates of 0.99 and 1.99 Hz, respectively^22,23.
   NOTE: The experimental details for SEM specimen preparation and image acquisition were discussed earlier²³.

Results

Figure 1 (upper panel) illustrates BINOL (R)- or (S)-titanium (IV) catalyst-mediated coordination-insertion polymerization leading to the (R)- and (S)-series of ethynylpolycarbodiimides with an altering ratio of the repeat units (i.e., aryl- and alkyne aryl). Monomers and catalysts were obtained as described elsewhere¹⁸. Both (R)- and (S)-family alkyne random copolymers were selected fo...

Discussion

In summary, the spin-coating deposition method represents a convenient way to reproducibly generate multiple-type morphologies, including fiber-like aggregates, ribbons, worm-like structures, fibrillar networks, looped fibers, toroids, and superhelices, from either alkyne polycarbodiimides or from their respective PS-derivatives (i.e., polycarbodiimide-g-polystyrenes). Thus, coordination-insertion polymerization, along with further functionalization using a "click" reaction followed by ATRP, pro...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
styrene	Sigma-Aldrich	S4972-1L	reagent
N,N,N′,N′′,N′′- Pentamethyldiethylenetriamine (PMDETA)	Sigma-Aldrich	369497-250ML	reagent
Copper(I) iodide	Sigma-Aldrich	215554-5G	reagent
Copper(I) chloride	Alfa-Aesar	14644, 5G	reagent
1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU)	Sigma-Aldrich	139009-100G	reagent
N,N-dimethylformamide (DMF)	Sigma-Aldrich	227056-100mL	solvent
Tetrahydrofuran (THF)	Acros-Organics	B0320346	solvent
Chloroform	Sigma-Aldrich	372978-100mL	solvent
Methanol	Fisher-Chemical	A411-20	solvent
20 mL glass scintillation vials	Cole-Palmer	UX-08918-03	glassware
1-Dram vials (15 x 45 mm)	Kimble-Chase	KIM-60965D-1	glassware
13 mm syringe filter with 0.45 µm PTFE membrane	VWR International	28145-493	membrane filter
Silicon wafer disks (25.4 ± 0.5 mm)	Wafer World, Inc	S076453	AFM substrate
Corning Stirrer/Hot Plate	Hot Plate	PC-420	heating device
single stage Unilab mBraun glove box	Unilab	12-109	glove box
Nanoscope IV-Multimode Veeco AFM-machine	Veeco	3100 Dimension V Atomic Probe Microscope	AFM-instrument