Synthesis of Cyclic Polymers and Characterization of Their Diffusive Motion in the Melt State at the Single Molecule Level

Summary

A protocol for the synthesis and characterization of diffusive motion of cyclic polymers at the single molecule level is presented.

Abstract

We demonstrate a method for the synthesis of cyclic polymers and a protocol for characterizing their diffusive motion in a melt state at the single molecule level. An electrostatic self-assembly and covalent fixation (ESA-CF) process is used for the synthesis of the cyclic poly(tetrahydrofuran) (poly(THF)). The diffusive motion of individual cyclic polymer chains in a melt state is visualized using single molecule fluorescence imaging by incorporating a fluorophore unit in the cyclic chains. The diffusive motion of the chains is quantitatively characterized by means of a combination of mean-squared displacement (MSD) analysis and a cumulative distribution function (CDF) analysis. The cyclic polymer exhibits multiple-mode diffusion which is distinct from its linear counterpart. The results demonstrate that the diffusional heterogeneity of polymers that is often hidden behind ensemble averaging can be revealed by the efficient synthesis of the cyclic polymers using the ESA-CF process and the quantitative analysis of the diffusive motion at the single molecule level using the MSD and CDF analyses.

Introduction

Cyclic polymers are unique in that they do not have chain ends. They often exhibit unusual behaviors which is distinct from their linear counterpart, including increased thermal stability of polymer micelles by a linear-to-cyclic conversion,^1,2 and spatial organization of DNA in bacterial cells by a loop formation.³ Topological interactions between the cyclic chains are believed to be the critical factor for such unusual behaviors.^4,5 Therefore, characterizing the motion and relaxation of cyclic polymers under entangled conditions has been an important research topic in polymer science for decades.⁶

Protocol

1. Synthesis of Monofunctional and Bifunctional Poly(THF)

1. Monofunctional poly(THF)
   1. Flame dry a 2-neck 100-ml round-bottom flask. Vacuum and fill the flask with nitrogen (3 cycles).
   2. Add distilled tetrahydrofuran (THF) (50 ml) to the flask. Put the flask in a water bath at 20 °C and equilibrate the temperature.
   3. Add methyl triflate (0.5 mmol) to the flask by a syringe. Stir the mixture for 5-10 min at 20 °C.
   4. Add N-phenyl pyrrolidine (4-6 equiv.) to the flask by a syringe. Stir the mixture for 30-60 min.
   5. Completely remove the solvent under reduced pressure (ca. 100 Torr)....

Results

The perylene diimide-incorporated 4-armed star and 8-shaped dicyclic poly(THF)s were synthesized using the electrostatic self-assembly and covalent fixation (ESA-CF) process (Figure 1, Figure 2). Time-lapse single-molecule fluorescence images were measured for the 4-armed (Figure 3a) and 8-shaped (Figure 3b) polymers. The time-lapse fluorescence images (Figure 3) show spatially isolated bright and sharp s.......

Discussion

The 4-armed and 8-shaped polymers were prepared via the ESA-CF protocol (Figure 1), which is a critical step for the synthesis.^12,24 Monofunctional and bifunctional linear poly(THF)s with N-phenylpiperidinium end groups were synthesized according to the previous procedure.¹¹ The ion exchange was carried out by reprecipitation of an acetone solution of a polymer precursor with triflate counteranions into an aqueous solution containing an excess amount of carboxylate.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Materials
THF	Godo
Wakosil C-300	Wako Pure Chemical Industries
Acetone	Godo
Toluene	Godo
n-Hexane	Godo
CHCl3	Kanto Chemical
Bio-Beads S-X1	Bio-Rad
Methyl triflate	Nacalai Tesque
Triflic anhydride	Nacalai Tesque
Potassium Hydroxide	Wako Pure Chemical Industries
Ethanol	Wako Pure Chemical Industries
Poly(tetrahydrofuran)	Aldrich
Chloroform	Wako Pure Chemical Industries
Immersion oil	Cargille	Type 37 / Type A
Equipment
2-Neck 100-ml round-bottom flask
Flask
Beaker
Funnel
Filter paper	Whatman
Reflux condenser
Syringe
Water bath
Magnetic stirrer
Rotary evaporator
Microscope cover slips (24 x 24 mm, No. 1)	Matsunami Glass	CO22241
Staining jar	AS ONE Corporation	1-7934-01
Ultrasonic cleaner	VWR International	142-0047
Inverted microscope	Olympus	IX71
Ar-Kr ion laser	Coherent	Innova 70C
Berek compensator	Newport	5540
Excitation filter	Semrock	LL01-488-12.5
Dichloric mirror	Omega optical	500DRLP
Emission filter	Semrock	BLP01-488R-25
Lens and mirror	Thorlabs
EM-CCD camera	Andor Technology	iXon
Objective lens (100X, N.A. = 1.3)	Olympus	UPLFLN 100XOP
Objective heater	Bioptechs
Preparative GPC	Japan Analytical Industry	LC-908