Article pubs.acs.org/Macromolecules
Synthesis of Heterotelechelic α,ω-Dye-Labeled Polymer and Energy Transfer between the Chain Ends Ye Sha,† Yunlong Xu,‡ Dongliang Qi,† Yuanxin Wan,† Linling Li,† Hong Li,‡ Xiaoliang Wang,† Gi Xue,*,† and Dongshan Zhou*,†,§ †
Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Key Laboratory of High Performance Polymer Materials and Technology (Nanjing University), Ministry of Education, State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructure, Nanjing University, Nanjing 210093, P. R. China ‡ State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210046, P. R. China § School of Physical Science and Technology, Xinjiang Key Laboratory and Phase Transitions and Microstructures in Condensed Matters, Yili Normal University, Yining 835000, P. R. China S Supporting Information *
ABSTRACT: We report on the synthesis of heterotelechelic α,ω-dyelabeled polystyrene and poly(methyl methacrylate) via a combination of site-specific atom transfer radical polymerization (ATRP) and click chemistry. By using the advantages of the living polymerization characteristics and a robust coupling efficiency, the Förster/fluorescence resonance energy transfer (FRET) pairs (i.e., carbazolyl and anthryl) were dictated to be at the near-stoichiometric chain ends. The distribution of the end-to-end distance was well described by the energy transfer response of the fluorescent groups between chain ends, which is in reasonable agreement with Gaussian statistics. The synthetic approach described here provides an opportunity to prepare polymeric materials with customized responsive elements and in-depth insight into the statistical scaling dimension of polymers.
■
labeling ratio.31 To overcome these problems, the synthesis of a narrowly distributed end-tagged polymer is needed to achieve statistical homogeneity due to the distance-dependent FRET efficiency.15,29 At the present time, controlled/living radical polymerizations (C/LRP) brings about an unprecedented facile opportunity to build well-defined polymer structures containing fluorescent end-groups.2 However, there is still an increasing synthetic challenge in preparing a well-defined polymer containing two different fluorophores because two independent reactions on each fluorophore modification have to be combined appropriately.3,12,15,32 Postmodification of polymer chain with a dye at a specific site is inevitable, and the reference dealing with this issue is very scarce.2 In addition, the stoichiometric single-chain labeling strategy is highly pursued for the undisputed quantitative analysis of the FRET terminal response.31 Thanks to the development of click chemistry, near-quantitative yields can be realized in a straightforward way because of the low susceptibility to electronic and steric effects of the groups linked to the alkyne or azide centers.33,34 To the best of our knowledge, the synthesis of stoichiometrically
INTRODUCTION Well-defined polymer structures, with a precisely customized functionality, molecular weight, and monodispersity, are becoming a prerequisite to match the requirements for the construction of versatile polymeric architectures.1−8 In particular, α,ω-heterotelechelic polymers, defined as polymer chains that have two different functional groups at their terminals, are receiving considerable attention because of their specificity for further modifications and the photoreactive endgroups.1,9−16 Microenvironmental changes, such as temperature, ion, pH, or miscibility, can be monitored efficiently by a single-molecule sensor placed between the stimuli-responsive fluorescent terminals of the heterotelechelic polymer using intramolecular Förster/fluorescence resonance energy transfer (FRET).17−24 FRET α,ω-dye-labeled macromolecules have been prepared for many years.22,25,26 However, quantitative conformational information basically focuses on biomacromolecules such as peptides,25,27 DNA,26 and protein.22,28 For conventional chemically synthesized polymer, quantitative analysis of FRET data remains mathematically challenging, and only qualitative analysis is suitable for stimuli-responsive polymers due to the plagued issues,29 such as diffusion-enhanced FRET efficiency,30 complicated end-to-end distance (ree) distribution,29 and poor © XXXX American Chemical Society
Received: August 8, 2016 Revised: October 4, 2016
A
DOI: 10.1021/acs.macromol.6b01726 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 1. Synthetic Routes for Dye-Labeled PS and PMMA
heterotelechelic α,ω-dye-labeled homopolymer for quantitative intramolecular FRET applications has been scarcely reported. On the basis of statistics of polymer physics, the statistical parameters and conformational distribution for classical model polymeric species such as polystyrene (PS) and polymethacrylate have been well established, thus offering a reliable testimony for the terminal response.35 For dye labels appended to polymer for the investigation of polymer physics by FRET, carbazolyl and anthryl are the most widely used FRET pairs owing to the good compatibility with bulk materials and thermostability.15,36−38 Herein, we report the synthesis of heterotelechelic PS and poly(methyl methacrylate) (PMMA) possessing FRET pairs (i.e., carbazolyl and anthryl) at stoichiometric chain ends based on the combination of sitespecific atom transfer radical polymerization (ATRP) and click chemistry. The chemical structure and functionality are verified by GPC, MALDI MS, FTIR, NMR, and UV−vis spectra and fluorescence spectra and decay. Finally, the ree distribution of the synthesized polymers is shown to obey Gaussian statistics by using intramolecular FRET.
■
Bruker TENSOR 27 spectrophotometer by using KBr slices in the 400−4000 cm−1 region. The melting point (mp) of the synthesized structure was obtained from a Mettler-Toledo DSC1 differential scanning calorimeter (DSC) instrument during the heating scan at 10 °C/min under a nitrogen atmosphere using zinc and indium as internal standards. The mass spectra were determined from a Micromass GC-TOF for EI-MS (70 eV). Elemental analysis concerning C, H, and N was performed by using an elementar vario EL III elemental analyzer. UV−vis spectroscopy was recorded from a MAPADA UV-1800 spectrophotometer. GPC was performed at a flow rate of 1.0 mL/min with tetrahydrofuran (THF) as moving phase on a PL-GPC 120 system equipped with a refractive index (RI) detector, and very narrow dispersed polystyrene species were employed as the molecular weight standard. MALDI-TOF MS was recorded on Bruker UltrafleXtreme MALDI TOF-TOF/MS with a nitrogen laser (337 nm, 2 nm pulse) using linear ion mode. All spectra of polymers were recorded in a reflectron mode (0−10 kDa). Dithranol (10 mg/mL in THF) and trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB, 10 mg/mL in THF) were used as the matrices and silver trifluoroacetate (10 mg/mL in THF) and sodium trifluoroacetate (10 mg/mL in THF) as the cationizing agents. Samples were prepared by mixing the matrix (45 μL), salt (5 μL), and polymer species (5 μL, 10 mg/mL in THF) solutions, spotting 1.5 μL onto the target, and drying at room temperature prior to analysis. The fluorescence spectra were recorded on a Photon Technology International (PTI) spectrofluorometer. The band-pass excitation and band-pass emission slits were both 1 nm. The fluorescence lifetime measurements were performed on a PTI QM-LS LaserStrobe Lifetime Enhancement module. Polymer films were prepared by the solutioncast method; after solvent evaporation, the films were quenched from the melt to room temperature. Synthesis Protocol. The synthesis and characterization details for small organic molecules and polymer are described in the Supporting Information.
EXPERIMENTAL SECTION
Materials. 2-(9H-Carbazol-9-yl)ethanol (Aldrich Co., Ltd.), anthracene-9-methanol (Aladdin Reagents), ethyl 2-bromoisobutyrate (EBiB, J&K Chemicals), methyl 2-bromopropionate (MBP, Aladdin Reagents), sodium azide (Aladdin Reagents), N,N-(dimethylamino)pyridine (DMAP, Aladdin Reagents), dicyclohexyl carbodiimide (DCC, TCI Chemicals), 2-bromo-2-methylpropanoic acid (J&K Chemicals), 2-bromopropanoic acid (J&K Chemicals), 4-pentynoic acid (J&K Chemicals), superdry DMF (water content
