Thiophene End-Functionalized Oligo-(D,L-Lactide) as a New Electroactive Macromonomer for the "Hairy-Rod" Type Conjugated Polymers Synthesis

Abstract

The development of the modern society imposes a fast-growing demand for new advanced functional polymer materials. To this aim, one of the most plausible current methodologies is the end-group functionalization of existing conventional polymers. If the end functional group is able to polymerize, this method enables the synthesis of a molecularly complex, grafted architecture that opens the access to a wider range of material properties, as well as tailoring the special functions required for certain applications. In this context, the present paper reports on α-thienyl-ω-hydroxyl-end-groups functionalized oligo-(D,L-lactide) (Th-PDLLA), which was designed to combine the polymerizability and photophysical properties of thiophene with the biocompatibility and biodegradability of poly-(D,L-lactide). Th-PDLLA was synthesized using the path of "functional initiator" in the ring-opening polymerization (ROP) of (D,L)-lactide, assisted by stannous 2-ethyl hexanoate (Sn(oct)₂). The results of NMR and FT-IR spectroscopic methods confirmed the Th-PDLLA's expected structure, while the oligomeric nature of Th-PDLLA, as resulting from the calculations based on ¹H-NMR data, is supported by the findings from gel permeation chromatography (GPC) and by the results of the thermal analyses. The behavior of Th-PDLLA in different organic solvents, evaluated by UV-vis and fluorescence spectroscopy, but also by dynamic light scattering (DLS), suggested the presence of colloidal supramolecular structures, underlining the nature of the macromonomer Th-PDLLA as an "shape amphiphile". To test its functionality, the ability of Th-PDLLA to work as a building block for the synthesis of molecular composites was demonstrated by photoinduced oxidative homopolymerization in the presence of diphenyliodonium salt (DPI). The occurrence of a polymerization process, with the formation of a thiophene-conjugated oligomeric main chain grafted with oligomeric PDLLA, was proven, in addition to the visual changes, by the results of GPC, ¹H-NMR, FT-IR, UV-vis and fluorescence measurements.

Keywords: electroactive macromonomers; grafted conjugated polymers; iodonium salt; oligothiophenes; photopolymerization; polylactide.

Figure 1

(A) ¹H-NMR and (B) ¹³C-NMR spectrum of Th-PDLLA registered in CDCl₃ (with * were denoted the impurities, possible traces of methanol).

Figure 5

FT–IR spectra of PEG₂₀₀₀ -substituted thiophene oligomers containing 3 (3T), 5 (5T), or 7 (7T) thiophene rings and the spectrum of OTh-PDLLA.

Scheme 2

The proposed hypothetical pathway for the synthesis of OTh-PDLLA by photo-induced oxidative hopomolymerization of Th-PDLLA.

Scheme 1

Synthesis pathway of Th-PDLLA.

Figure 2

Absorption (A) and emission (B) spectra of Th-PDLLA in organic solvents of different nature (c = 1 mg/mL) (λ_ex = 330 nm).

Figure 3

DSC trace of treated (A) and TGA trace of untreated (B) Th-PDLLA macromonomer.

Figure 4

The ¹H-NMR spectrum of the OTh-PDLLA recorded in CDCl₃.

Figure 6

UV–vis and fluorescence spectra of OTh-PDLLA in chloroform (c = 1 mg/mL) (λ_ex = 330 nm).