Main chain selective polymer degradation: controlled by the wavelength and assembly

Abstract

The advent of reversible deactivation radical polymerization (RDRP) revolutionized polymer chemistry and paved the way for accessing synthetic polymers with controlled sequences based on vinylic monomers. An inherent limitation of vinylic polymers stems from their all-carbon backbone, which limits both function and degradability. Herein, we report a synthetic strategy utilizing radical ring-opening polymerization (rROP) of complementary photoreactive cyclic monomers in combination with RDRP to embed photoresponsive functionality into desired blocks of polyvinyl polymers. Exploiting different absorbances of photoreactive cyclic monomers, it becomes possible to degrade blocks selectively by irradiation with either UVB or UVA light. Translating such primary structures of polymer sequences into higher order assemblies, the hydrophobicity of the photodegradable monomers allowed for the formation of micelles in water. Upon exposure to light, the nondegradable blocks detached yielding a significant reduction in the micelle hydrodynamic diameter. As a result of the self-assembled micellar environment, telechelic oligomers with photoreactive termini (e.g., coumarin or styrylpyrene) resulting from the photodegradation of polymers in water underwent intermolecular photocycloaddition to photopolymerize, which usually only occurs efficiently at longer wavelengths and a much higher concentration of photoresponsive groups. The reported main chain polymer degradation is thus controlled by the irradiation wavelength and the assembly of the polymers.

Scheme 1. Triblock copolymer synthesis and its stepwise photodegradation: a diblock copolymer consisting of a PDMA nondegradable block and photodegradable copolymer of the coumarin cycloadduct and DMA block is prepared by green light-initiated RAFT polymerization. Chain-extension of this polymer with RAFT co-polymerization of DMA and the cyclic monomer resulted from intramolecular [2 + 2] cycloaddition of styrylpyrene under blue light yields a triblock polymer with the copolymer of DMA and styrylpyrene cycloadduct as the third block. Under UVA, the styrylpyrene cycloadduct experiences [2 + 2] cycloreversion, leading to the fragmentation of the third block. Subsequent UVB irradiation initiates the degradation of the second block as the coumarin dimer in the polymer backbone undergoes [2 + 2] cycloreversion.

Fig. 1. Transformation from linear monomer SC4 to cyclic monomer SC5: (A) reaction; (B) absorption spectrum of reaction solution under λ = 465 nm irradiation (0.05 mg mL⁻¹ in THF); (C) SEC chromatogram of reaction solution (0.05 mg mL⁻¹ in THF) before photoreaction and after 150 min of λ = 465 nm irradiation; (D) ¹H NMR of the monomer before and after irradiation showing the appearance of cyclobutane proton resonances after the irradiation.

Fig. 2. (A) Copolymerization of styrylpyrene based monomer SC5 and DMA, and photodegradation of the obtained copolymer P1 under UVA irradiation. Protons whose resonances change in the ¹H NMR are denoted with orange letters; (B) ¹H NMR spectra of PDMA, cyclic monomer SC5 and copolymer P1 P(DMA-co-SC5); (C) SEC traces of P1 in DMAc after UVA irradiation; (D) degradation upon UVA irradiation over time: reduction of the average molecular weight (M_n) of P1 represented by the ratio of M_n under t min UVA irradiation to the M_n of pristine P1 plotted vs. UVA irradiation time.

Fig. 3. (A) Synthesis of diblock copolymer P2, PDMA-b-(DMA-co-C3), of coumarin based monomer C3 and DMA, and its photodegradation under UV light. Protons whose resonances change in the ¹H NMR are denoted with orange letters; (B) ¹H NMR spectra of PDMA, cyclic monomer C3 and copolymer P2 PDMA-b-(DMA-co-C3); (C) overlayed SEC traces of the first PDMA block, diblock copolymer P2 and UVB degraded P2 in DMAc; (D) SEC traces of polymer P2 in DMAc solution over the course of UVA and UVB irradiation.

Scheme 2. Chain-extension of PDMA with coumarin cyclic monomer C3 and DMA, followed by chain-extension with styrylpyrene cyclic monomer SC5 and DMA, via RAFT polymerization gives rise to triblock copolymer P3.

Fig. 4. (A) Stacked ¹H NMR spectra of the homopolymer PDMA, diblock copolymer PDMA-b-(DMA-co-C3) and triblock copolymer P3 PDMA-b-(DMA-co-C3)-b-(DMA-co-SC5); (B) DOSY NMR of polymer P3 showing similar diffusion coefficients for SC5 and DMA resonances, indicative of successful chain extension and copolymerization; (C) overlayed SEC traces of the first block PDMA, diblock polymer and triblock polymer P3; (D) overlayed SEC traces of triblock polymer P3, UVA degraded P3, and UVA–UVB degraded P3; (E) schematic illustration for stepwise UV-degradation of P3 corresponding to photolabile functional groups in each block of the polymer.

Fig. 5. A dispersion of diblock copolymer P2 in water (1.5 mg mL⁻¹) was irradiated with UVB for 90 min. (A) Experimental SANS data (scattering intensity plotted versus momentum transfer q) of P2 dispersion in water irradiated with UVB light for 0 min, 30 min and 90 min and their fit with a spherical polymer micelle model via SasView. Note that the data and fit of P2 irradiated with UVB for 30 min and 90 min are offset by 10 and 100, respectively; (B) SEC traces of polymers obtained from UVB irradiation of the P2 dispersion at different time points; (C) overlay SEC traces of the polymer obtained from 90 min UVB irradiation of P2 dispersion in water and 30 min UVB irradiation of its solution in DMAc; (D) schematic representation of the postulated photodegradation process of P2 in water. (E) Effect of the environmental confinement on telechelic coumarin terminated PDMA segments, shifting their photostationary state under UVB irradiation.