Conjugated cross-linked phosphine as broadband light or sunlight-driven photocatalyst for large-scale atom transfer radical polymerization

Published in Chemistry
Conjugated cross-linked phosphine as broadband light or sunlight-driven photocatalyst for large-scale atom transfer radical polymerization
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The development of reversible deactivation radical polymerization (RDRP) has provided for industrial chemists with a feasible pathway to scale up the production of advanced functional polymeric materials via radical polymerization, such as polymers with predetermined molecular weight, narrow distribution, pre-designed block sequence, complex architecture, etc. With the global concern for carbon-neutral policies, polymer scientists are emulating the solar-driven protein biosynthesis with sequence control in biological systems and integrate photocatalysis with RDRP techniques.

However, there are several typical barriers in RDRP using sunlight directly. Unlike laboratory light sources, the illumination intensity of sunlight is much higher in practical applications. This can lead to the saturation of most organic dyes, resulting in the irreversible deterioration of the excited molecules and their photobleaching. Meanwhile, the high-energy photons in the solar spectrum can also cause side reactions such as self-initiation of monomers and degradation of polymers containing unstable groups in main chains. These features greatly limit the application of dyes or related compounds under sunlight irradiation.

In organocatalyzed atom transfer radical polymerization (ATRP), polymerization using high reductive molecules as photocatalysts under sunlight is less efficient than that under white light. Inorganic semiconductors such as ZnO and TiO2 nanoparticles were not compatible with sunlight-driven photopolymerization due to the broad energy band (UV region) and low solar utilization efficiency (~5%). Although carbon-based materials (carbon dots (CDs), or conjugated microporous polymers (CMPs), ) possess fascinating photophysical properties and photostability, they were very challenging to achieve ideal monomer conversions (99%) within a single solar illumination period (such as 6 h). Metal organic frameworks (MOFs) with various functional building blocks and dimensions have been employed as highly efficient heterogenous photocatalysts in stereolithographic 3D printing. However, sunlight was not applied in these polymerizations. Although the localized photothermal effect of photocatalysts (Ag3PO4, N-doped CDs, etc.) facilitated nearly quantitative monomer conversions under sunlight, they were subjected to many disadvantages such as the tedious separation process, narrow choice of monomers (only highly reactive monomers can be used), and relatively broad dispersity of the synthesized polymers.

The solar spectrum covers wavelengths from 250-2500 nm, with IR energy making up nearly 50% of solar energy, while photons from visible light and NIR constitute 95% of the solar flux. Broadband and NIR absorbance are necessary to achieve efficient solar energy utilization. Hence, photocatalysts applied for large-scale photocatalyzed RDRP under sunlight not only requires low toxicity, high activity, low cost, suitable redox potential, and a balanced size to ensure surface area and recyclability, but also requires wide absorption and NIR catalytic performance to harness the long-wavelength visible and NIR photons for improved penetration of photons in most reaction media and to avoid the competitive absorption of reactants.

In this work, we report the synthesis of a phosphine-based conjugated hyper crosslinked polymer (PPh3-CHCP) photocatalyst and its application for the persistent large-scale sunlight-driven Cu-ATRP deoxygenation procedure (Fig. 1). Increasing the scale of π-conjugation increases the maximum absorption peak (λmax). The raw materials and the reaction conditions for the synthesis of the photocatalyst are feasible to be upscaled at an industrial level. Most polymerizations were completed at 99% monomer conversions with good control, which were conducted over a wide range of 450-940 nm light irradiation due to the inherent broad light absorption of PPh3-CHCP, and the light intensities applied were very low (Fig. 2). The heterogeneous nature of the photocatalyst allowed for easy separation and reuse in multiple cycles with the retention of high photocatalytic efficiency. In sunlight-driven polymerizations, monomers (e.g., methyl acrylate (MA) and methyl methacrylate (MMA)) could achieve near quantitative conversion in a single solar illumination period (6 h) at scaled-up production of up to 200 mL (the largest reaction scale being reported as of now), with good control over dispersity. The block copolymer of PMA-b-PMMA could be in-situ synthesized at 400 mL scale using the pre-prepared PMA macroinitiator under blue light irradiation (also the largest scale being reported until present). The development of this photocatalyst and the related protocol is very likely to positively impact photocatalyzed radical polymerization processes, together with the potential industrial application of green and sustainable RP processes.

Fig. 1 Development of photocatalyst for large scale sunlight-driven Cu-ATRP. (a) Strategy for the fabrication of PPh3-CHCP photocatalyst. (b) Generation of ATRP activators via photoredox reactions. (c) Photoinduced Cu-ATRP in the presence of PPh3-CHCP (inserted photo: 200mL reaction scale of PMA under sunlight irradiation).

Fig. 2 (a) Monomer conversions of MA and MMA using PPh3-CHCP as photocatalysts under blue, green, orange, red, white, 730 nm, 760 nm, 800 nm, 850 nm, 940 nm, and sunlight irradiation respectively. Experimental details are provided in the supplementary materials. SEC traces of synthesized (b) PMA and (c) PMMA using PPh3-CHCP under series light irradiation. (d) SEC traces of 200 mL scale of PMA macroinitiator (in blue) and 400 mL scale of PMA200-b-PMMA170 copolymer (in red, and 192.9 g block copolymer was obtained finally) upon in situ chain extension showing high chain-end fidelity and successful chain extension. The polymers were synthesized using PPh3-CHCP in the absence of external deoxygenation.

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