Research on visible light-controlled cationic polymerization published on Nature Communications

On June 24, 2022, the renowned academic journal Nature Communications published an article of Prof. Yongfeng ZHOU’s research team and A. Prof. Shaodong ZHANG from the School of Chemistry and Chemical Engineering and the Frontiers Science for Transformative Molecules, Shanghai Jiao Tong University, entitled “Visible light-controlled living cationic polymerization of methoxystyrene”.

Fig. 1: The novel visible light-driven photo-controlled living cationic polymerization system in this work.In recent years, the combination of living polymerizations and photooxidation reduction has endowed light-controlled polymerization with great capacity on spatio-temporal control, which attracts considerable attention. Such light-controlled living polymerization can synthesize polymers with complex topological structure, controllable molecular weight and narrow molecular weight distribution. Moreover, the growth of the polymer can be controlled by a light switch: turn on the light source to make the chain grow; turn off the light source to stop the chain growth. Although light-controlled polymerization has been a great success, it is mainly focused on free radical polymerization and progress on light-controlled living cationic polymerization is very limited. There are only a few photocatalysts and chain transfer agents suitable for light-controlled cationic polymerization, and monomers are limited to vinyl ethers. In addition, residual photocatalysts can lead to polymer color contamination, heavy metal contamination and possible side reactions, thus limiting their application in advanced optoelectronics, biomaterials, etc.

Prof. Yongfeng ZHOU’s research team and A. Prof. Shaodong ZHANG therefore report on a new photocatalytic system composed of OPC tris(2,4-dimethoxyphenyl)methylium tetrafluoroborate and phosphate chain transfer agents (CTAs), so to realize visible light-controlled living cationic polymerization by using 4-methoxystyrene (p-MOS) as a proof-of-concept monomer (Fig. 1).

The system has the following advantages.

(1) Simple reaction. Organic photocatalysts and CTA raw materials are easily available, simple to synthesize and metal-free.

(2) Visible light-controlled living polymerization. Under green LED light irradiation, the polymerization reaction exhibits obvious living polymerization characteristics, including molecular weight predictability up to 50 KD, narrow molecular weight distribution (Đ = 1.25) and good living chain fidelity (Fig. 2).

(3) Outstanding light control ability. The reaction can be switched on and off by turning on and off the light. After the light source is turned off, it exhibits the longest 36-hour dormant period of light-controlled cationic polymerization systems to date (Fig. 3).

Fig. 2: Evidence of photo-induced polymerization of p-MOS.(4) Decolorizability. By adding a small amount of alkali after the polymerization, the residual photocatalyst can be easily deactivated and decolorized, which can effectively avoid the pollution and side reactions caused by the residual catalyst.

Fig. 3: Temporal control of polymer chain growth via on-off light switching. The polymerization immediately ceased upon removal of light stimulus. The dormant period reached up to 36 h. The reaction was woken again after the green light was turned on.

Fig. 4: Proposed mechanism for the photo-controlled living cationic polymerization. a) Photo-induced initiation, propagation and formation of adduct. b) Visible light-mediated activation and deactivation.Possible mechanism for the photo-controlled living cationic polymerization was proposed and validated (Fig. 4).

Fig. 5: Decoloration of the OPC 1 and proposed mechanism of decoloration. 

This visible light-controlled living cationic polymerization system has good scalability. Firstly, the photophysical and photochemical properties of the triarylmethyl cation can be adjusted by changing the substituent groups on the aromatic ring, which allows this type of photocatalyst to be applied to more catalytic systems. Secondly, due to the relatively weak P(O)O–R bonds, phosphate CTAs can be applied in more low-energy-barrier reactions, and expand the applicable monomer range of light-controlled cationic polymerization. In addition, the photo-controlled cationic polymerization mechanism can also be used for the photocatalytic synthesis of organic small molecules.

Lei WANG, PhD student of Prof. Yongfeng ZHOU, is the first author. Prof. Yongfeng ZHOU and A. Prof. Shaodong ZHANG are the co-corresponding authors. Prof. Chunyang YU provided technical support for the DFT. This work was financially supported by the National Natural Science Foundation of China and the Program for Basic Research of Shanghai Science and Technology Commission.

Translator: Chenyun SUN

Reviser: Xiaoke HU