Photopolymerization mechanism of photopolymerization reaction

Photopolymerization is essentially a chain reaction, in which chemically active liquid substances are rapidly transformed into solid States by initiators under illumination. Similar to traditional thermal polymerization, once initiated, the reaction proceeds at a rapid polymerization rate.

According to the different reaction mechanisms, photopolymerization can be divided into free radical photopolymerization and cationic photopolymerization. Free radical photopolymerization refers to the reaction that produces free radicals and initiates polymerization after illumination. According to the different photoinitiation mechanisms, free radical photoinitiators can be divided into cracking photoinitiators and hydrogen-absorbing photoinitiators.

The free radicals produced by light irradiation of cracking photoinitiator are highly active. After absorbing light energy, they jump to the excited singlet state and then jump to the excited triplet state through the system. When they excite singlet or triplet States, the molecules are very unstable, and the weak bonds in them are evenly split, generating primary active free radicals and initiating the polymerization of vinyl monomers. Most of them can be combined with monomers or resins in the curing formula, but the relative molecular weight of photolysis products produced in the curing process is mostly lower than that of the original photoinitiator, which is more volatile and causes odor problems. When bimolecular reaction occurs between hydrogen-absorbing photoinitiator and amine co-initiator, free radicals will also be generated [Figure 2 (Ⅱ)], which absorbs light energy, and bimolecular will react with * * * initiator (hydrogen donor) in the excited state to generate active free radicals. Generally speaking, tertiary amine is a common initiator. Taking benzophenone as an example, it takes hydrogen to generate keto radical, which has no initiating activity, while the primary radical (R) generated by * * * initiator has initiating activity, which can react with monomer or resin in the curing formula to form a cured product, while the carbonyl radical has low activity. In the end, two identical free radicals will be disproportionally generated into benzophenone and benzhydryl alcohol, or tetraphenylphenol ether will be generated through double base coupling, or combined with chain free radicals as polymerization terminator. Although the molecular weight of photolysis products after light curing is higher than that of the original photoinitiator, the trend of migration or capture from cured products is still high.

The mechanism of cracking initiation polymerization is shown in the figure.

The active fragments of cationic photopolymerization are protonic acid and free radical, in which protonic acid plays the main initiating role. The basic characteristic of cationic photoinitiator is that it is activated to excited state by light, and the molecules undergo a series of decomposition reactions, eventually producing super strong protonic acid (also known as Br? The anions paired with acid centers are generally BF4-, PF6-, AsF6- and SbF6- plasmas, which have weak nucleophilicity and strong proton acid. Cationic photoinitiator is a very important kind of initiator, including diazonium salt, diaryl iodide salt, triaryl sulfonium salt, alkyl sulfonium salt, aromatic iron salt and so on. Among them, diaryl iodonium salt and triaryl sulfonium salt are the most representative and have been reported in many practical applications. This is because these two initiators have the advantages of good thermal stability and high initiation activity.

The polymerization rate of cationic photopolymerization is slower than that of free radical photopolymerization, and the growth rate constant kp and termination rate constant kt are both one order of magnitude smaller than the latter. In free radical photopolymerization, the chain termination reaction is mainly two-group termination; However, in cationic photopolymerization, onium ions with the same charge and polymer carbocation can't react because of the same repulsion, so it is difficult to have chain termination reaction. Therefore, cationic photopolymerization is also called continuous polymerization or living polymerization. (1) If there are no strong nucleophilic substances such as amine and mercaptan in the system, the active species of protonic acid or Lewis acid are chemically stable and will not disappear through coupling like free radicals, but can only be added to the monomer to initiate polymerization and maintain this ionic activity.

(2) Free radical polymerization is fast, almost not limited by temperature, and once active free radicals are generated, it can quickly initiate polymerization; Cationic polymerization is different. At the same time of irradiation or after light activation, it is sometimes necessary to increase the temperature appropriately to accelerate polymerization.

(3) Although free radical photopolymerization is faster, if the light source is suddenly cut off during photopolymerization, the polymerization rate will drop rapidly, and the polymerization conversion rate will still rise a little slowly, and finally tend to be stable. It can be considered that for free radical photopolymerization, when the light stops, the polymerization will stop almost immediately; In the process of cationic polymerization, if the light source is suddenly cut off, the polymerization rate will not decrease rapidly, but will continue to increase at a faster rate. In other words, cationic photopolymerization is immortal polymerization, and the latter dark polymerization can be carried out smoothly as long as it is irradiated with light at the initial stage.

(4) Free radical photopolymerization is particularly sensitive to molecular oxygen, prone to oxygen polymerization inhibition, and insensitive to nucleophilic reagents such as water and amine base; There is no problem of oxygen polymerization inhibition in cationic photopolymerization, but nucleophilic substances such as water vapor and amino group will combine with cationic active center stably, resulting in polymerization inhibition.

(5) After cationic photopolymerization is completed, protonic acid may still remain in the coating, which is harmful to the coating itself and the substrate for a long time.