1.1 Reaction mechanism of urethane acrylate
 
The isocyanate contains highly unsaturated double bonds and its electronic resonance structure is shown in Figure 1. As can be seen from Figure 1, the electron cloud on the oxygen and nitrogen atoms is very dense and electronegative.


Among them, the oxygen atom has the largest electronegativity and is a nucleophilic center. It easily reacts with an active hydrogen compound to form a hydroxyl group. The hydroxyl group is unstable on an unsaturated carbon atom and rearranges into a carbamate or urea.


The electron cloud density of carbon atoms is very low, showing a strong positive charge. It is an electrophilic center and is easily attacked by nucleophiles in active hydrogen compounds to undergo nucleophilic addition polymerization.


1.2 Synthesis route of urethane acrylate


Polyurethane acrylate is prepared by the reaction of polyisocyanate, hydroxy acrylate and long-chain diol. Since both hydroxy acrylate and polyol contain hydroxyl groups which can react with isocyanate, there are two different schemes for the synthesis route:


1 first chain extension and re-esterification, reaction with an excess of isocyanate and polyol to carry out chain extension, synthesis of isocyanate-terminated polyurethane prepolymer, and then react with hydroxy acrylate;


2 first esterification and then chain extension, isocyanate and hydroxy acrylate first single molecule addition, and then add polyol to extend the chain to obtain PUA prepolymer. Both routes have their own advantages and disadvantages. In practical applications, the ideal synthetic route can be selected according to the specific use and processing properties of the prepolymer.
 
Chen Yihong et al. synthesized polyether PUA with 2,4-toluene diisocyanate (TDI), hydroxyethyl acrylate (HEA) and polyethylene glycol (PEG) (reaction formula shown in Figure 2).

Studies have shown that in the synthesis of PUA, as the reaction temperature and molecular weight of the prepolymer increase, the flexibility of the cured film increases, and the solvent and catalyst content have little effect on the mechanical properties.


Yu Zongping et al. selected a reasonable synthetic route by changing the order of addition of hydroxyethyl acrylate (HEA) and polyester polyols by comparing the feasibility of reaction operation and product performance.


The results show that the process of adding hydroxyethyl acrylate (HEA) and then adding polyester polyol facilitates the mass distribution and molecular structure of the molecule, making the reaction process easy to control.