Question

Explain the role of the photochemical reactions of carbonyl com-
pounds in the photoinitiated polymerisation of vinyl monomers and cross - linking in
polymers.

Answer 1

Photochemical reactions are involve absorption of light, energy absorb on type of electronic transitions. The presence of light is primary activate the reaction to take place. There is no or very little effect on the rate of photochemical reactions and intensity of light has very much effect on the rate of photochemical reactions. Some of photochemical reactions are initiated by the presence of photosensitizer. Photosensitizer works in different way than a catalyst.

Initially all molecules are present in the ground State (S​​​​​​0​​​​​) .when it radiated the light beam then G.S. molecules reached into the excited state are denoted as S​​​​​​1 and T​​​​​​1​. In singlet excited State in which excited electron and original electron has opposite spin and in triplet excited state in which excited electron and original electron has same spin either both up or either both down spin. Triplet excited state has parallel spin and it's spin multiplicity =3 and it have paramagnetic , low energy state. Singlet excited state have unpaired electrons with opposite spin and spin multiplicity =1 and diamagnetic in nature and have high energy state.

Triplet state is normally more stable than singlet state .

Photo initiator responsible for initiation step in photopolymerization reactions. Carbonyl compounds have two oxygen centered nonbonding electrons. Many chemical species that are capable of generating radical photochemically. Most of phoTopolymerization process requires addition of initiators such as radical initiator etc.