Whic type of reaction is used in the production of nylon

The reaction that takes place in the production of nylon is a polymerization reaction. This type of reaction is reacting monomer molecules together to form a polymer chain. For the production of nylon, the monomers are adipoyl chloride and hexamethylene diamine.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.