The earliest efforts at producing a permanent wave hairdo involved heat and various chemicals. For decades, thioglycolate has been the chemical of choice. Two industry standards have been the alkaline sodium or ammonium thioglycolate and the acidic glyceryl monothioglycolate.
Do you recognize the term thio in both reagents? This refers to sulfur. The chemical formula for thioglycolic acid is HS-CH₂-COOH. The element sulfur in the form of disulfide linkages is the cause of curl.
Make, Then Break
In producing a permanent wave (called a permanent or perm) the hair is wet. It is then manipulated into the desired shape, perhaps using rods. Existing curl is then removed by breaking present disulfide linkages. Thioglycolate is applied. It severs all the disulfide linkages between peptide bonds in the keratin of the hair.
Now to reset the curl of each individual hair! Use of a neutralizer deactivates the thioglycolate, permanently setting the hair in the new configuration by reforming the disulfide bonds.