E09. Determine whether a material that has a degree of conversion around 60% also must have a high concentration of free monomer.

Degree of conversion relates to number of consumed double bonds in relationship to the original number of double bonds. For example, if we start with 100 double bonds (before cure) and end up with 30 double bonds (after cure), we have consumed 70% of all double bonds. The degree of conversion is said to be 70%.  

(Courtesy of IE Ruyter)

This slide shows how a dimethacrylate polymerizes. A dimethacrylate molecule can cross-link. However, it does not always react which reacts in free monomer. It may also react with one end (methacrylate group) only. It is possible to how well a material has reacted by determining how many percent of the double bonds that are still available. Degree of conversion is defined as 100% minus remaining percent double bonds. Typical conversion values for dental composites are 50 - 70%. Remember that it does not mean that 50-30% monomer is still left, only how many percent double bonds that are still present. Many molecules are as mentioned bonded at one end only.

From the above we can draw another important conclusion. If we have a degree of conversion that is 50%, we can, at least theoretically have one single polymer chain along which the second methacrylate group of each dimethacrylate molecule with its carbon double bond form a side group. In this particular case, we have not a single monomer molecule present despite a degree of conversion of only 50 %. This example is of course an extreme situation, and if the degree of conversion is as low as 50% we should expect to have some leachable monomer molecules present.

Dental composites have often a degree of conversion that ranges from 50 to 75%. The larger the amount of stiffer molecules (e.g. bisGMA), the lower the degree of conversion. The reason is simply that stiff molecules can not as easily rotate and match with other reactive methacrylate groups during polymerization.