Expanding monomer

They can be added to monomer formulations to counteract the usual volume shrinking (during polymerization) to manufacture products with higher quality and durability.

Volume Shrinkage is in first line for the unmeltable thermosets a problem, since those are of fixed shape after polymerization completed.

[1]: 2 The quality of thermosets (crosslinked polymers) is determined by a numerous factors such as the purity of the used monomer, polymerization time and temperature, stoichiometry of comonomers (when used) or type and quantity of catalyst or initiator.

Expandable monomers occupy after polymerization a greater volume than before and were designed to counteract the volumetric shrinkage upon addition.

Nonetheless, for some applications even a small shrinkage can be desirable as for one-piece molds, to accomplish an easy removal.

A minor role is playing furthermore the entropy change while polymerization and the package density, as the polymer is more closely packed than the monomer.

In step-growth polymerization (condensation reaction) small molecules are eliminated, which are also contributing to shrinkage when removed.

At elevated temperatures also the thermal aging plays a role, where unreacted monomer can polymerize and degradation products and other small molecules are released.

are reducing the shrinkage in proportion to the used amount since the volume stable filler replaces the shrinking polymer.

The viscosity increase which is caused by fillers is disadvantageous since it restricts the flow of resins and mold fill.

A hypothetical ring-opening process of cyclobutane to n-butane would result in a volumen expansion of approximately 15%.

The volume change during ring-opening polymerization is also influenced by the number of polymerizable rings per monomer.

Derived from the ring effects the design of the expanding monomers is based on bicyclic compounds.

A net expansion is reached when for each bond which undergoes a shift from covalent to van der Waals distance at least two bonds are shifting from covalent to near van der Waals distance, as it is shown in the following picture.

Usually a Lewis acid like boron trifluoride etherate is used for both, the synthesis of the orthoester and the polymerization.

Expanding monomers are interesting for application as in matrix resins in radically polymerized dental fillings,[5][6] high-strength composites (e. g. in epoxy resins[7]), adhesives, coatings, precision castings, and sealant materials to counteract shrinking during polymerization.

[8] This can be necessary in case of dental fillings since polymerization shrinkage and subsequent contraction stress in the resin composite and at the bonding interface may lead to debonding, microleakage, post-operative sensitivity, a compromise in the material's physical properties and even cracks in healthy tooth structure.