Cyclic Acetals as Expanding Monomers to Reduce Shrinkage

Area of Science:

• Polymer Chemistry
• Radical Reactions
• Materials Science

Background:

• Polarity-reversal catalysts (PRCs) are known in radical chemistry but underutilized in photopolymerization.
• Traditional photopolymerization often suffers from shrinkage and associated stress.

Purpose of the Study:

• To introduce and investigate the use of thiols as dual-function reagents (thiol-ene click and PRC) in photopolymerization.
• To develop a method for reducing polymerization-induced shrinkage and stress in thermosets.
• To explore the application of this system in additive manufacturing.

Main Methods:

• Utilizing thiols as both thiol-ene click reagents and polarity-reversal catalysts (PRCs).
• Employing radical-mediated redox rearrangements of benzylidene acetals to cyclic benzoate esters.
• Investigating the volumetric expansion during the rearrangement reaction.
• Characterizing the impact on network properties and polymerization stress.

Main Results:

• Cyclic benzylidene acetals rearrange to benzoate esters, causing significant molecular volumetric expansion.
• This expansion effectively reduces thermoset shrinkage and polymerization stress.
• The (thermo-)mechanical properties of the cross-linked networks remain largely unaffected.
• The process simplifies photopolymerization by eliminating the need for sensitizers or mixed initiator systems.

Conclusions:

• Thiols can act as effective polarity-reversal catalysts (PRCs) in photopolymerization, enabling shrinkage reduction.
• The dual functionality of thiols simplifies the process and allows for spatial and temporal control.
• This approach offers a promising platform for additive manufacturing applications requiring low-stress materials.