# Evaluation of Various Thiourea Derivatives as Reducing Agents in Two-Component Methacrylate-Based Materials

## TL;DR

This study examines how different thiourea derivatives affect the performance of redox initiator systems in dental materials, aiming to improve polymerization efficiency and mechanical properties.

## Contribution

The study reveals the dual role of thiourea in redox systems and identifies specific derivatives that enhance mechanical properties in dental composites.

## Key findings

- Thiourea acts as both a reducing agent and a ligand in copper complexes, influencing radical generation and polymerization.
- Hexanoyl thiourea, N-benzoylthiourea, and 1-(pyridin-2-yl)thiourea produced composites with flexural strength >100 MPa and modulus >6000 MPa.
- Adjusting Cu(acac)2 allows working time to be set between 100 and 200 seconds.

## Abstract

Two-component dental materials are commonly used by the dentist for various applications (cementation of indirect restorations, filling of a cavity without layering, etc.). These materials are cured by redox polymerization. The (hydro)peroxide/thiourea/copper salt redox initiator system is well established and can be found in a wide range of commercially available dental materials. The thiourea is a key component of the initiator system. This study explores the influence of the nature of the thiourea reducing agent on the reactivity and efficiency of redox initiator systems. In this work, six different thiourea structures were investigated, in combination with copper(II) acetylacetonate and cumene hydroperoxide (CHP), to understand their impact on polymerization kinetics and mechanical properties of methacrylate-based materials. Various experimental techniques, including mass spectrometry (MS) and spectroscopic analyses, were employed to elucidate the underlying mechanisms governing these redox systems. The results highlight that thiourea plays a dual role, acting both as a reducing agent and as a ligand in copper complexes, affecting radical generation and polymerization efficiency. Structural modifications of thiourea significantly influence the initiation process, demonstrating that reactivity is governed by a combination of factors rather than a single property. Self-cure dental flowable composites exhibiting excellent flexural strength (>100 MPa) and modulus (>6000 MPa) were obtained using hexanoyl thiourea, N-benzoylthiourea, or 1-(pyridin-2-yl)thiourea as a reducing agent. The adjustment of the Cu(acac)2 enables to properly set the working time in the range of 100 to 200 s. These findings provide valuable insights into the design of the next generation of redox initiating systems for mild and safe polymerization conditions.