Addition-fragmentation Chain Transfer Reagent Research Articles

The influence of vinyl activating groups on β-allyl sulfone-based chain transfer agents for tough methacrylate networks

In radical polymerization of monofunctional monomers, addition fragmentation chain transfer (AFCT) agents are well known to regulate polymerization and yield polymers with lower molecular weights and narrower molecular weight distributions. Papers concerning bulk photopolymerization of monomer mixtures with AFCT agents are rarely found in literature. In this article, AFCT reagents based on β-allyl sulfones with different vinyl activating groups were synthesized and compared. The compounds were tested in mono- and difunctional monomer systems providing information about the influence on photoreactivity, molecular weight, as well as thermal and mechanical properties of the resultant polymers. Where more potent activating groups (-Ph, -CN) markedly influenced polymerization at lower concentrations, the AFCT reagent with an ester activating group reacted at a similar rate to the methacrylate monomer (CT ≈ 1) and provided the best overall performance. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1417-1427

Allyl sulfides and α‐substituted acrylates as addition–fragmentation chain transfer agents for methacrylate polymer networks

ABSTRACTThe rapid and uncontrolled nature of network formation from di(meth)acrylate monomers produces high shrinkage stress and results in polymers with oftentimes brittle mechanical properties. Methods for regulating polymerization and network formation are sought. One option is the use of addition–fragmentation chain transfer (AFCT) agents, which are well known to control molecular weight and molecular weight distribution of monofunctional (meth)acrylates. A series of novel and previously described AFCT reagents were synthesized and screened with laser flash photolysis to determine reactivity. Well‐performing AFCT reagents were then tested in polymerizations with monofunctional and difunctional methacrylates. With monofunctional monomers, the molecular weight and polydispersity of the resultant linear polymers tend to decrease with the addition of AFCT agent. In copolymerization with dimethacrylate monomers, the AFCT agents were found to substantially lower and sharpen the glass transition. Sharpness of the glass transition is here indicative of a more regular and homogenous network. After coupling of the instruments, photorheology was performed simultaneously with real‐time IR to show an increase in monomer conversion at the time of gelation, which appears to have a positive effect on reducing shrinkage stress. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 394–406

β-Allyl Sulfones as Addition–Fragmentation Chain Transfer Reagents: A Tool for Adjusting Thermal and Mechanical Properties of Dimethacrylate Networks

Dimethacrylates are known to have good photoreactivity, but their radical polymerization usually leads to irregular, highly cross-linked, and brittle polymer networks with broad thermal polymer phase transitions. Here, the synthesis of mono- and difunctional β-allyl sulfones is described, and those substances are introduced as potent addition–fragmentation chain transfer (AFCT) reagents leading to dimethacrylate networks with tunable properties. By controlling the content and functionality of said AFCT reagents, it is possible to achieve more homogeneous networks with a narrow glass transition and an adjustable glass transition temperature (Tg), rubber modulus of elasticity (Er), and network density. In contrast to dimethacrylate networks containing monomethacrylates as reactive diluents, the network architecture of the β-allyl sulfone-based dimethacrylate networks is more homogeneous and the tunability of thermal and mechanical properties is much more enhanced. The reactivity and polymerization were investigated using laser flash photolysis, photo-DSC, and NMR, while DMTA and swellability tests were performed to characterize the polymer.