Mechanistic Insights into Regioselectivity and Its Evolution in On-Surface Polymerization.

Area of Science:

• Surface Science
• Polymer Chemistry
• Organic Synthesis

Background:

• Surface-catalyzed polymerization is vital for chemical science and industry.
• Achieving regioselective radical polymerization on surfaces presents a significant challenge.

Purpose of the Study:

• To demonstrate and understand the regioselective Ullmann polymerization of 2,8-dibromoquinoline (DBQ) on an Au(111) surface.
• To elucidate the molecular-level mechanisms governing regioselectivity and its temperature dependence.

Main Methods:

• Scanning tunneling microscopy (STM) for surface imaging.
• Density functional theory (DFT) calculations for mechanistic insights.
• Kinetic modeling to analyze polymerization dynamics.

Main Results:

• DBQ monomers form covalent dimers via head-to-head (HtH) coupling at 348-368 K.
• At higher temperatures (390-473 K), oligomers and polymers form with emerging head-to-tail (HtT) linkages dominating.
• Regioselectivity evolves from HtH to HtT with increasing temperature.

Conclusions:

• The study reveals temperature-driven regioselectivity evolution in surface-catalyzed Ullmann polymerization.
• A sequential monomer addition and shifting reactive site distribution explain the HtH to HtT transition.
• Provides crucial molecular-level insights into surface polymerization regiochemistry.