High Refractive Index Aromatic and Ether‐Containing Polythioureas: Improving Transparency and Mechanical Properties via Reciprocal Hydrogen Bonds

Abstract A series of aromatic‐aliphatic polythiourea copolymers bearing highly polarizable poly(4,4′‐phenylsulfide thiourea) and hydrogen bond (H‐bond) acceptor poly(ether thiourea) units are synthesized. The copolymers are highly refractive ( n D = 1.78 at maximum) as a result of high sulfur content and lack of crystallinity and exhibit improved visible transparency (over 95 %T in 1 µm thickness) compared to all‐aromatic polythioureas. Further, the copolymers display higher fracture strength upon introducing 20–25 mol% of the poly(ether thiourea) unit, despite the inherent flexibility of ether groups. A detailed spectroscopic investigation indicates that this unique toughening behavior is attributed to the reciprocal (well compatible) intermolecular H‐bonds intersected between aromatic and aliphatic thiourea moieties, as well as dynamic H‐bond slipping/swapping between ether units and thiourea counterparts during the stretching process. These findings provide a rational strategy for simultaneously satisfying superior optical and mechanical properties, broadly applicable to various H‐bond containing high refractive index polymers.