On November 13, 1937, chemist Otto Bayer and his team at IG Farben's Leverkusen lab filed German patent DRP 728,981 — "A Process for the Production of Polyurethanes and Polyureas." The US counterpart, US 2,292,443 (filed 1938, granted August 1942), described what Bayer called the diisocyanate polyaddition process: react a diisocyanate with a polyol, and watch a new class of polymer form itself, molecule by molecule, with no byproducts to boil off.

This was chemically radical. Wallace Carothers had just invented nylon at DuPont using condensation — reactions that spat out water and required extreme conditions. Bayer's polyaddition route was clean: the isocyanate group (–N=C=O) is so hungry that it grabs a hydroxyl (–OH) and bonds instantly, incorporating every atom. You could tune the recipe. Stiff polyol + aromatic isocyanate → rigid plastic. Flexible polyol + a splash of water → the water reacts with isocyanate to release CO₂, which foams the polymer as it cures. One family, infinite materials.

IG Farben's initial goal was mundane: a nylon substitute Germany could make without infringing DuPont's patents. Wartime shortages pushed the team to explore foams as rubber replacements. By 1941 they had rigid foams for aircraft; by 1952 Bayer AG (the postwar successor) was shipping flexible foam commercially.

What Bayer actually claimed in the patent was startlingly broad — the general reaction between polyfunctional isocyanates and any compound bearing active hydrogens (alcohols, amines, water, carboxylic acids). Essentially every polyurethane made since fits inside those claims. Chemists still call it the Bayer reaction.

Where it hides today:

• Your sneakers. Nike React, adidas Boost, and every polyurethane midsole use tuned flexible PU foams. Boost's TPU pellets are Bayer chemistry, steam-expanded.
• Your refrigerator and freezer. The insulation between the inner and outer walls is rigid PU foam blown in place. Without it, your fridge would be the size of a wardrobe to hit the same efficiency.
• Your car. Seats, headliners, dashboards, steering wheels, bumpers, and sound-deadening panels are almost all polyurethane variants.
• Your mattress. Memory foam is a viscoelastic polyurethane — NASA developed it in 1966 to cushion astronauts, then Tempur-Pedic commercialized the recipe.
• Building envelopes. Spray polyurethane foam is the highest R-value-per-inch insulation in wide use. Passive House construction depends on it.
• Medical implants. Polycarbonate-urethane is used in artificial heart valves, catheters, and 3D-printed patient-specific implants because it flexes like tissue and resists blood clotting.
• Wind turbine blades. Modern 100-meter blades use PU-based adhesives and leading-edge coatings that survive rain erosion at 300 km/h tip speeds.
• Additive manufacturing. Carbon's Digital Light Synthesis printer — the one Adidas uses to print Futurecraft midsoles — cures liquid polyurethane resins under UV light. Bayer's 1937 reaction, running at 100 μm resolution.

Why it still matters: polyurethanes are one of the only polymer classes where the chemist can independently dial hardness, elasticity, density, and thermal response by swapping monomers. That tunability is why they haven't been displaced in 90 years. Global production now exceeds 25 million tonnes per year — roughly 3 kg of new polyurethane per human, annually.

The catch Bayer never anticipated: isocyanates are toxic to make, and PU is hard to recycle because those instant, permanent bonds don't want to un-bond. The frontier now is covalent adaptable networks — polyurethanes engineered to reversibly break and re-form, so a mattress can be dissolved back into feedstock. The chemistry that made the material class un-recyclable is being rewritten to make it circular.