What Are the Basic Reactions in the Production of Flexible Polyurethane Foam?

There are two main reactions in the production of flexible polyurethane foam:

Reaction 1: Reaction between isocyanate and hydroxyl compounds

Reaction 2: Reaction between isocyanate and water

There are also two side reactions:

Reaction 3: Reaction between urethane and isocyanate

Reaction 4: Reaction between substituted urea and isocyanate

Reactivity order of active hydrogen compounds in the four reactions:
Primary hydroxyl > Secondary hydroxyl ~ Water > Urea group > Urethane

Under a catalytic system, the reactivity order during production becomes:
Water ~ Primary hydroxyl > Secondary hydroxyl > Urea group > Urethane