A.
The definition and characteristics are explained in DGR 3.4.1.4.1.1.
“Polymerizing substances are substances which, without stabilization, are liable to undergo a strongly exothermic reaction resulting in the formation of larger molecules or resulting in the formation of polymers under conditions normally encountered in transport.”

“Polymerization” means a chemical formation of more than two molecules of the same single chemical compound resulting to a new macromolecule chemical compound.

“Polymerizing reaction” is to make a polymerizing substance by chemically connecting monomers or polymers.

For example, “polyethylene”, a polymer, is made by chemically connecting (polymerizing)

multiple “ethylene” monomers. When multiple “vinyl chloride” (monomer) is polymerized, a polymer “polyvinyl chloride” is produced, the plastic bags you receive at a supermarket.

Nylon is a “polyamide” produced by polymerizing “amine” and “carboxylic acid” molecules.
The “polyamide” produced is used to make synthetic fibre and synthetic resin.
“Protein” which compose our human body is a “polyamide” (a polymer) of multiple “amino acid” monomers.
Macromolecules (high polymer) use a prefix of “poly-“.

3.4.1.4.1.2 A mixture meeting the criteria of polymerizing substances must be classified as a polymerizing substances in Division 4.1. There are four new entries in the blue pages.

UN 3531 Polymerizing substance, solid, stabilized, n.o.s. ★ Class 4.1 PI 459
UN 3532 Polymerizing substance, liquid, stabilized, n.o.s.★ Class 4.1 PI 459
UN 3533 Polymerizing substance, solid, temperature controlled, s.o.s.★ Class 4.1 Forbidden
UN 3534 Polymerizing substance, liquid, temperature controlled, n.o.s.★ Class 4.1 　　 Forbidden

3.4.1.4.1.3 Polymerizing substances are subject to temperature control in transport if their self-accelerating polymerization temperature (SAPT) is 50℃ or less in the packaging in which the substances is to be transported.

Substances of Class 2, Class 3, Div. 6.1 and Class 8 which have the possibility of a dangerous decomposition or polymerization under normal conditions of transport are defined in DGR 3.2.6; 3.3.6; 3.6.1.9; and 3.8.4. “Chemically unstable substances are forbidden for transport unless the necessary precautions have been taken to prevent the possibility of a dangerous decomposition or polymerization under normal conditions of transport. For the precautions necessary to prevent polymerization, see Special Provision A209. To this end particular care must be taken to ensure that receptacles and tanks do not contain any substances liable to promote these reactions.”

Special Provision A209:- “When chemical stabilization is employed, the person offering the packaging for transport must ensure that the level of stabilization is sufficient to prevent the substance in the packaging from dangerous polymerization at a bulk mean temperature of 50℃. Where chemical stabilization becomes ineffective at lower temperatures within the anticipated duration of transport, temperature control is required and the substance is forbidden for transport by air.”