Massive-area single-crystal monolayers of two-dimensional (2D) supplies comparable to graphene and hexagonal boron nitride (h-BN) will be grown by chemical vapour deposition (CVD). Nevertheless, the excessive temperatures and quick timescales at which the conversion from a gas-phase precursor to the 2D materials seem, make it extraordinarily difficult to concurrently comply with the atomic preparations. We utilise helium atom scattering to find and management the expansion of novel 2D h-BN nanoporous phases in the course of the CVD course of. We discover that previous to the formation of h-BN from the gas-phase precursor, a metastable (3×3) construction is shaped, and that extra deposition on the ensuing 2D h-BN results in the emergence of a (3×4) construction. We illustrate that these nanoporous constructions are produced by partial dehydrogenation and polymerisation of the borazine precursor upon adsorption. These steps are largely unexplored in the course of the synthesis of 2D supplies and we unveil the wealthy phases throughout CVD progress. Our outcomes present vital foundations for 2D supplies engineering in CVD, by adjusting or fastidiously controlling the expansion situations and thus exploiting these intermediate constructions for the synthesis of covalent self-assembled 2D networks.