Method

We have performed DFT calculations, within a pseudopotential total-energy scheme for periodic systems [36]. We use the local-density approximation (LDA) [37,38] for the exchange-correlation energy, Troullier-Martins pseudopotentials [39] for boron, carbon and nitrogen and a modified Kerker pseudopotential [40] for hydrogen, and a plane-wave basis-set for the wavefunction expansion up to a kinetic energy cutoff of 60 Ry. The polymers were modelled in a periodic cell with a large distance between neighbouring chains to minimize their interaction. The Brillouin zone was sampled using twenty $k$-points. Both the atomic coordinates and the cell degrees of freedom were fully relaxed.