This paper presents a multiscale approach to model the dynamic transition between macro, micro, and homogenized atomistic scales in 2D polycrystalline materials. At the macroscale, the material is assumed as an isotropic homogeneous medium. The material at the microscale, in contrast, is characterized by anisotropic grains with stochastic morphologies and random crystalline orientations. The Boundary Element Method (BEM) is used to model both macro and micro scales using isotropic and anisotropic formulations, respectively. To connect both scales, the macro transient responses at internal points are prescribed as boundary conditions to the microscale analysis. The mechanical behavior at the interfaces between grain boundaries is assessed using Cohesive Interface Elements (CIEs). Based on the Multiscale Cohesive Zone Model (MCZM), this approach transfers the homogenized strain energy evaluated in the CIE during the microscale analysis to an atomistic arrangement. In order to define a failure criterion from the atomistic interpretation, the extended Finnis-Sinclair potential is applied to describe the interactions between atoms. Finally, we present some examples of intergranular crack propagation resulting from the rupture of atomic bonds due to the interatomic forces.