Split mesh faces C#

How can i split mesh faces without losing faces materials assignments. The code i used is here (the other variables you can't see are declared in the main code), it works but it loses materials assignments and gets the final mesh with only a material:

 void defragMesh()
     {
         myMesh = new Mesh();
         
         myMesh.vertices = startMesh.vertices;
         myMesh.triangles = startMesh.triangles;
         myMesh.uv = startMesh.uv;
         myMesh.normals = startMesh.normals;
         
         newVertices = new Vector3[myMesh.triangles.Length];
         int[] newTris = new int[myMesh.triangles.Length];
         Vector2[] newUV = new Vector2[myMesh.triangles.Length];
         Vector3[] newNormals = new Vector3[myMesh.triangles.Length];
         
         for(int i = 0; i < myMesh.triangles.Length; i++)
         {
             newVertices[i] = myMesh.vertices[myMesh.triangles[i]];
             newUV[i] = myMesh.uv[myMesh.triangles[i]];
             newNormals[i] = myMesh.normals[myMesh.triangles[i]];
             newTris[i] = i;
         }
         
         myMesh.vertices = newVertices;
         myMesh.triangles = newTris;
         myMesh.uv = newUV;
         myMesh.normals = newNormals;
         
         normals = new Vector3[myMesh.triangles.Length / 3];
         int index = 0;
         for(int i = 0; i < normals.Length; i++)
         {
             Vector3 v = newVertices[index++];
             normals[i] = Vector3.Cross(newVertices[index++] - v, newVertices[index++] - v).normalized;
         }
         
     }