void Generate() {
    for (int i = 0; i < 20; i++) {
       for(int j = 0; j < 20; j++) {
         clone = Instantiate(grid,new Vector2(i*tileWidth, j*tileHeight),Quaternion.identity)as GameObject;
         clone.renderer.material = tiles[Random.Range(0,tiles.Length)];
       }
    }   
}

I have a grid generating and I make the material random. How do I keep that grid going when I get to the edge of what is already generated? Also, another small question is it possible to change material at runtime?