What do you mean by boundary?
If you want the actual jagged cell boundary, probably use polygon to raster Polygon to Raster (Conversion)—ArcGIS Pro | Documentation and either symbolise by outline, or convert to line.
Otherwise if you mean the rectangular bounds (envelope) - Feature Envelope To Polygon—Help | ArcGIS for Desktop
If it is a raster I think David means
You will end up with a multipart featureclass, so you might want to run
to get separate shapes for each polygonal area
You might want to try the polyline option first but if it isn't satisfactory, you can always convert the polygons to polylines if you use the raster to polygon option,
I tried with this procedure, it works for most of the cases, here is one exception as shown in the following figure:
As you can see, when two type of grids is nearby each other, one of their boundaries will be missing. Because the polygon and polyline of the nearby part is overlapped.
Any suggestions for this problem?
I might do a negative buffer of the polygons you have, then erase the original polygon with that buffer. Then run the polygon to raster tool, ensuring you have an appropriate Cell Size which will not destroy the original boundary when re-rasterized.
Why are you converting this boundary back to a raster? It seems unorthodox.
If it just the boundaries you are concerned about, you dont need to worry about them being different classes. When represented as a polygon or polyline, their 1 cell thickness will vanish and the features will share an edge.