Kripke on Mind-Brain Identity

Kripke notoriously argues against the mind as being identical to the brain roughly as follows. 

Saul Kripke

1. My being in a pain is identical my brain state.

2. If my being in a pain state is identical to my brain state, then necessarily my being in a pain state is identical to my brain state. (rigid designation)

3. Hence, necessarily my being in a pain state is identical to my brain state.

4. ​However, we can imagine imagine disembodied minds or brain-states with no pains.

5. But, it follows that it is not necessary that my being in a pain state is identical to my brain state.

6. Therefore, my being in a pain state is not identical to my brain state. 

Questions: How would a materialist respond to this argument? How would they "explain away" the appearance of contingent identity mentioned in (4)?