It can probably be done. We would just need an obscene amount of power to fully reduce it to its base elements, something like a hydrogen blast furnace. The economics wouldn't add up until we it is basically free power or there is some market value to doing so. People aren't going to invest the capital to do so purely out of altruism, somebody needs to be earning something of value.
It also depends on the type of plastic as well, anything with chlorine or flourine in it would add extra complexity.