RE: Process simulation of HIPS 
Hi impact polystyrene is formed in two steps. First, polybutadiene homopolymer is produced. Each repeat unit of butaidiene has an internal double bond. This can be CIS, TRANS, or pendant. Apparently Cis/Trans is much more common.
In the second stage, the rubber is crumbed and dissolved in styrene. A "grafting initiator" is added to the mix. This initiator decomposes and reacts with the internal double bonds, causing them to start polymerizing.
Add an appropriate reaction (maybe "Graft initiation") where
R* + Dn( butadiene segment ) --> Pn*( butadiene segment ) [Increments LCB]
No additional reactions are required, although the BD is an internal segment could be treated as a live end for practical purposes. Increments live sflow and eflow, no impact on higher moments.
Need to think through implications for BLOCKN as it seems this assumes linear molecules today; need to make a correction for LCB.