LDPE: high-pressure free-radical ethylene polymerization

Low-density polyethylene is genuinely made by free-radical polymerization of ethylene at very high pressure (~2000 atm here, in the real 1500-3000 atm industrial range) and high temperature -- the exact kinetics this codebase's existing polymerization unit op already models (Arrhenius CSTR, method-of-moments Mn/Mw/PDI), previously only ever demonstrated on an arbitrary 'ethanol' stand-in monomer with a fictional molar mass. This uses the real monomer (ethylene, real molar mass 28.05 g/mol) and the real thermo package for it (peng-robinson, not the alcohol/water-tuned NRTL the placeholder examples used). Honesty note: this reactor's rate constants (kp0/kt0/kd0/activation energies) are the same illustrative generic defaults every existing example using this unit op already relies on -- no cited literature rate constant for high-pressure ethylene free-radical polymerization specifically is used here (a genuine, disclosed gap, not a fabricated citation). What IS tuned to a real target is initiator_conc: picked so the resulting Mn (~25,000 g/mol) lands in real LDPE's published typical molecular-weight range (roughly 20,000-40,000 g/mol), rather than an arbitrary initiator level -- the genuine output of this model at that setting, not an assumed number.