Aminosilane Coupling Agents at the Boron Carbide/Poly(Vinyl Chloride) Interface: Substitution, Minor Dehydrochlorination and C–Cl Bond Weakening

Boron carbide in a hydrogenous polymer is a standard thermal-neutron shield. Under a preliminary cross-section estimate, poly(vinyl chloride) (PVC) reaches the absorption of high-density polyethylene carrying 10 wt% B4C with 6.2 wt%, and so needs less filler and less coupling agent. Whether an aminosilane coupling agent is inert toward such a matrix was examined computationally. Substitution and HCl elimination by the pendant amine at a backbone C–Cl unit were located as verified transition states for free and grafted models, sampled conformationally and compared over a range of dielectric environments. The semiempirical GFN2-xTB and GFN1-xTB methods favor elimination through the zero-point advantage of a synchronous proton transfer. Dispersion-corrected hybrid density functional theory (DFT) does not: its elimination transition state is asynchronous, the zero-point advantage falls from 16 to 7 kJ mol−1, and substitution is favored by 12 to 19 kJ mol−1 (single transition-state conformers, mixed DFT/GFN2-xTB Hessians). The semiempirical prediction of dominant elimination is therefore unreliable; the coupling agent most likely reacts with PVC mainly by N-alkylation, although no product ratio for a real composite follows. At the DFT level, the half-life of a grafted amine at 453 K is on the order of hours to days, an order-of-magnitude estimate for an idealized bimolecular encounter. The minor elimination channel leaves allylic C–Cl bonds 76.5 kJ mol−1 weaker than the backbone. For PVC-matrix shields, non-basic methacryloxypropyl and glycidoxypropyl silanes, which carry no nucleophilic nitrogen, are the safer choice where amine-induced HCl loss and ammonium chloride formation at the interface are to be avoided, as far as the amine-mediated pathways studied here are concerned.