Phenylation of PVC with triphenylaluminum and related model reactions

Abstract

It was theorized that the heat stability of PVC may be improved by exchanging the “weak” (allylic and tertiary) chlorines by groups of higher bond strength, e.g., phenyl. Thus PVC was phenylated by ϕ₃Al in carbon disulfide slurry for various times (1 to 5 hrs) and temperatures (from −30 to +45°C). The ϕ₃Al concentration, temperature and time of ϕ₃Al treatment greatly affect the ultimate heat stability of PVC. Significant improvement in the heat stability was obtained by treating PVC in carbon disulfide at 25°C for 1 hr. The mechanism of phenylation was studied by model experiments in which the possible structural irregularities in PVC were simulated by small molecule chlorine compounds, i.e., 3‐chloro‐1‐butene, 1,3‐dichloro‐1‐butene, 4‐choro‐2‐pentene were models for terminal and inchain allylic chlorines, t‐butyl chloride was the model for tertiary chlorine at branch site, and isopropyl chloride simulated regular secondary chlorines. The reactions between ϕ₃Al and these chlorides were carried out at 35°C for various times and the NMR spectra were analyzed. The ϕ₃Al + tBuCl reaction in carbon disulfide gives t‐butylbenzene (Wurtz coupling) and isobutylene oligomers. Evidently ϕ₃Al (or the ϕ₂AlCl formed) dehydrochlorinates tBuCl to isobutylene which rapidly oligomerizes in the presence of Lewis acids. In aromatic diluents, i.e., benzene, chlorobenzene, and o‐chlorotoluene, Friedel‐Crafts t‐butylation of the diluent occurs; in mesitylene t‐butylation is absent presumably because of steric hindrance and small amounts of t‐butylbenzene are formed by Wurtz coupling. All the allyl chlorides examined are rapidly phenylated by ϕ₃Al in carbon disulfide at 35°C and yield the expected products: 3‐chloro‐1‐butene → 1‐phenyl‐2‐butene; 1,3‐dichloro‐1‐butene → 1‐chloro‐3‐phenyl‐1‐butene; 4‐chloro‐2‐pentene → 4‐phenyl‐2‐pentene. The kinetics of phenylation by ϕ₃Al of model compounds have been studied and the following rate constants were obtained: k_allyl = too fast to measure, k_tBuCl = 1.0, and k_iPrCl = 9 × 10⁻⁴ l/mole min. Evidently the rate of the ϕ₃Al + RCl reaction is mainly determined by the stability of the R^⊕ intermediate: substituted allylic^⊕ ≫ tertiary^⊕ ≫ secondary^⊕.