AbstractA model equation derived for the viscosity‐average degree of polymerization (P̄v) for the incremental addition of modifiers,P̄v=K{(exp{arx1} − 1) + 2/(2 + exp {x1r})a(exp{arx2} − exp{arx1})}1/aindicates a minimum should occur in a curve ofP̄vversus conversion at which the increments are added,x1. Incremental addition of commercialtert‐dodecyl mercaptan in the laboratory to SBR recipes at 5°C. showed effects contrary to those predicted by the model equation, but incremental addition experiments withtert‐nonyl mercaptans resulted in a minimum in viscosity versusx1curves, changed the molecular weight distribution, and required 34 wt.‐ less mercaptan than did thetert‐dodecyl mercaptan control (all thetert‐dodecyl mercaptan added initially). Incremental addition oftert‐nonylmercaptan in pilot plant 20‐gal. autoclave reactors gave essentially the same results found in the laboratory bottle experiments. The stress‐strain and mixing properties of the incrementally modified polymer and thetert‐dodecyl mercaptanmodified control were essentially the same within experimental error. Conditions required for successful application of the incremental m
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