Influence of Molecular Weight on Conversion and exo-Selectivity in Thermal PIBSA Synthesis from Highly Reactive Polyisobutylene

Polyisobutylene succinic anhydride (PIBSA) is an important intermediate for ashless dispersant additives used in lubricant formulations, where conversion and exo-selectivity strongly influence product performance. In this work, the thermal synthesis of PIBSA derived from highly reactive polyisobutylene (HR-PIB) 1300 and 2300 was systematically investigated using a full factorial experimental design to evaluate the effects of feed molar ratio, reaction temperature, and stirring rate on conversion and exo-PIBSA yield. Structural confirmation was performed using 1H NMR spectroscopy, gel permeation chromatography (GPC), and saponification value analysis. HR-PIB 1300 achieved conversions of 60–94%, while HR-PIB 2300 achieved conversions of 75–88% under the investigated conditions. A clear molecular-weight-dependent trade-off between conversion and exo-selectivity was observed. For PIBSA 1300, higher temperatures increased conversion but reduced selectivity, resulting in exo-PIBSA yields of 24–56%. In contrast, PIBSA 2300 achieved yields of up to 71%, highlighting the importance of mixing and mass-transfer effects in higher-viscosity systems. These findings demonstrate that optimal thermal maleation conditions are strongly dependent on polymer molecular weight and provide mechanistic insight for future optimization of industrial PIBSA production.