A liquid phase polymerization process for preparing PIB polymer composition, said process comprising: providing a feedstock comprising isobutylene; providing a catalyst composition comprising a complex of BF 3 and a catalyst complexing agent thereof; introducing said feedstock and said catalyst composition either separately or as a single mixed stream into a residual reaction mixture in a loop reactor reaction zone; intimately intermixing said residual reaction mixture, said feedstock and said catalyst composition to present an intimately intermixed reaction admixture in said reaction zone; maintaining the intimately intermixed reaction admixture in its intimately intermixed condition and keeping it at a reaction temperature of at least 16 °C (60 °F) while the same is in said reaction zone; to thereby cause the isobutylene therein to undergo polymerization to form said polyisobutylene; maintaining a ratio of complexing agent to BF 3 in said reaction admixture that is no more than 1.3:1; withdrawing a product stream comprising said PIB polymer composition from said reaction zone; controlling the introduction of said feedstock into said reaction zone and the withdrawal of said product stream from the reaction zone such that the residence time of the isobutylene components undergoing polymerization in the reaction zone is no more than 4 minutes; characterized in that the ratio of complexing agent to BF 3, the reaction temperature, and the residence time are so adjusted that the PIB polymer composition withdrawn from the reaction zone is a mid-range vinylidene content PIB polymer composition comprising PIB molecules, wherein a first portion of said PIB molecules have alpha position double bonds and a second portion of said PIB molecules have beta position double bonds, wherein said first and second portions together include at least 90% of the PIB molecules of the composition, wherein said first portion includes from 20% to less than 70% of the PIB molecules of the composition, wherein said second portion includes from 30% to 80% of the total molecules present in the product, and wherein said mid-range vinylidene content PIB polymer composition has a molecular weight in the range of from 350 to 5000. providing a feedstock comprising isobutylene; providing a catalyst composition comprising a complex of BF 3 and a catalyst complexing agent thereof; introducing said feedstock and said catalyst composition either separately or as a single mixed stream into a residual reaction mixture in a loop reactor reaction zone; intimately intermixing said residual reaction mixture, said feedstock and said catalyst composition to present an intimately intermixed reaction admixture in said reaction zone; maintaining the intimately intermixed reaction admixture in its intimately intermixed condition and keeping it at a reaction temperature of at least 16 °C (60 °F) while the same is in said reaction zone; to thereby cause the isobutylene therein to undergo polymerization to form said polyisobutylene; maintaining a ratio of complexing agent to BF 3 in said reaction admixture that is no more than 1.3:1; withdrawing a product stream comprising said PIB polymer composition from said reaction zone; controlling the introduction of said feedstock into said reaction zone and the withdrawal of said product stream from the reaction zone such that the residence time of the isobutylene components undergoing polymerization in the reaction zone is no more than 4 minutes; characterized in that the ratio of complexing agent to BF 3, the reaction temperature, and the residence time are so adjusted that the PIB polymer composition withdrawn from the reaction zone is a mid-range vinylidene content PIB polymer composition comprising PIB molecules, wherein a first portion of said PIB molecules have alpha position double bonds and a second portion of said PIB molecules have beta position double bonds, wherein said first and second portions together include at least 90% of the PIB molecules of the composition, wherein said first portion includes from 20% to less than 70% of the PIB molecules of the composition, wherein said second portion includes from 30% to 80% of the total molecules present in the product, and wherein said mid-range vinylidene content PIB polymer composition has a molecular weight in the range of from 350 to 5000.