AbstractThe kinetics and the molecular weight distributions (MWD) in the anionic polymerization oftert‐butyl acrylate (tBuA) in tetrahydrofuran at 23 ± 3°C were investigated.Tert‐butyl α‐lithioisobutyrate (tBiB‐Li) was used as the initiator in the absence and presence of the additives lithium chloride and lithiumtert‐butoxide (tBuOLi). A flow tube reactor was used which allowed very rapid mixing of reagents, within milliseconds. The polymerization is extremely fast, half‐lives being 0,01 s for the system without additive, 0,05 s for LiCl and 3 s for tBuOLi as an additive. The rate of termination was estimated from the strong UV absorption of the enolized β‐oxoester end group formed. The number‐average degree of polymerization is a linear function of conversion, the absolute values, however, demonstrate initiator efficiencies of only ca. 30%. The MWD's of the polymers formed are narrow (Mw/Mn= 1,35 without additive,M/Mn= 1,08 in the presence of LiCl andMw/Mn= 1,77 in the presence of tBuOLi). After precipitation these values are reduced to 1,17; 1,06 and 1,32, respectively. Experiments in a stirred tank reactor and in ampoules lead to much broader MWD's. This shows that the rate of mixing has a strong effect on the MWD and is of substantial importance in these ra