Three modes of operation are considered for the free-radical solution polymerisation of vinyl monomers in semi-batch reactors. By using dimensionless variables and equations, the regions of validity for different assumptions of stationarity are determined. These regions depend on the reactor filling policy and have been compared with those for batch and continuous-flow back-mixed reactors. Using a simple example, it is shown that monomer conversions depend on both the filling policy and the relative stability of the initiator. With reactive initiators, addition of initiator to monomer produces the highest polymer yield and a filling tank operation can produce higher yields than a batch reactor. High molecular mass polymer products are expected from semi-batch schedules involving the delayed addition of initiator. Cumulative molecular masses from a semi-batch reactor can be higher than those from a batch reactor when a relatively reactive initiator is used. Semi-batch operation tends to produce wider molecular mass distributions than those from corresponding batch polymerisations.