We use density functional theory to investigate the self-directed growth mechanism of molecular nanowires on the Si (100)-2x1 monohydride surface from the molecular precursors styrene (H2C=CH-C6H5) and propylene (H2C=CH-CH3). The reaction is initiated using a scanning tunneling microscope tip to create a Si dangling bond on the surface. This dangling bond then attacks the C=C pi bond to form a Si-C bond and a C radical. Next, the C radical abstracts a H atom from a neighboring surface site, which results in a new Si dangling bond to propagate the chain reaction. For the case of H2C=CH-C6H5 the predicted hydrogen abstraction barrier of 18.0 kcal/mol from a neighboring dimer along the dimer row for C-H bond formation is smaller than H2C=CH-C6H5 desorption energy of 22.6 kcal/mol. On the other hand, for the case of H2C=CH-CH3 the predicted hydrogen abstraction barrier of 10.8 kcal/mol for C-H bond formation from a neighboring dimer is significantly larger than H2C=CH-CH3 desorption barrier of 2.7 kcal/mol. Consequently, the predicted barriers indicate that the self-directed growth of nanowires on (100) silicon using styrene occurs while a self-directed chain reaction using propylene should not occur, in agreement with experimental observations.