The time-resolved fluxes of negative polysilicon hydride ions from a power-modulated rf silane plasma have been measured by quadrupole mass spectrometry and modeled using a simple polymerization scheme. Experiments were performed with plasma parameters suitable for high-quality amorphous silicon deposition. Polysilicon hydride anions diffuse from the plasma with low energy (approximately 0.5 eV) during the afterglow after the electron density has decayed and the sheath fields have collapsed. The mass dependence of the temporal behavior of the anion loss flux demonstrates that the plasma composition is influenced by the modulation frequency. The negative species attain much higher masses than the positive or neutral species and anions containing as many as sixteen silicon atoms have been observed, corresponding to the 500 amu limit of the mass spectrometer. This suggests that negative ions could be the precursors to particle formation. Ion-molecule and ion-ion reactions are discussed and a simple negative ion polymerization sheme is proposed which qualitatively reproduces the experimental results. The model shows that the densities of high mass negative ions in the plasma are strongly reduced by modulation frequencies near 1 kHz. Each plasma period is then too short for the polymerization chain to propagate to high masses before elementary anions are lost in each subsequent afterglow period. This explains why modulation of the rf power can reduce particle contamination. We conclude that for the case of silane rf plasmas, the initiation steps which ultimately lead to particle contamination proceed by negative ion polymerization.