The Gilch polymerization is one of the most popular routes toward high molecular weight alkoxy-substituted poly(p-phenylenevinylenes) (PPV) applied in, for instance, organic electronics and bioimaging. As the interplay between optoelectronic performance and (synthesis-related) defects represents an active area of research, control over the polymerization is of utmost importance. In this work we quantify for the first time the rate constants of the reaction steps of the Gilch polymerization. We obtain these values by fitting concentration transients of various key intermediates, measured by in situ low-temperature (-68 degrees C) H-1 NMR spectroscopy, to kinetic models based on sets of coupled rate equations. The modeling not only accounts for the usual processes of initiation, propagation, transfer, and radical recombination but also involves the side reaction cascade associated with the presence of residual water in the reaction mixture. The results demonstrate that chain growth initiation by active monomer dimerization is slow and rate determining. We show that a low temperature suppresses the occurrence of bisbenzyl and bisbromobenzyl coupling defects. The initiation rate is reduced by orders of magnitude compared to the propagation rate. Hence, fast chain growth occurs at a relative low concentration of radical intermediates, which suppresses defect formation due to both active monomer dimerization and radical radical recombination.