The main reaction routes that lead to the formation of structural defects in PVC are studied on a theoretical basis with the BMK/6-311+G(3df,2p)//B3LYP/6-31+G(d) method. All studied reactions can be classified into four classes: the reactions following a head-to-head addition, intramolecular H-transfer (backbiting), and chain transfer reactions to polymer and to monomer. The head-to-tail propagation is the reference reaction for estimating the probability of the reaction routes leading to defect formation. Variations of chain length of the reacting polymer chain were taken into account in the calculations, leading to more than 100 studied reactions. The ab initio kinetic parameters, combined with typical monomer and polymer concentrations during suspension polymerization, serve as an input for the calculation of the defect concentrations that can be compared to the experimental data. This work supports the overall mechanism of defect formation during vinyl chloride polymerization as established experimentally.