Catalytic chain transfer polymerization (CCTP) of methacrylates is an efficient method to produce low molecular weight poly(methacrylates). The mechanism is such that the products each have a terminal vinyl group which is then available for subsequent further reaction. This study involves the use of matrix assisted laser/desorption ionization time-of-flight mass spectrometry (MALDI TOF MS) to investigate the effect of the addition of methyl methacrylate macromonomers as prepared by CCTP to the polymerization of perdeutero-methyl methacrylate and n-butyl methacrylate in the presence and absence of catalytic chain transfer agent. The experiments have demonstrated that methyl methacrylate dimer as prepared by CCTP may copolymerize with deutero-methyl methacrylate. Addition fragmentation is demonstrated to dominate over CCT, contrary to that predicted from a consideration of C-s values. No evidence for copolymerization with butyl methacrylate has been observed. Inevitably this will result in small amounts of branching during CCTP taken to high conversion.