The present invention concerns a process for the preparation of a copolymer comprising the steps of copolymerizing under suitable reaction conditions at least one first type of olefin monomer and at least one second type of functionalized olefin monomer using a catalyst system to obtain a polyolefin main chain having one or multiple functionalized short chain branches, the catalyst system comprising: i) a single-site catalyst or catalyst precursor comprising a metal selected from Ti3+ or Cr3+; ii) a co-catalyst; iii) optionally a scavenger.