Process for the production of copolymers comprising: (i) forming a reaction mixture comprising an elastomer and a vinyl aromatic compound; (ii) introducing the reaction mixture to a reaction vessel; and (iii) performing a polymerisation reaction wherein steps (i) - (iii) are performed in that order thereby obtaining a copolymer comprising: (a) a phase comprising a reaction product of the elastomer with the vinyl aromatic compound; and (b) a phase comprising a reaction product of the vinyl aromatic compound wherein a sample of the reactor contents is obtained during step (iii) to determine of the formed copolymer: • the content of phase (b); and/or • the molecular weight of phase (b) wherein the determination of the content of phase (b) in the formed copolymer and the molecular weight of phase (b) is performed by a method comprising isolation of phase (b) from the sample of the formed copolymer and subjecting the obtained phase (b) to molecular analysis; wherein the method of isolation of phase (b) from the sample of the formed copolymer involves the steps of: preparing a solution of the formed copolymer in a solvent; and removing the insoluble fraction from the product obtained in step by a filtration wherein the molecular analysis is performed by gel permeation chromatography in accordance with ASTM D5296-11; wherein • the elastomer is one or more selected from poly(butadiene), poly(styrenebutadiene), poly(butylacrylate), poly(butadiene-acrylonitrile) or poly(isoprene) and/or • the vinyl aromatic compound is selected from styrene, α-methyl styrene, halogenated styrene, vinyltoluene, butylstyrene, p-hydroxystyrene or methoxystyrene; and wherein the reaction mixture further comprises a comonomer selected from acrylonitrile, itaconic acid, acrylamide, methacrylamide, maleic anhydride, maleimide, substituted maleimides, methacylonitrile, α-chloroacrylonitrile, β-chloroacrylonitrile, α-bromoacrylonitrile, acrylic acid, methyl(meth)acrylate, ethyl(meth)acrylate, n-butyl(meth)acrylate, t-butyl(meth)acrylate, n-propyl(meth)acrylate, isopropyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, or glycidyl(meth)acrylates, wherein phase (a) comprises a reaction product of the elastomer with the vinyl aromatic compound and the comonomer, and wherein phase (b) comprises a reaction product of the vinyl aromatic compound and the comonomer. (i) forming a reaction mixture comprising an elastomer and a vinyl aromatic compound; (ii) introducing the reaction mixture to a reaction vessel; and (iii) performing a polymerisation reaction (a) a phase comprising a reaction product of the elastomer with the vinyl aromatic compound; and (b) a phase comprising a reaction product of the vinyl aromatic compound • the content of phase (b); and/or • the molecular weight of phase (b) preparing a solution of the formed copolymer in a solvent; and removing the insoluble fraction from the product obtained in step by a filtration • the elastomer is one or more selected from poly(butadiene), poly(styrenebutadiene), poly(butylacrylate), poly(butadiene-acrylonitrile) or poly(isoprene) and/or • the vinyl aromatic compound is selected from styrene, α-methyl styrene, halogenated styrene, vinyltoluene, butylstyrene, p-hydroxystyrene or methoxystyrene; and