A general model for calculating the average properties of the copolymer formed between two polydisperse reactive polymers is developed. (M) over bar(n),(M) over bar(w),(M) over bar(z),(M) over bar(z+1) and other higher average molecular weights can all be described as a function of the reaction conversion and the average properties of two reactive polymers directly without calculation of the complete distribution. The predictive capacity of the model is limited to reaction before gelation and can be used for prediction of the gel point. The model is explicitly formulated and can therefore be readily applied.