1. A method for reducing transition time and amount of off-specifications polymer produced during a change of specification from one homo- or co-polymer grade to another in a double loop slurry reactor with a Ziegler-Natta or a metallocene catalyst system, wherein the polymer is prepared from olefin monomer(s), the method comprising, prior to transition from a first polymer grade to a second polymer grade: selecting n input variables, linked to reaction conditions; defining a constrained neural network model of general form: f 1 =1/(1+exp(−(a 11 *X 1 +a 12 *X 2 +a 13 *X 3 +. .. +b 1 ))) f 2 =1/(1+exp(−(a 21 *X 1 +a 22 *X 2 +a 23 *X 3 +. .. +b 2 ))) f 3 =1/(1+exp(−(a 31 *X 1 +a 32 *X 2 +a 33 *X 3 +. .. +b 3 ))). .. Res=1/(1+exp(−(a (n+1)1 *f 1 +a (n+1)2 *f 2 +a (n+1)3 *f 3 +. .. +b (n+1) ))) wherein the X i 's are n normalised input variables, the a ij 's and b i 's are numerical coefficients, the f i 's are intermediate functions, and Res is a resulting scaled polymer property estimate corresponding to the reaction conditions defined by the n input variables; adjusting numerical coefficients of the constrained neural network model to minimise error on Res under constraints, such constraints being imposed by equalities or inequalities on X i 's, a ij 's, b i 's, f i 's, Res, any partial derivative X i 's of any order, any partial derivative a ij 's of any order, any partial derivative b i 's of any order, any partial derivative f i 's of any order, any partial derivative Res of any order, or combinations thereof, wherein the partial derivatives measure variations of results derived from the constrained neural network model when only one of the input variables is changed by an infinitesimal step; predicting a density and a melt flow index of polymer leaving the double loop slurry reactor from a time evolution of the n input variables in the constrained neural network model; inferring values for any combination of two input variables X i and X j , knowing the other (n−2) variables, and knowing desired values of density and melt flow index; from a given law of time evolution of the n input variables between two steady-state conditions corresponding to two product specifications, calculating a time at which at least one polymer characteristic leaves product specifications of the first polymer grade and a time at which a latest polymer characteristic reaches product specifications of the second polymer grade, wherein the input variables are determined before transition from the first polymer grade to the second polymer grade; calculating a time of off-specifications production by computing a difference between the time at which the at least one polymer characteristic leaves product specifications of the first polymer grade and the time at which the latest polymer characteristic reaches product specifications of the second polymer grade; and minimising the time of off-specifications production by modifying the law of time evolution of the n input variables.