A method for determining parameters for a transition from an initial reaction for producing a product whose properties meet an initial specification set, to a target reaction for producing the product with properties that meet a target specification set, said method including the steps of: (a) identifying, as a primary property, one of the product's properties that can cause production of more off-grade product during the transition than any of the other properties wherein identifying the primary property of the product produced in a reactor includes obtaining data indicative of instantaneous and average values for each of at least two different properties of the product before, during, and after the transition, where each of said different properties is compliant with the initial specification set at the start of the transition; and (b) determining initial conditions for the transition, including an initial value of the primary property, that significantly reduce the amount of off-grade product produced during the transition; and determining process control variables for implementing the transition commencing with the initial conditions determined in step (b) such that the product produced during the transition goes off-grade due to the primary property at a first time and goes off-grade due to each of at least one non-primary property of the product at a time at least substantially equal to the first time, the primary property first complies with the target specification set at a second time, and at least one said non-primary property first complies with the target specification set at a time at least substantially equal to but not later than the second time wherein the parameters include initial reaction parameters for commencing the transition and parameters for changing product parameters during the transition, and wherein step (b) includes the steps of: (c) determining a time constant, for change of the primary property during the transition, that causes the product produced during the transition to go off-grade due to the primary property at a first time and causes the primary property to first comply with the target specification set at a second time; and (d) determining an initial value for each of at least one non-primary property of the product, and a time constant for change of each said non-primary property during the transition, that cause the product produced during the transition to go off-grade due to each said non-primary property at a time at least substantially equal to the first time; wherein each said initial value and each said time constant determined in step (d) cause each said non-primary property to first comply with the target specification set at a time at least substantially equal to but not later than the second time. (a) identifying, as a primary property, one of the product's properties that can cause production of more off-grade product during the transition than any of the other properties wherein identifying the primary property of the product produced in a reactor includes obtaining data indicative of instantaneous and average values for each of at least two different properties of the product before, during, and after the transition, where each of said different properties is compliant with the initial specification set at the start of the transition; and (b) determining initial conditions for the transition, including an initial value of the primary property, that significantly reduce the amount of off-grade product produced during the transition; and determining process control variables for implementing the transition commencing with the initial conditions determined in step (b) such that the product produced during the transition goes off-grade due to the primary property at a first time and goes off-grade due to each of at least one non-primary property of the product at a time at least substantially equal to the first time, the primary property first complies with the target specification set at a second time, and at least one said non-primary property first complies with the target specification set at a time at least substantially equal to but not later than the second time wherein the parameters include initial reaction parameters for commencing the transition and parameters for changing product parameters during the transition, and wherein step (b) includes the steps of: (c) determining a time constant, for change of the primary property during the transition, that causes the product produced during the transition to go off-grade due to the primary property at a first time and causes the primary property to first comply with the target specification set at a second time; and (d) determining an initial value for each of at least one non-primary property of the product, and a time constant for change of each said non-primary property during the transition, that cause the product produced during the transition to go off-grade due to each said non-primary property at a time at least substantially equal to the first time; wherein each said initial value and each said time constant determined in step (d) cause each said non-primary property to first comply with the target specification set at a time at least substantially equal to but not later than the second time.