Prediction of molecular weight distribution in chain growth polymerizations
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Prediction of molecular weight distribution in chain growth polymerizations Ramiro Infante-Martínez, Enrique Saldívar-Guerra, Odilia Pérez-Camacho, Maricela García-Zamora, Víctor Comparán-Padilla Centro de Investigación en Química Aplicada, Saltillo, Coahuila. México Presenting author’s email: [email protected]
ABSTRACT This work presents results on the prediction of the molecular weight distributions (MWD) of chain growth polymerization using conventional software and hardware tools. The investigation focuses on two kinds of polymerization processes: free radical batch and continuous polymerization processes with application to low density polyethylene synthesis (LDPE); and coordination polymerization via metallocenes with application to high density polyethylene synthesis (HDPE). For both processes, kinetic models, consisting of sets of differential equations describing the dynamic behavior of all the chemical species in the reaction media, are presented. From these sets is possible to obtain the molecular weight distribution of the polymer1,2,4 Strategies for the simulation of the polymerization models were developed and results of these simulations are presented. On the free radical polymerization case, the next results are highlighted: i) It was confirmed that the chain transfer to polymer step produces strong asymmetries on the MWD as well as a high polydispersity index; ii) It’s possible to calculate the MWD in the CSTR process through a simulation strategy that permits the decoupling of the live and dead chains populations. On the metallocene polymerization case, it was demonstrated that the coordination standard model represents well the system experimentally studied and it can be employed to directly calculate the molecular weight distribution. These results confirm the idea that the complete MWD can be directly calculated with conventional hardware and software tools. INTRODUCTION As an example of a model for the chain-growth polymerization, the free-radical polymerization mechanism is illustrated1. Initiator decomposition: kd I 2R· Chain initiation: ki M + R· P1·
87
Propagation: kp Pn· + M
Pn+1·
Termination by combination: Pn· + Pm·
ktc
Dn+m
Terminación by disproportion: ktd Pn· + Pm· Dn + Dm Chain-transfer to polymer: Pn· + Dm
kfp
Dn + Pm·
Chain-transfer to small species: kfA Pn· + A Dn + A· Where I = Initiator, R = Free-radicao, M = Monomer, Pi = Live chain of length i, Di = Dead chain of i units, A = Chain-transfer agent (monomer, solvent, etc.), ki = Kinetic constant for reaction i. Description of the full molecular weigth distribution is to translate the kinetic mechanism known or proposed in equations describing the evolution of the concentrations of all the sizes of polymeric species. This results in a system of ordinary differential equations (ODE´s) which must be resolved to know the evolution in time of the complete MWD2,5. EXPERIMENTAL PART For the polymerization via metallocenes case, results of an experimental program conducted to obtain data for modelling purposes3 are
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