Molecular Weight Distribution in Composition Controlled Emulsion Copolymerization CLAUDIA SAYER, 1,2 ENRIQUE L. LIMA, 2 JOSE CARLOS PINTO, 2 GURUTZE ARZAMENDI, 3 JOSE ´ M. ASUA 1 1 Institute for Polymer Materials “POLYMAT” and Grupo de Ingenierı ´a Quı ´mica, Universidad del Paı ´s Vasco, Apdo. 1072, 20080 San Sebastia ´n, Spain 2 Programa de Engenharia Quı ´mica/COPPE, Universidade Federal do Rio de Janeiro, Cidade Universita ´ria, CP: 68502, CEP 21945-970, Rio de Janeiro, Brazil 3 Departamento de Quı ´mica, Universidad Pu ´ blica de Navarra, Campus de Arrosadı ´a, 31006 Pamplona, Spain Received 21 July 1999; accepted 5 January 2000 ABSTRACT: The effect of different strategies for copolymer composition control on the molecular weight distribution (MWD) and gel fraction in the emulsion copolymerization of methyl methacrylate and butyl acrylate was investigated. Starved and semistarved processes for copolymer composition control were both considered. For gel-forming systems it was found that the starved process gave more gel and lower molecular weights than the semistarved process. The feasibility of simultaneous control of the copolymer composition and the MWD was assessed. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1100 –1109, 2000 Keywords: emulsion polymerization; methyl methacrylate; butyl acrylate; molecular weight distribution; composition control; dodecanethiol INTRODUCTION Film-forming emulsion polymers suitable for wa- ter-borne coatings are prepared by emulsion co- polymerization that usually involves “hard” (high glass-transition temperature, T g ) and “soft” (low T g ) monomers. These monomers usually have dif- ferent reactivity ratios. Because the film forma- tion and the properties of the film both depend on the copolymer composition, 1,2 several strategies for copolymer composition control have been de- veloped. 3–22 The simplest strategy is the starved process in which the monomers are slowly fed into the reactor in such a way that the process is controlled by the addition rate and the composi- tion of the copolymer equals that of the feed. The main drawback of this strategy is that long pro- cess times are usually needed. 3–7 A way of over- coming this limitation is to work under semi- starved conditions in which the ratio of the con- centrations of the monomers in the polymer particles is kept at the value that yields the de- sired copolymer composition but the overall con- centration of the monomers, and hence the poly- merization rate, are substantially higher than in the starved process. 8 The calculation of the over- all monomer concentration profile is usually car- ried out using an optimization algorithm, 8 –13 al- though for some cases the optimal monomer feed profile can be easily inferred. Thus, the minimum- time optimal monomer feed profile in a reactor without heat transfer limitations is to initially charge all the less reactive monomer plus the amount of the more reactive monomer needed to keep the monomer ratio at the value that ensures the formation of the desired copolymer composi- tion. Then the rest of the more reactive monomer is fed in such a way that the required monomer ratio is maintained during the process. 8 –11 The implementation of these processes can be carried out in open loop 8 –13 as well as in closed loop. 14 –22 Copolymer composition is critical in important application properties such as minimum film forming temperature, 23 but molecular weight dis- tribution and gel fraction are equally critical in other application properties such as adhesion per- formance 24 and paper coating. 25 However, the lit- erature about the effect of the strategies for co- polymer composition control on molecular weight and gel formation is scarce. Arzamendi et al. 26 Correspondence to: J. M. Asua (E-mail: qppasgoj@sq.ehu.es) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 38, 1100 –1109 (2000) © 2000 John Wiley & Sons, Inc. 1100