Molecular Weight Development in Emulsion Copolymerization of n-Butyl Acrylate and Styrene Nuria Zoco, 1 Lourdes Lo ´ pez de Arbina, 1 Jose ´ R. Leiza, 2 Jose ´ M. Asua, 2 Gurutze Arzamendi 1 1 Departamento de Quı ´mica Aplicada, Universidad Pu ´ blica de Navarra, Edif. Acebos, Campus de Arrosadı ´a, 31006 Pamplona, Spain 2 Institute for Polymer Materials (POLYMAT), and Grupo de Ingenierı ´a Quı ´mica, Departamento de Quı ´mica Aplicada, Facultad de Ciencias Quı ´micas, University of the Basque Country, Apdo. 1072, 20080 Donostia-San Sebastia ´n, Spain Received Received 24 October 2001; accepted accepted 29 April 2002 ABSTRACT: The seeded emulsion copolymerization of n- butyl acrylate and styrene in a weight ratio of 50/50 was investigated. The effect of the type of process (batch vs. semicontinuous) and the amounts of initiator and emulsifier charged into the reactor on the time evolution of the frac- tional conversion, number of polymer particles, and weight- average molecular weight (M w ) was analyzed. It was found that the M w depends to a slight extent on the type of process and the emulsifier concentration and to a larger extent on the initiator concentration. The molecular weight distributions (MWDs) and the gel content of the final latexes were also analyzed. In the absence of chain transfer agents (CTAs), the fraction of gel was higher in the semicontinuous processes. It was also found that the gel content increased with increas- ing initiator concentration in the recipe. The addition of 1 wt % CTA avoided gel formation and led to an important reduction of the M w . Nevertheless, the MWDs presented a shoulder or even a second peak at high molecular weights that was due to reactions of chain transfer to the polymer. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1918 –1926, 2003 Key words: emulsion polymerization; styrene; butyl acry- late; kinetics; molecular weight distribution INTRODUCTION The properties of coatings based on acrylic resins are usually correlated with two variables, the molecular weight and the comonomer ratio. The mechanical properties resistance such as the toughness, abrasion resistance, or impact resistance of the final coatings improve with the increase of the molecular weight of polymers. 1 Nevertheless, the viscoelastic properties related to the mobility of chains such as tack 2 and wettability in adhesive applications require the pres- ence of low polymer chains in the formulation. The incorporation of a comonomer 1,3– 6 commonly sup- poses the modification of the glass-transition temper- ature (T g ) of the formulation, and it can also modify the molecular weight 7 and hence the mechanical prop- erties of the final product. This is the case of n-butyl acrylate (n-BA), which is often copolymerized with styrene (St) in order to improve the hardness, 1 al- though this copolymerization decreases the flexibility of coatings and the tack in adhesive applications such as pressure sensitive adhesives (PSAs). Another factor that has also been shown to influence the above-men- tioned properties is the presence of crosslinked poly- mer chains, which in coatings leads to an increase in both the film hardness and its chemical resistance. Also, for PSA applications, gel fractions up to 30% significantly increases the shear resistance. 2 Polyacry- lates for coatings and adhesives are mainly synthe- sized by emulsion polymerization, 8,9 because the latex obtained through this technique can be directly ap- plied. Despite the industrial importance of n-BA/St copol- ymers, the literature about the relationship between polymerization conditions and molecular weights is scant. 5,7,10 –12 Most studies on n-BA/St copolymers deal with the kinetic aspects. 13–21 Yang and Yang 10 studied the influence of the comonomer ratio, initia- tor, emulsifier, and temperature on the polymerization rate and molecular weight under batch conditions. They found (in line with other authors 5,11 ) that an increase of n-BA content in the formulation led to an increase in both the polymerization rate and the mo- lecular weights; the increase of temperature and initi- ator led to an increase of the polymerization rate and a decrease of the molecular weight. Cruz-Rivera et al. 11 obtained higher molecular weights in batch emul- sion polymerization than in semicontinuous polymer- ization. Chra ´stova et al. 12 observed that polymeriza- tion using a water insoluble initiator produced much higher molecular weights than using a water soluble Correspondence to: G. Arzamendi (garzamendi@unavarra.es). Contract grant sponsor: CICYT; contract grant number: MAT1999-1049-C03-02. Contract grant sponsor: Universidad Pu ´ blica de Navarra (to N.Z.). Journal of Applied Polymer Science, Vol. 87, 1918 –1926 (2003) © 2003 Wiley Periodicals, Inc.