Citation: Castellanos, S.G.; Fernández-Escamilla, V.V.A.; Corona-Rivera, M.Á.; González-Iñiguez, K.J.; Barrera, A.; Moscoso-Sánchez, F.J.; Figueroa-Ochoa, E.B.; Ceja, I.; Rabelero, M.; Aguilar, J. Coagulative Nucleation in the Copolymerization of Methyl Methacrylate–Butyl Acrylate under Monomer-Starved Conditions. Polymers 2023, 15, 1628. https://doi.org/10.3390/ polym15071628 Academic Editor: Alberto Romero García Received: 6 March 2023 Revised: 20 March 2023 Accepted: 20 March 2023 Published: 24 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Coagulative Nucleation in the Copolymerization of Methyl Methacrylate–Butyl Acrylate under Monomer-Starved Conditions Sujey G. Castellanos 1 , V. Vladimir A. Fernández-Escamilla 1 , Miguel Á. Corona-Rivera 2 , Karla J. González-Iñiguez 3 , Arturo Barrera 1 , Francisco J. Moscoso-Sánchez 3 , Edgar B. Figueroa-Ochoa 3 , Israel Ceja 4 , Martín Rabelero 5 and Jacobo Aguilar 1, * 1 Departamento de Ciencias Tecnológicas, Centro Universitario de la Ciénega, Universidad de Guadalajara, Av. Universidad No. 1115, Col. Lindavista, Ocotlán 47819, Mexico 2 Ingeniería Química, Coordinación Académica Región Altiplano (COARA), Universidad Autónoma de San Luis Potosí, Carretera a Cedral Km 5+600, San Jose de las Trojes, Matehuala 78700, Mexico 3 Departamento de Química, Centro Universitario de Ciencias Exactas en Ingenierías, Universidad de Guadalajara, Blvd. Gral. Marcelino García Barragán No. 1421, Col. Olímpica, Guadalajara 44430, Mexico 4 Departamento de Física, Centro Universitario de Ciencias Exactas en Ingenierías, Universidad de Guadalajara, Blvd. Gral. Marcelino García Barragán No. 1421, Col. Olímpica, Guadalajara 44430, Mexico 5 Departamento de Ingeniería Química, Centro Universitario de Ciencias Exactas en Ingenierías, Universidad de Guadalajara, Blvd. Gral. Marcelino García Barragán No. 1421, Col. Olímpica, Guadalajara 44430, Mexico * Correspondence: jacobo.aguilar@academicos.udg.mx; Tel.: +52-392-92-594-00 Abstract: Coagulative nucleation in the copolymerization of methyl methacrylate–butyl acrylate (MMA-BA) via semicontinuous emulsion heterophase polymerization (SEHP) under monomer- starved conditions in latexes with high solid content (50.0 wt %) and low concentrations of surfactant is reported. The SEHP technique allows the obtention of latex with high colloidal stability and has potential industrial application in polymer synthesis. High instantaneous conversions (>90%) and a high-ratio polymerization rate/addition rate (R p /R a ) ≥ 0.9 were obtained at low times until the final copolymerization, which confirmed the starved conditions in the systems at the highest surfactant concentrations. The particle size exhibited a linear size increment at conversions between 0 and 40% induced by homogeneous nucleation, a transition region between 40 and 50%, and non-linear behavior at higher conversions by coagulative nucleation. These three behaviors were also observed in the particle surfactant coverage area (Sc), Z-potential, particle coagulation rate (dN p /dt) by the Smoluchowski model, final particle size (Dp z ), and number particle (N p ) through the reaction. By means of transmission electron microscopy (TEM) images, the onset of coagulation was observed from 50% of conversion until the end of the reaction. In addition, in both processes of copolymerization, tacticity was displayed (mainly syndiotacticity). Keywords: monomer-starved conditions; homogeneous nucleation; coagulative nucleation; methyl methacrylate; butyl acrylate 1. Introduction Polymeric particle formation mechanisms have been widely studied in emulsion- polymerization-modifying reaction factors such as the surfactant and initiator concentra- tion [1–3], solid content [4,5], and monomer addition rate [2,6–8]. Several models have been proposed for the particle formation during the nucleation stage such as homogeneous nu- cleation, micellar nucleation, and coagulative nucleation. The synthesis mechanism can be tailored to a particular polymeric reaction system by adjusting the reaction factors enlisted previously. The semicontinuous emulsion heterophase polymerization (SEHP) process under monomer-starved conditions involves the continuously controlled feeding of the monomer or the comonomer into a monomer-free surfactant and initiator aqueous solution. In this polymerization process, monomer molecules are first in contact with the continuous Polymers 2023, 15, 1628. https://doi.org/10.3390/polym15071628 https://www.mdpi.com/journal/polymers