Special Issue: Innovative Processes and Enabling Technologies

Polymers are ubiquitous in the materials that surround us. They are used for packaging, coatings, adhesives, fibers, pharmaceutical delivery, and other medical applications. While design of new specialty polymers is challenging, especially when striving to achieve architectures and functions prevalent in nature, even commodity products require continued research due to remaining unsolved challenges. Design of synthetic polymer products, and the engineering knowledge required to produce them at large scale, stimulates our appetite for discovery and innovation. Over the years, polymer reaction engineers have advanced the field by making simplifying assumptions over a broad design space. For example, bulk rate coefficients have been applied in emulsified reaction environments and chemically inert solvents have been treated as if they are also inert from a process perspective. Focusing a critical eye on long-studied systems reveals many high-impact questions that remain to be solved. It is exciting to discover the individuality of particular systems, while enhancing our understanding of the mechanistic similarities among them. Modeling, in particular, has at its core the need to develop base assumptions where one only gains confidence by experimental validation. Challenging of assumptions leads to refined understanding of key process details. Herein lies the beauty of polymer reaction engineering, a field where the story is incomplete unless both experimentation and modeling are melodically in tune. Polymer reaction engineers are contributing to new knowledge in many ways. New processes are being developed for hybrid and composite polymeric materials enabling both physical and reactive compatibility of components, with the resulting materials benefitting from their synergistic properties. Our community is developing greener processes to generate water-borne coatings with performance that is comparable to or surpasses that of solvent-borne counterparts developed in the past. Ionic liquids are being leveraged as effective water-tolerant catalysts to achieve ultra-high molecular weight polymer molecules in nano-scale colloidal particles. As new materials and processes are developed, new products arise for the benefit of society. As the title implies, this special issue of Macromolecular Reaction Engineering involves two interrelated themes. In the first half (one review and four articles), the focus is on Innovative Processes where often multiple techniques are hybridized to afford new materials, processes, and mechanisms. The second half of the issue, Enabling Research (also one review and four articles), then captures powerful examples of systems revisited for clearer understanding of mechanism toward optimization of existing and/or future novel products. These important topics were the focus of the recent Polymer Reaction Engineering X conference where we enjoyed seeing many of the contributors to this issue. For Innovative Processes, we start with a review article by Dubé and coworkers.1 Over the last several decades, there has been a concerted effort to develop greener water-based coatings and adhesives that meet or surpass the performance of their solvent-based counterparts. Many of these efforts are captured here, showcasing a new trend where composite particles from emulsion-produced latex are enhanced by including non-latex nanoparticles. The review provides examples of the impact of shape, size and compatibilizing groups on product performance. Nanoclays, carbon nanotubes, and cellulose nanocrystals are among the most promising new nanoparticles hybridized with latex. Coatings derived from dispersed particles inherently suffer challenges during coalescence to form films. This review shows that, with proper distribution of high-modulus nanoparticles, their small size offering effective surface area for interaction with the host matrix, effective coalescence can be achieved while ensuring improved coating modulus and durability. An alternative approach to increasing film modulus is to leverage crosslinking and curing reactions. This approach is challenging for coatings derived from waterborne polymer particles, because achieving sufficient interparticle coalescence to eliminate interstitial cavities is difficult; moreso when crosslink density within the particles restricts those interparticle deformations. One way to improve the formation of crosslinked films is to introduce a latent reactive component into the continuous aqueous phase, which is subsequently triggered during film formation to form interparticle bonds. Unfortunately, this approach can suffer from incomplete conversion of that component, leaving it free to migrate in the final film. In their contribution to this special issue, Minari and coworkers2 incorporate a new strategy where the interparticle crosslinking reaction relies on a nanocomposite approach, wherein hydroxyl groups are present on one component and a hydrophobic co-reactant (iso-butylated melamine (iBMF)) is present on the complementary particles. In this way, a thermosetting particulate system is produced where crosslink density and kinetics can be tuned via the iBMF concentration, inherently without any ungrafted polymer. Non-aqueous dispersions are produced commercially yet the structure and dispersity of the stabilizing agent has been shown to play a critical role in control of the particle characteristics.3 As with above, a key here is to minimize any ungrafted stabilizer free to diffuse post film formation. Specifically, the most effective approach to maximize incorporation is for the dispersant to have a terminal double bond. Here, Hutchinson and Zhang4 have experimentally shown this to be most effective if a sequential one-pot reaction starting with atom transfer radical polymerization (ATRP) and finishing with cobalt catalytic chain transfer polymerization (CCTP) is employed, which not only maximizes the terminal double bond but also reduces the dispersity of the dispersant chain length. Mechanistic insights were gained by leveraging Predici software modeling to validate their hypothesis that the dominating termination mechanism in CCTP versus ATRP is key to minimizing the fraction of chains with the undesired chain end functionality and simultaneously affecting the overall dispersity. Shipp and coworkers5 also describe the importance of having the stabilizer strongly interacting with the dispersed particle surface, yet this work focuses on micron sized crosslinked poly(thioether) particles generated from suspension polymerization stabilized by a Pickering interaction at their surface with 80 nm silica nanoparticles. A key to the approach was in determining the proper costabilizer to aid in restricting the silica nanoparticles to the external surface. Cetyl alcohol was found most effective and produced the desired raspberry-like morphology. Keeping with the trend of developing innovative new processes by hybridizing techniques, Sayer and coworkers6 leverage an ionic liquid as an efficient water-tolerant catalyst in the cationic emulsion polymerization of styrene. Moreover, this novel approach results in quite high weight average molecular weights, above 106 Da, for chains confined within 100 nm colloidal particles. The ionic liquid catalyst was found to play a critical role in maintaining high molecular weight even at small particle size. This work systematically probes the role of both surfactant and ionic liquid concentrations and finds that even a relatively small concentration of ionic liquid relative to the monomer enhances the achievable molecular weight. Shifting our attention to our Enabling Research section, we start with a review article on condensed mode cooling, a well-known approach for improving heat removal that has allowed polyolefin manufacturers to increase production rates significantly. McKenna7 offers a broad review of both the academic and patent literature on the impact of liquid components injected into the reactor vapor phase and the misconception that these induced condensing agents (ICA) are inert. While not chemically reactive, ICA’s can be highly influential on process aspects key to maintaining high production rates, such as fluidization and reduction of fouling in the reactor. McKenna reminds us that a holistic view of the entire process enables improved decisions on reaction and process design. Polymerization of ethylene with single-site coordination catalysts such as metallocenes can produce long-chain branching (LCB) microstructures in solution polymerization. Three mechanisms for LCB have been proposed (i.e., terminal branching, CH bond activation, and intramolecular random incorporation) but, until now, it has not been clear which mechanism dominates. In their contribution, Soares and coworkers8 develop models to probe the influence of these proposed mechanisms, comparing simulation results with observed trends in literature data. They conclude that a terminal branching mechanism is the most consistently suited mechanism for LCB formation across a broad set of experimental data and conditions, providing valuable information for researchers and practitioners who seek to control LCB in polyethylene products. Shifting gears from polyolefin production, McAuley and coworkers9 used models and literature data to study complex equilibrium behavior of nylon 6 and nylon 6,6 polymerizations, which has been a concern for many years. They reveal that side reactions involving hydration of carboxyl ends and dehydration of amine ends can explain the non-linear dependence of water on the polycondensation equilibrium constant. Insights and parameter estimates gained from this study will be valuable for predicting reaction rates and degree of polymerization in industrial nylon 6/6,6 copolymerizations. In their contribution, Teymour and coworkers10 show that crosslinked hydrogel nanoparticles produced by inverse-miniemulsion polymerization are effective for controlled and targeted delivery of polyphosphate salts for healing of gastrointestinal injuries. The impact of parametric variations, from characteristics of the salts to crosslink density and stabilizer choices, are evaluated so that release profiles can be tuned. Design and optimization of this particle-based delivery system aims to mitigate past challenges (e.g., uncontrolled delivery rates and non-specific targeting) toward effective administration of polyphosphate solutions to the gut. The final contribution to this special issue involves pressure sensitive adhesives (PSA). While PSA’s have long been produced using copolymers of n-butyl acrylate (BA), 2-ethylhexyl acrylate (EHA), and methyl methacrylate (MMA). Gabriel and Dubé11 note that several critical copolymer reactivity ratios, specifically involving EHA, have not yet been published. Their contribution describes how copolymer reactivity ratios were determined for comonomer pairs using low-conversion data and subsequently validated experimentally at high conversion. More importantly, the copolymer reactivity ratios were successfully used to predict and validate the relevant terpolymer compositions in high conversion experiments. Now with these key reactivity ratios established, creative combinations of process and composition can be more easily explored to design new PSA products. We trust that you will enjoy the articles within this special issue of Macromolecular Reaction Engineering. John G. Tsavalas earned his B.Sc. in Chemical Engineering from the University of Virginia in 1996 and his Ph.D. in Chemical Engineering from the Georgia Institute of Technology in 2001. He is a polymer reaction engineer with particular research emphasis in multiphase polymer colloids. He is currently an associate professor of Chemistry and Materials Science at the University of New Hampshire (UNH) and the Director of the UNH Latex Morphology Industrial Consortium. Kimberley B. McAuley earned her B.A.Sc. in Chemical Engineering from the University of Waterloo in 1987 and her Ph.D. in Chemical Engineering from McMaster University in 1992. She specializes in fundamental modeling of polymerization reactors and in development of statistical methods for designing experiments and estimating parameters. She is a professor of Chemical Engineering at Queen’s University in Kington, Canada and the Lab Leader for the Chemical Process Mathematics Lab of the Fields Centre for Quantitative Analysis and Modeling.

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