Polymerization Reactions

Polymerization reactions are widely investigated and have led to high-value, high-performance engineered materials that are in our homes, automobiles and even our bodies. For these polymers, a thorough understanding of polymerization reactions and control of all reaction variables is crucial to producing a material that meets the end-use requirements and specifications.

Polymers are macromolecules that consist of smaller repeating monomeric sub-segments that are linked together to form chains. Polymers that exist in nature, such as polypeptides and polysaccharides, are critical components of living organisms. Synthetic polymers, such as nylon and polyurethane, have transformed how we manufacture and use commercial products. These latter polymers are typically formed by adding monomer segments together via free radical addition processes, or through linking the segments together by condensation reactions that produce the polymer along with water or another small molecule.

The two general classes of polymerizations are addition reactions and condensation reactions. In addition reactions, also described as chain growth polymerizations, the intact monomer links together to form linear or branched chains.

In addition polymerizations (A.), the entire monomer molecule becomes a segment of the polymer. Addition polymers form via several different mechanisms including free radical polymerizations, anionic polymerizations, cationic polymerizations, etc. Common polymers formed by addition polymerizations include polyolefins, polystyrene and polyvinylchloride. Another type of addition reaction is ring-opening polymerization used in the preparation of polymers, such as polycaprolactam, as well as highly tailored siloxane polymers.

In condensation reactions (B.), both polymer and a by-product molecule, such as water, or HCl, are formed when monomers join. If the monomers have two or more reactive functional groups, then more branched polymers form. Condensation reactions are commonly described as step-growth polymerizations because initially dimers are formed, then trimers, eventually leading up to longer chain oligomers. Common polymers formed by condensation reactions include polyamides, polyester and polycarbonate.

The kinetics and energetics of addition and condensation reactions are quite different. Addition reactions require an initiator, which can be, for example, UV light or elevated temperatures/pressures. The monomers making up an addition polymerization react rapidly and energetically to form high molecular weight chains. In condensation reactions, the step-wise growth of the polymer occurs more slowly than in addition reactions, but eventually longer chain polymers form.

In either addition or condensation reactions, careful control of the polymerization reaction enables physical and chemical properties to be tailored to specific product specifications. For this latter reason, in-situ FTIR spectroscopy, particle size analyzers and the highly controlled reaction conditions achieved with chemical reactors are valuable for developing and controlling polymerization reactions. 

There are a variety of techniques employed for polymerization reactions. All techniques have the same goal to achieve specific structural and physical properties for the polymer.

Techniques for Addition Reactions include:

• Bulk Polymerization – Neat, liquid monomers are reacted to form polymers, including PVC or LDPE.
• Solution Polymerization – Monomer and initiator are dissolved in an appropriate solvent and the polymer forms in solution. The polymer can be recovered by evaporation, or be left in solution to be used in coatings and adhesives. Polymers, such as polyacrylic acid, are commonly made via solution polymerization.
• Suspension Polymerization – Insoluble monomers are dispersed in an aqueous solution and after reaction initiation, small polymer spheres form in the solution. Small, uniform sized polystyrene beads are formed by suspension polymerizations.
• Emulsion Polymerization – Insoluble monomer is dispersed in water as an emulsion in the presence of a surfactant, and individual high molecular weight, polymeric microparticles are formed (latex). Paints and coatings, as well as adhesives, are widely used products formed by emulsion polymerizations.

Techniques for Condensation Reactions include:

• Melt Polymerization – All reactants are mixed together neat and reaction temperature is elevated above the melt temperature of the resulting polymer. Polyester and polyamides fibers are formed from melt condensations.
• Solution Condensation – All reactants (monomer, catalyst) are mixed together in a solvent that does not react. Synthetic elastomers are formed by solution condensation.
• Interphase Condensation – The polymerization reaction takes place and the resultant polymer resides in the interface of two immiscible phases. Polyanilines and polyimides are often formed by interphase condensation.

Whether a polymerization proceeds via addition as a chain reaction or condensation in a step reaction, it is essential to fully understand the chemistry in order to advance research and/or quickly bring new polymers to market.

This understanding involves factors including:

• Reaction conversion
• Monomer conversion rates and reactivity ratios
• Relationship and influence of reaction parameters on the molecular weight and distribution
• Thorough understanding of reaction mechanism in initiation, propagation and termination phases
• Overall polymer structure for target application needs

In more complex polymerizations such as copolymer or multi-polymer, measuring the individual polymerization reaction rates of the different monomers allows researchers to both tune and ensure the physical properties of the final product. Understanding critical polymer reaction parameters can lead to precise control of multi-step polymerizations, real-time residual monomer measurements and ultimately improved end-use polymer properties.

Well-regulated polymerization reactions yield molecules that are well-characterized with respect to composition, molecular weight, molecular weight distribution, structural and physical properties. A thorough understanding of these elements ensure that the synthesized polymer is “fit-for-purpose” in its intended use. To achieve this, it is necessary to understand and carefully control the many chemical and reaction parameters associated with the synthetic process. Infrared spectroscopy has proven to be highly valuable for meeting this challenge. Real-time, in-situ spectroscopy has proven particularly valuable to provide insight into key kinetic, mechanistic and chemical structure information, while eliminating the difficulties associated with offline measurements of polymerizations reactions.

Over the past three decades, the investigation of polymerization reactions from the lab through scale-up to production has been one of the most prolific and valued uses for in-situ FTIR and Raman spectroscopy.

Real-time, in-situ FTIR and Raman Spectroscopy provide enhanced knowledge and improved performance in the investigation of polymerization reactions:

• Analyze broad range of polymerization reactions, including homogeneous (e.g. free radical and condensation) and heterogeneous (e.g. emulsion and microemulsion)
• Measure monomers, pre-polymers and polymers accurately across a wide concentration range as a result of characteristic mid IR spectral bands
• Acquire data for reaction kinetics, monomer conversion rates, reactivity ratios, activation energies, role of initiators, intermediates and by-product formation
• Track individual monomer conversion rates and overall polymer composition in copolymer and multicomponent polymerizations
• Investigate chain growth, crosslinking and curing
• Understand mechanistic role of catalysts in polymerization reactions; determine catalyst active species and kinetics
• Monitor and proactively adjust reaction conditions as required to ensure compliance with intended end-product specifications
• Measure residual monomer levels and ensure that they meet product and regulatory requirements. Adjust charge ratios and other reaction variables to minimize the amounts present.

Schultz, A. et al., Virginia Tech, “Living anionic polymerization of 4‐diphenylphosphino styrene for ABC triblock copolymers”, Polymers International, vol. 66 issue 1, 52-58, (2017).

Anionic polymerization is a widely used chain growth method for producing thermoplastic elastomers and many hundred thousand tons of material is made every year using this process.

In this article, the scientists report the development of a new class of phosphorus-containing styrenic ABC triblock copolymers.

ABC triblock copolymers are formed by linking three different monomers and in this case, those monomers are styrene (S), isoprene (I) and 4-diphenylphosphino styrene (DPPS). The scientists report that sequential addition of these monomers via anionic polymerization yields a high-performance polymer that can be fine-tuned with regard to molecular weights and molecular weight uniformity.

By tracking IR peaks for the individual propagating monomers (S, 908 cm⁻¹; I, 912 cm⁻¹; DPPS, 918 cm⁻¹) with ReactIR, they confirmed the living synthesis of the poly(S-b-I-b-DPPS) and were able to gain insight into the kinetics of each propagation step. Understanding kinetics and adjusting fine-tuning reaction variables is critical to produce a material with targeted performance.

In-situFTIR monitors the living anionic polymerization forming an ABC triblock polymer poly(styrene-b-isoprene-b-diphenylphosphino styrene) [poly(S-b-I-b-DPPS)]

• The mid-IR = CH₂ wag mode is used to track each of the individual monomers to develop an absorbance vs. time plot
• Disappearance of vinyl group rate revealed differences in monomer propagation times
• Pseudo first order kinetics reflects differences in rate for the different monomers
• In-situ FTIR aided in finding optimal conditions for the synthesis of the triblock polymer 

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Dow Toray Co., LTD, “Novel Silicone Synthesis via Precisely Controlled Polymerization”, METTLER TOLEDO, 2020.

For a wide range of industries, silicone's diverse properties enable companies to design products with specific, fit-for-purpose characteristics. These products exploit the varied properties of silicone rubbers such as strength, thermal resistivity and stability. Typically, silicone is produced via hydrolysis of a chlorosilane followed with a terminal functional group addition, or through polycondensation of a cyclic siloxane. Each of these methods are equilibrium reactions that produce low-molecular-weight products with a wide range molecular weight distribution. 

Dow researchers have developed an alternate means of producing silicone, based on a precisely controlled polymerization, to yield product with targeted, uniform chain lengths. In this synthesis, a lithium-based reactant serves to open a cyclic tri-siloxane ring, followed by addition of another cyclic siloxane reagent, to yield a monodispersed silicone polymer.

This novel silicone polymerization, which results in monodispersed product with precisely controlled chain lengths, is tracked by ReactRaman, eliminating the delays and reaction uncertainties associated with offline GC analysis. Reaction initiation, progress and kinetics are all readily measured by the Raman method, providing continuous, real time verification that the reaction is proceeding as expected.

Dr. Long describes how in-situ FTIR impacted polymer synthesis research. The technology allowed his group to determine real-time kinetics, reactivity ratios and activation energies on the polymerizations reactions studied. This presentation highlights the in-situ FTIR monitoring of various chain growth polymerization processes for the determination of reactivity ratios during copolymerization. FTIR is well suited for chain growth addition involving olefinic monomers. Plus, the addition of various nucleophiles using click reactions with a focus on the Michael addition reaction is described. Spectroscopy during peroxide decomposition also permits the determination of half-life times during nitroxide mediated polymerization. In addition to chain growth polymerizations, in-situ FTIR is well suited for the monitoring of isocyanate composition in the formation of urethanes. 

In-situ FTIR and Raman Spectroscopy provide continuous monitoring of key polymerization species (monomers and polymers), and provides valuable information on polymerization reaction kinetics. With real-time, in-situ ReactIR or ReactRaman, the individual monomers used in co- and ter-polymerization reactions can be tracked in real time enabling decisions about the reaction to be made immediately throughout the experiment.

Proven benefits of in-situ FTIR and Raman spectroscopy for investigating polymerization reactions:

• Eliminate the time delays of sample extraction and offline analyses
• Enable proactive control of reaction parameters
• Minimize the need for labor-intensive offline analyses (e.g. gravimetric measurements, gel phase chromatography, NMR)
• Eliminate the introduction of air and moisture or disturbance of reaction resulting from sample extraction
• Eliminate sample irreproducibility and inaccuracy that is typical when extracting a viscous sample for offline analysis
• Minimize operator exposure to toxic chemicals, potentially energetic reactions or hazardous reaction conditions

Process Development and Scale-up workstations provide thermodynamic data in real time, the ability to investigate the impact of changing conditions on heat and mass transfer and support studies related to concentrations, temperature or kinetics.

Reaction Calorimeters allow researchers to measure heat generated by a polymerization reaction and to control the reaction based on heat output.

The control of the relevant parameters including additions can be automated and pre-programmed, so experiments can be safely run while recording all polymerization reaction parameters, 24 hours a day. The individual steps of the process of the polymerization reaction together with the experimental data are continuously recorded and stored securely making them available for evaluation and interpretation. Due to the safe, accurate and precise measurement and control, the number of experiments required is reduced making scale-up efficient.

In polymerization reactions, the impact of process parameters on droplet size are important factors to consider. Traditionally, this has been estimated using offline methods. However, such an approach can be can be difficult and unsafe.

Inline monitoring with particle size analyzers allow droplets to be monitored in real time and enable operators to act decisively in the plant environment to ensure product specifications are met. Key particle mechanisms, such as coalescence and breakage, can be quantified in real time enabling users to understand the impact of changing process parameters and ensure batch-to-batch repeatability.

Recent articles describing the use of ReactIR in-situ FTIR  spectroscopy in polymerization reactions:

• Dapeng Zhang, Yang Zhang, Yujiao Fan, Marie-Noelle Rager, Vincent Guérineau, Laurent Bouteiller, Min-Hui Li, Christophe M. Thomas, “Polymerization of Cyclic Carbamates: A Practical Route to Aliphatic Polyurethanes”, Macromolecules 2019, 52, 7, 2719-2724.
  
  
• Kenson Ambrose, Jennifer N. Murphy, Christopher M. Kozak, “Chromium Amino-bis(phenolate) Complexes as Catalysts for Ring-Opening Polymerization of Cyclohexene Oxide”, Macromolecules 2019, 52, 19, 7403-7412.
  
  
• Sarah N. Ellis, Anna Riabtseva, Ryan R. Dykeman, Sam Hargreaves, Tobias Robert, Pascale Champagne, Michael F. Cunningham, Philip G. Jessop, “Nitrogen Rich CO2-Responsive Polymers as Forward Osmosis Draw Solutes”, Industrial & Engineering Chemistry Research, 2019, 58, 50, 22579-22586.
  
  
• Jinghan Zhang, Yibo Wu, Kaixuan Chen, Min Zhang, Liangfa Gong, Dan Yang, Shuxin Li, Wenli Guo, “Characteristics and Mechanism of Vinyl Ether Cationic Polymerization in Aqueous Media Initiated by Alcohol/B(C6F5)3/Et2O”, Polymers 2019, 11(3), 500.
  
  
• Timothy S. Anderson, Christopher M.Kozak, “Ring-opening polymerization of epoxides and ring-opening copolymerization of CO2 with epoxides by a zinc amino-bis(phenolate) catalyst”, European Polymer Journal, 2019, 120, 109237.
  
  
• Raissa Gabriela M. Reis Barroso, Sílvia B. Gonçalves, Fabricio Machado, “A Novel Approach for the Synthesis of Lactic Acid-based Polymers in an Aqueous Dispersed Medium”, Sustainable Chemistry and Pharmacy, 2020, 15, 100211.
  
  
• Rafał Januszewski, Ireneusz Kownacki , Hieronim Maciejewski, Bogdan Marciniec, “Transition metal-catalyzed hydrosilylation of polybutadiene – The effect of substituents at silicon on efficiency of silylfunctionalization process”, Journal of Catalysis, 2019, 371, 27-34.
  
  
• Kenson Ambrose, Katherine N. Robertson, Christopher M. Kozak, “Cobalt amino-bis(phenolate) complexes for coupling and copolymerization of epoxides with carbon dioxide”, Dalton Trans., 2019, 48, 6248-6260.
  
  
• Kori A. Andrea, Francesca M. Kerton, “Triarylborane-Catalyzed Formation of Cyclic Organic Carbonates and Polycarbonates”, ACS Catal. 2019, 9, 3, 1799-1809.

Recent articles describing the use of Automated Reactors in polymerization reactions:

• Marco Oliveira , Sabrina Lewin Behrends , Ivan Reis Rosa, Cesar Liberato Petzhold, “Use of a trithiocarbonyl RAFT agent without modification as (Co)stabilizer in miniemulsion polymerization J. Polym. Sci.
  
  
• Anderson Nogueira Mendes, Lívia Alves Filgueiras, Monica Regina Pimentel Siqueira, Gleyce Moreno Barbosa, Carla Holandino, Davyson de Lima Moreira, José Carlos Pinto, Marcio Nele, “Encapsulation of Piper cabralanum (Piperaceae) nonpolar extract in poly(methyl methacrylate) by miniemulsion and evaluation of increase in the effectiveness of antileukemic activity in K562 cells”, Int. J. Nanomedicine, 2017; 12: 8363–8373
  
  
• Peter Rodiča, Ingrid Miloševa, Maria Lekkab, Francesco Andreattab, Lorenzo Fedrizzi, “Corrosion behaviour and chemical stability of transparent hybrid sol-gel coatings deposited on aluminium in acidic and alkaline solutions”, Progress in Organic Coatings,  2018, 124, 286-295
  
  
• Sankaranarayanan, S., Likozar, B., Navia, R. “Real-time Particle Size Analysis Using the Focused Beam Reflectance Measurement Probe for In Situ Fabrication of Polyacrylamide–Filler Composite Materials” Sci Rep, 2019, 9, 10126 https://doi.org/10.1038/s41598-019-46451

Recent articles describing the use of Inline Particle Size Analyzers in polymerization reactions:

• Xiongli Liu, Yangbing Wen, Jialei Qu, Xin Geng, Bin Chen, Bing Wei, Binbin Wu, Shuo Yang, Hongjie Zhang, Yonghao Ni, “Improving salt tolerance and thermal stability of cellulose nanofibrils by grafting modification”, Carbohydrate Polymers, 2019, 211, 257-265. 
  
  
• Yu Huang, Xiaogang Xue, Kaiqiao Fu, “Application of Spherical Polyelectrolyte Brushes Microparticle System in Flocculation and Retention”, Polymers 2020, 12, 746.