Today, significant portions of commercial products, vehicles used for transportation, and the built environment are made using plastics or elastomers with vinyl polymers. These polymers are typically produced by free radical polymerization involving controlled reactions using vinyl monomers, radical initiators, chain transfer agents, and a solvent.
One reason vinyl polymers are so prevalent is because of the high degree of durability and versatility they provide to materials. However, these characteristics are dependent on the quality of the polymer, requiring manufacturers to optimize molecular weight, purity, and morphology. In many cases, these properties can be optimized and controlled quite easily if azonitrile-based radical initiators are used.
What Are Azonitrile Initiators?
Azonitrile initiators are organic compounds with a central N2 linking group, as shown in the chemical decomposition scheme below. These compounds are incredibly useful as a free radical source for polymerization initiation because it is thermodynamically favorable for the compound to drive the release of N2 molecules as nitrogen gas. This reaction also forms two radicals from the azonitrile compound.
These radicals then initiate polymerization by attacking the double bond of vinyl monomer molecules, forming a new radical species that repeats this process with other vinyl monomers, ultimately propagating the polymer chain.
Benefits of Using an Azonitrile Initiator
Besides azonitrile compounds, organic peroxides are commonly used initiators for radical initiation because they are relatively inexpensive. As a result, however, they force disadvantages in terms of polymer quality, workplace safety, and simplicity of use. Therefore, many vinyl polymer manufacturers invest in azonitrile initiators to gain better polymer yields, spend less on hazard response and mitigation programs, and increase their manufacturing efficiency. Details pertaining to how azonitrile initiators specifically provide these benefits in comparison to organic peroxides are explored below.
Optimized Polymer Quality
Organic peroxide initiators decompose into hydroxyl radicals, which are extremely strong oxidizers. These hydroxyl radicals can easily abstract hydrogen atoms from growing polymer chains, creating radicals at unwanted sites. When this occurs, unexpected polymer branching or cross-linking may occur, presenting significant challenges to controlling the molecular weight and morphology of the polymer. On the other hand, azonitrile radical initiators do not abstract hydrogen from growing polymer chains. This reduces the chances of undesired polymer branching and cross-linking, giving the manufacturer more control over the final polymer structure. Therefore, azonitrile radical initiators enable crucial quality improvements for manufacturers who produce specialty polymers with well-defined molecular weight and quality goals.
Increased Safety
Since organic peroxides are incredibly volatile initiators, they can undergo a rapid exothermic reaction through decomposition if they are not stored under strictly controlled temperatures. This, of course, can lead to explosion risks that pose serious workplace hazards. When compared to peroxides with similar half-lives, azonitrile radical initiators have higher decomposition temperatures, making them less likely to decompose accidentally and endanger workplaces. Also, azonitriles are not shock sensitive like organic peroxides. Therefore, the risk of explosion during transportation and storage is reduced. In addition, azonitriles have lower health hazards because they pose low oral toxicity and are not skin sensitizers.
Simplified Reaction Conditions
Due to their extremely high reactivity, organic peroxides present challenges in setting up the reaction conditions that maximize polymer yield and quality. For example, peroxides may react with redox-sensitive agents, solvents, and dyes in a reaction medium, making it difficult to adjust reaction condition requirements to promote effective use. In comparison, azonitriles will not undergo redox reactions and will not be compromised by redox-sensitive reagents. Azonitrile radical initiators also will not react with solvents, dyes, or pigments. Highlighting a separate issue, organic peroxides have a high tendency to undergo radical-induced decomposition, cannibalizing other initiators and making polymerization less efficient. Comparatively, azonitriles do not decompose in the presence of free radicals but rather only in the presence of heat or light. This allows them to be used with other types of radical initiators if desired.
Vazo™ Free Radical Initiators
Vazo™ Free Radical Initiators are substituted azonitrile compounds from Chemours. These free radical initiators are ideal for initiating polymerization and creating polymers with strictly defined molecular weights and morphologies. Vazo™ Free Radical Initiators are available in six grades that vary in decomposition temperature and ideal fit in specific reaction conditions.
The right Vazo™ grade for an application is decided by two main factors: the desired reaction temperature and the reaction medium.
Reaction Temperature
The most important factor in determining which Vazo™ Free Radical Initiator to use is the reaction temperature, typically determined by the specific monomer used in the reaction. In general, the Vazo™ portfolio spans a realistic operating window between 65°C and 140°C. The number in each Vazo™ grade denotes the approximate 10-hour half-life temperature. As a guideline, a suitable Vazo™ grade may be selected based on a reaction temperature range that is 10°C to 30°C above or below the corresponding grade number.
Reaction Medium
The solubility of the initiator in the desired reaction medium is critical to polymerization efficiency. All Vazo™ grades are soluble in common reaction mediums such as toluene and acetone. However, Vazo™ 67 exhibits better solubility in a broader range of organic solvents. For reactions in which water is present, Vazo™ 56 WSP and Vazo™ 68 WSP are recommended.
Applications for Vazo™ Free Radical Initiators
Examples of vinyl polymers ideal for initiation via Vazo™ free-radical initiators include polyethylene (PE), polymethyl methacrylate (PMMA), polystyrene (PS), polyvinyl chloride (PVC), polyvinyl acetate (PVA) polyacrylonitrile (PAN), and polybutadiene (BR). As alluded to in the beginning of this article, these polymers are prevalent in various components and structures. Below are some of the use cases for these polymers and the benefits Vazo™ initiators provides that increase performance.
Minimal impurity formation
Minimal chain defects FDA approval in food contact applications
Hair Spray Binders
PVP, PVA
Binders for Aspirin Coatings
PVP, PVA
Electronics
Computer Housings
PS
Minimal chain defects
No oxygen-containing groups
Electronic Circuits
PMMA, Polyethylene naphthalate
Copper Laminates
PE, PTFE
Photoresist Coatings
PMMA, Cyanoacrylates
LCD Backlight Guide Plate
PMMA
Paper and Textiles
Coatings
LDPE, PET, PTFE
Minimal chain defects Minimal color-forming impurities
FDA approval in food contact
Dyes
Acrylates
No oxygen-containing groups
Release Agents
PTFE, PE
Minimal chain defects
Acrylic Fibers
Acrylic
Minimal chain defects
Summary
Manufacturers of vinyl polymers often use organic peroxide initiators for free radical polymerization because they are relatively low in cost. However, organic peroxides contribute to polymer quality, workplace safety, and manufacturing efficiency issues. Vazo™ azonitrile-based radical initiators provide manufacturers a better initiator alternative that creates polymers with less chain defects and tighter molecular weight specifications. Vazo™ initiators are also less susceptible to accidental degradation and require fewer reaction conditions for effective use. Thus, they are much safer and permit higher efficiency and ease of use than organic peroxide initiators.
Do you have any questions about Vazo™ or need help selecting the right grade? Click below to have one of our product specialists guide you to the right Vazo™ radical initiator solution for your application.
