What is a radical initiator?
A radical initiator is a compound that contains a weak bond and undergoes homolytic cleavage under heat, light, or radiation to generate radicals, thereby initiating polymerization of monomers. Unlike catalysts, initiators are consumed during the reaction; their fragments typically become polymer chain end groups, so they are not regenerable.
Two key steps of initiation:
· Decomposition to generate primary radicals (generally the rate-determining step)
· Addition to a monomer to form a monomer-derived/propagating radical (usually faster)
Overview of initiator types
Radical initiators are diverse and can be classified by decomposition mechanism or chemical structure.
· By decomposition mode: thermally decomposing, photodecomposing, and redox (“red-ox”) types.
· By chemical structure:
o Peroxides: including organic peroxides such as benzoyl peroxide (BPO) and inorganic peroxides such as potassium persulfate (KPS) and ammonium persulfate (APS);
o Azo compounds: e.g., azobisisobutyronitrile (AIBN);
o Others: specialized photoinitiators, radiation sources, etc.
A brief summary of the features and mechanistic highlights of each class follows.
1. Peroxide initiators
Peroxide initiators include organic peroxides and inorganic peroxides. Common choices are benzoyl peroxide (BPO), lauroyl peroxide (LPO), tert-butyl peroxybenzoate (TBPB), etc. Inorganic peroxide initiators are represented by persulfates, including potassium persulfate (KPS) and ammonium persulfate (APS).
Peroxide initiators contain a peroxy (–O–O–) group that undergoes homolysis upon heating or irradiation to generate radicals. Taking diacyl peroxides as an example:
· Step 1: O–O homolysis
R–C(=O)–O–O–C(=O)–R → 2 R–C(=O)–O· (acyl-oxy radicals)
· Step 2: (Partial) decarboxylation
A portion of the acyl-oxy radicals further decarboxylate to give R· and CO₂.
R–C(=O)–O· → R· + CO₂
2. Azo initiators
Azo initiators are typically used in the 45–80 °C range. Their decomposition is essentially first-order, yielding a single radical species; nitrogen gas is evolved on cleavage and no induction period is observed. They are comparatively stable and can be stored safely on their own.
The most widely used examples are azobisisobutyronitrile (AIBN) and azobis(2,4-dimethylvaleronitrile) (ABVN). Taking AIBN as an example (see scheme): upon heating, the N═N bond undergoes homolytic cleavage, generating two identical 2-cyanoprop-2-yl radicals (·C(CH₃)₂–CN) with concomitant release of N₂—the textbook azo-initiation mechanism.
3. Redox systems
Inorganic peroxides, high-valent transition-metal ions, or other high-oxidation-state species, when combined with a small amount of reductant, generate primary radicals via oxidation–reduction reactions to initiate polymerization; such initiator sets are referred to as water-soluble redox initiation systems.
Most organic peroxides are insoluble in water but dissolve in common organic solvents and in many vinyl monomers; thus, when formulating a redox pair, an oil-soluble reductant should be selected. A widely used class is the organic peroxide–tertiary amine system, with benzoyl peroxide (BPO) and N,N-dimethylaniline (DMA) as the textbook example. This pair decomposes much faster than BPO alone.
Compared with peroxide and azo initiators discussed above, redox systems exhibit lower activation energies, enabling initiation at lower temperatures (at or below room temperature).
Note that residual metal ions in the polymer typically degrade certain material properties.
Examples of redox initiation:
· H₂O₂ + Fe²⁺ → HO• + OH⁻ + Fe³⁺: classic Fenton reaction (most effective under mildly acidic conditions).
· ROOH + Fe²⁺ → RO• + OH⁻ + Fe³⁺: Fenton-like reaction generating alkoxy radicals.
· S₂O₈²⁻ + S₂O₃²⁻ → SO₄²⁻ + SO₄•⁻ + S₂O₃•⁻: in industry, S₂O₈²⁻/HSO₃⁻ (or S₂O₅²⁻) systems are even more common, producing SO₄•⁻ and SO₃•⁻.
How to select an initiator?
From a thermodynamic standpoint, most olefinic monomers are predisposed to polymerize; whether polymerization actually occurs, however, is ultimately governed by kinetics—namely, whether a sufficient rate can be achieved. Consequently, judicious initiator selection is critical.
Guiding principles for selecting an initiator
· Match to the polymerization method: bulk and suspension polymerizations typically use oil-soluble initiators (e.g., peroxides, azo compounds); emulsion and aqueous-solution polymerizations use water-soluble initiators (e.g., persulfates).
· Avoid incompatibilities with the formulation: in reducing systems, avoid peroxide initiators to prevent unintended redox side reactions.
· Align half-life with the temperature profile: choose an initiator whose decomposition rate (t₁/₂) matches the planned polymerization temperature to control reaction rate.
· Blend initiators when needed: in practice, initiators with different half-lives are often combined to maintain a uniform, stable rate throughout the run.
· Safety and residues: prefer initiators whose decomposition products are low in toxicity and that are safe to store. Also consider tendency to color/discolor, inherent toxicity, ease of handling, and cost-effectiveness.
Initiator options by temperature
Temperature range (°C) | System / Initiator | Abbrev. | CAS | Notes / Remarks |
−10–30 | Persulfate + bisulfite / metabisulfite (redox) | KPS/APS + NaHSO₃/Na₂S₂O₅ | Aqueous, low-temperature redox start; operable at ambient temperature. Temperature is strongly influenced by ratio / pH / metal ions. | |
−10–30 | TBHP + SFS | TBHP + SFS | Common low-temperature redox system for emulsions/adhesives; mature industrial practice. | |
30–100 | Benzoyl peroxide | BPO | Widely used thermal initiator; 10 h t₁/₂ ≈ 72–73 °C (solvent-dependent). | |
30–100 | Azobisisobutyronitrile | AIBN | 78-67-1 | Evolves N₂ on decomposition; 10 h t₁/₂ ≈ 65 °C (medium-dependent). |
30–100 | 4,4′-Azobis(4-cyanovaleric acid) | ACVA | 5109-65-9 | Common for aqueous polymerization (anionic azo acid). |
30–100 | Ammonium/potassium persulfate (thermal) | APS/KPS | Thermal runs typically 75–95 °C; pairing with a redox system can markedly lower the temperature. | |
> 100 | Cumene hydroperoxide | CHP | Common for high-temperature initiation / post-cure. | |
> 100 | tert-Butyl hydroperoxide | TBHP | Can serve as a high-temperature thermal initiator or as the oxidant in a redox pair. | |
> 100 | Dicumyl peroxide | DCP | 80-43-3 | High-temperature initiation/crosslinking (PE/EPDM, etc.); 10 h t₁/₂ ≈ 116 °C. |
40–65 | Di(2-ethylhexyl) peroxydicarbonate | EHP | 16111-62-9 | Low-temperature initiator; common in PVC suspension polymerization; 10 h t₁/₂ ≈ 40 °C (solvent/concentration-dependent). |
Common thermal initiators
Name | Type | CAS No. | 10-h half-life & medium | Solubility | Features |
Benzoyl peroxide (BPO) | Organic peroxide | ≈ 73 °C (benzene/toluene; solvent-dependent) | Soluble in aromatics, chloroform, ethers; sparingly soluble in ethanol; practically insoluble in water | Classic oil-soluble radical initiator; suitable for styrene and (meth)acrylates; forms redox pairs with tertiary amines for room-temperature curing. | |
Dicumyl peroxide (DCP*) | Organic peroxide | 80-43-3 | ≈ 115 °C (aromatic solvents) | Soluble in aromatics, cumene, ethers; insoluble in water | High-temperature initiation/crosslinking; widely used for PE/EPDM crosslinking or late-stage, high-temperature polymerization. *Also referred to as “diisopropylbenzene peroxide.” |
Di(2-ethylhexyl) peroxydicarbonate (EHP) | Organic peroxide | 16111-62-9 | ≈ 45–47 °C (aromatics/esters; medium-dependent) | Readily soluble in aliphatic/ester solvents; insoluble in water | Low-temperature initiator; used in suspension/emulsion polymerizations (e.g., VCM); requires low-temperature storage. |
Azobisisobutyronitrile (AIBN) | Azo | 78-67-1 | ≈ 65 °C (aromatic media) | Soluble in methanol, ethanol, acetone, acetonitrile; insoluble in water | “Clean” decomposition (N₂ evolved); common for solution/bulk/suspension polymerizations. |
Azobis(2,4-dimethylvaleronitrile) (ABVN/AMVN) | Azo | 4419-11-8 | ≈ 52 °C (aromatic media) | Soluble in methanol, ethanol, acetone; insoluble in water | Decomposes at lower temperature than AIBN; suitable for initiation near ambient temperature. |
Potassium persulfate (KPS) | Inorganic peroxide | (Aqueous) ≈ 60 °C for 10-h t₁/₂ | Soluble in water; insoluble in ethanol | Common in emulsion/aqueous-solution polymerizations; use chelators to suppress metal-ion effects. | |
Ammonium persulfate (APS) | Inorganic peroxide | At 50 °C, t₁/₂ ≫ 10 h; at 70 °C, ≈ 12 h (pH-dependent) | Highly soluble in water; poorly soluble in ethanol (moderate in methanol) | Highly water-soluble; can pair with (bi)sulfite/metabisulfite to form redox systems for low-temperature initiation. |
Common Photoinitiator Types
Name | CAS No. | Type | Solubility | Absorption (λ range) | Features |
1-Hydroxycyclohexyl phenyl ketone | Type I, α-hydroxyketone (cleavage) | Oil-soluble | ~244/280/330 nm (solvent-dependent) | High efficiency, low odor, low yellowing, good hardness (limited thick-film penetration). | |
Phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO) | Type I, phosphine oxide (cleavage) | Oil-soluble | ~350–420 nm, λ_max ≈ 370 nm | Broad-spectrum, high efficiency; absorption extends beyond 400 nm; suited to thick films / colored systems. | |
2,4,6-Trimethylbenzoyl diphenylphosphine oxide (TPO) | Type I, phosphine oxide (cleavage) | Oil-soluble | ~350–400 nm, λ_max ≈ 380 nm | High activity, low yellowing, long-wavelength response; widely used in white / light-colored systems. | |
Benzophenone (BP) | Type II, hydrogen-abstraction (requires amine co-initiator) | Oil-soluble | Mainly < 350 nm (solvent-dependent) | Low cost but needs amine synergist; potential odor/migration. | |
Isopropylthioxanthone (ITX) | Type II, hydrogen-abstraction | Oil-soluble | Sensitive to long-wave UV | Commonly paired with amines/sensitizers; relatively deep coloration. | |
Iodonium salts (e.g., diphenyliodonium hexafluorophosphate) | 58109-40-3 (example; many variants) | Cationic photoinitiators (generate strong acid; initiate epoxies, etc.) | Depends on counter-anion | Intrinsic absorption mainly in deep UV; can be sensitized to 365–405 nm | Suitable for cationic systems; sensitive to amines/moisture. |
Common redox systems
Common Redox Systems | Oxidant CAS | Reductant CAS | Medium | Features |
Ammonium persulfate (APS) + sodium bisulfite (NaHSO₃) | Aqueous | Classic low-temperature aqueous redox pair; single-electron transfer generates SO₄•⁻/SO₃•⁻; widely used in emulsion/solution polymerizations. | ||
Potassium persulfate (KPS) + ferrous sulfate (FeSO₄) | Aqueous | Fe²⁺-activated persulfate redox system (polymerization context); sensitive to adventitious metal ions/impurities—use chelators and control pH. | ||
tert-Butyl hydroperoxide (TBHP) + sodium formaldehyde sulfoxylate (SFS) | Aqueous / Emulsion | Efficient low-temperature initiation; common in emulsion polymerization and adhesives; TBHP typically used as an aqueous solution. | ||
Benzoyl peroxide (BPO) + N,N-dimethylaniline (DMA) | Oil-soluble | Classic oil-phase redox pair; fast room-/low-temperature curing (e.g., acrylates, UPR). | ||
Methyl ethyl ketone peroxide (MEKP) + cobalt naphthenate (Co-naphthenate) | Oil-soluble | UPR ambient-cure system; cobalt acts as promoter; tends to color; observe safety and exotherm. | ||
Cumene hydroperoxide (CHP) + Fe²⁺ (e.g., ferrous sulfate) | 7720-78-7 (example) | Water/oil interface | Fenton-like system; suitable for strong-oxidation, low-temperature initiation; monitor metal-ion residues and stability. |
Emerging Initiators and Development Trends
With environmental regulations tightening and application spaces expanding, initiators are evolving toward higher efficiency, safety, and sustainability:
· Macromolecular photoinitiators: low migration and low odor; suitable for food-contact packaging.
· Visible-light photoinitiators: lower energy consumption and stronger penetration; suitable for thick-film curing and biomedical applications.
· Aqueous redox systems: environmentally friendly; suitable for low-temperature emulsion polymerization.
· Enzyme-catalyzed / bio-based initiators: initiator systems that are genuinely green.
Practical Application Cases
· PVC suspension polymerization: industry practice centers on peroxydicarbonates (e.g., EHP) mainly in the 40–65 °C window, often blended with diacyl peroxides (e.g., LPO, lauroyl peroxide) to cover fast/slow stages.
· Acrylate emulsions: KPS or APS are commonly used as water-soluble initiators; pairing with redox systems enables low-temperature polymerization and improves molecular-weight distribution uniformity.
· UV-curable coatings: α-hydroxy ketones and phosphine oxides (184/TPO/BAPO) are mainstream; TPO/BAPO are LED-friendly at 365–405 nm and excel in thick-film curing.
Conclusion
Radical initiators determine the controllability, efficiency, and safety window of polymerization. Selection should balance target temperature vs. half-life, solubility/compatibility with the medium, product performance, and safety & environmental factors. Where appropriate, employ blends and redox strategies to achieve low-temperature, uniform, and high-conversion processes.
References
1. Pan Zuren; Sun Jingwu. Polymer Chemistry. Chemical Industry Press, 1980.
2. Pan Zuren; Ding Zaizhang. Free-Radical Polymerization. Chemical Industry Press, 1983.
3. Wang Jian; Zhang Qian; Zhao Jinyuan; et al. “Research Progress on Initiators for Free-Radical Polymerization.” Plastics Additives, 2022(04): 29–34+40.
4. Xu Cheng; Tang Huadong. “Advances in Radical Polymerization Initiators.” Zhejiang Chemical Industry, 2015, 46(06): 34–37.
5. Li Xuechun; Sun Fang. “An Introduction to and Research Progress of Radical-Type Photoinitiators.” University Chemistry, 2021, 36(06): 5–14.
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