Introduction

Atom transfer radical polymerization (ATRP) initiator refers to the chemical process in which certain chemical substances react with monomers to form intermediate compounds, which are continuously connected with a large number of new monomers to form polymeric compounds. The role of initiators in atom transfer radical polymerization is to determine the number of growing chains in the polymer. If the rate of initiation is fast enough, chain termination and chain transfer are negligible, the number of growing chains is constant and equal to the concentration of the initiator, and the molecular weight of the polymer after the complete reaction of monomers can be determined by the amount of initiator. Traditional ATRP initiators include alkyl halide initiators, α-halide ester initiators, benzyl halide initiators, sulfonyl halide initiators, and α-halide ketone initiators.

Types of ATRP Initiators

• Alkyl Halide Initiators

Alkyl halide initiators are commonly used in atom transfer radical addition reactions and are one of the earliest initiators used in atom transfer radical polymerization.^{[1, 2]} Commonly used alkyl halide initiators are CHCl₃, CCl₄, etc.

• α-Halide Ester Initiators

Common α-halogenated ester initiators are very good ATRP initiators, such as methyl α-chloropropionate, ethyl α-chloropropionate, methyl α-bromopropionate, ethyl α-bromopropionate, etc.

• Halogenated Benzyl Initiators

Halogenated benzyl has a similar structure to styrene monomers, so it is often used as an ARTP initiator initiated by styrene monomers. Meanwhile, benzyl halides are the most effective initiators for styrene monomers, but cannot be used for more reactive monomers.

• Sulfonyl Halide Initiators

According to researchers,^{[3, 4]} sulfonyl halide initiators can effectively initiate polymerization and react with monomers, while substituted aryl sulfonyl chloride containing S-Cl bond is a universal initiator for acrylate, methacrylate, and styrene monomers.

• α-Halide Ketone Initiators

Polyhalogenated ketone are one of the most effective initiators for ATRP of methyl methacrylate catalyzed by Ru-catalyst, and commonly used ones are CCl₃COCH₃, CHCH₂COPh, etc.

Selection Criteria for ATRP Initiators

1. Structural similarity.
2. Ensure that the initiation rate is greater than the chain growth rate.
3. Minimize side effects.

References

1. Wang, J.S. et al. Controlled/"living" radical polymerization. atom transfer radical polymerization in the presence of transition-metal complexes. Journal of the American Chemical Society. 1995, 117(20): 5614-5615.
2. Kato, M. et al. Polymerization of methyl methacrylate with the carbon tetrachloride/dichlorotris-(triphenylphosphine) ruthenium (II)/methyl aluminum bis (2,6-di-test-butylphenoxide) initiating system: possibility of living radical polymerization. Macromolecules. 1995, 28(5): 1721-1723.
3. Granel, C. et al. Controlled Radical Polymerization of Methacrylic Monomers in the Presence of a Bis(ortho-chelated) Arylnickel(II) Complex and Different Activated Alkyl Halides. Macromolecules. 1996, 29(27): 8576-8582.
4. Uegaki, H. et al. Living radical polymerization of methyl methacrylate with a zerovalent nickel complex, Ni(PPh3). Journal of Polymer Science Part A: Polymer Chemistry. 1999, 37(15): 3003-3009.