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Chain-growth or addition polymerization is successive addition reactions of monomers with a polymer chain. In radical chain-growth polymerization, the reaction proceeds via a free-radical intermediate. The free radical is formed from radical initiators, which spontaneously generate free radicals by homolytic fission. Organic peroxides (such as dibenzoyl peroxide, as shown in Figure 1) or azo compounds are popular radical initiators. A low concentration ratio of radical initiator to monomer is...

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Polymers

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The word polymer is derived from the Greek words “poly” which means “many” and “mer” which means “parts”. Polymers are long chains of molecules composed of repeating units of smaller molecules, known as monomers. They either occur naturally, such as DNA and proteins, or can be constructed synthetically, like plastics. They have varied structural characteristics, such as linear chains, branched chains, or complex networks, that contribute to the properties that they exhibit. Additionally,...

Polymers

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The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the π bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the π bond generates a carbocation stabilized by the electron‐donating group. In the propagation step, the π bond of the second monomer acts as a nucleophile and attacks the generated carbocation,...

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Ethylene Polymerizations Using Parallel Pressure Reactors and a Kinetic Analysis of Chain Transfer Polymerization

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Chain Length Dependence of Chemically Controlled Reactions in Polymerization.

Yue Mu¹, Zhen Liu¹, Michelle L Coote²

¹School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China.

Journal of the American Chemical Society

|July 14, 2026

Summary

Chemically controlled polymerization reactions show unexpected chain length effects due to entropy. This challenges the assumption of independent rate coefficients, impacting reversible addition-fragmentation chain transfer (RAFT) polymerization and other polymer-polymer reactions.

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Area of Science:

Polymer Chemistry
Chemical Kinetics
Thermodynamics

Background:

Traditionally, rate coefficients in chemically controlled polymerization are assumed to be chain-length independent.
This assumption is critical for understanding polymerization kinetics and thermodynamics.

Purpose of the Study:

To challenge the assumption of chain-length independent rate coefficients in chemically controlled polymerization.
To investigate the impact of chain length on the kinetics and thermodynamics of polymer-polymer reactions, using reversible addition-fragmentation chain transfer (RAFT) polymerization as a case study.

Main Methods:

Utilized multiscale modeling to simulate polymerization processes.
Compared chain length convergence of reaction entropy for polymer-polymer RAFT agent reactions versus low molecular weight RAFT agent reactions.
Analyzed Diels-Alder step-growth polymerization with varying chain lengths.

Main Results:

Demonstrated significant chain length effects in chemically controlled polymer-polymer reactions.
Observed delayed entropy convergence in RAFT polymerization involving polymeric RAFT agents (persisting to DP > 100) compared to low molecular weight agents (equilibrium within 10 monomer additions).
Found that entropy contributions scale logarithmically with chain length, leading to chain length-dependent equilibrium constants.

Conclusions:

Chain length effects are significant in the kinetics and thermodynamics of chemically controlled polymer-polymer reactions due to fundamental entropic considerations.
Findings provide a mechanistic basis for resolving experimental discrepancies in rate coefficients and understanding rate retardation in RAFT systems.
Highlights the importance of considering chain length dependence in polymerization modeling and analysis.