Study of radical intermediate cleavage on RAFT polymerization of methyl acrylate and methyl methacrylate

doi:10.11949/0438-1157.20250215

Abstract

Abstract:

Although methyl acrylate (MA) and methyl methacrylate (MMA) share similar monomer structures, they exhibit significant behavioral differences in reversible addition-fragmentation chain transfer (RAFT) polymerization. This study focuses on investigating the intrinsic mechanisms underlying the differences in molecular weight control and dispersity between MA and MMA by comparing four distinct chain transfer agents (CTAs). Combining polymerization kinetics experiments with density functional theory (DFT) calculations, the fragmentation tendency (φ) of intermediate radicals and the interaction strength between CTAs and propagating radicals (control index, C) were analyzed. The results show that the dissociation tendency of the intermediate free radical and its interaction strength with CTA are key factors affecting the polymerization rate and molecular weight distribution. When φ approaches 1 (e.g., in MA/CTA 2-4 systems), the molecular weight aligns well with theoretical predictions. In contrast, for MMA systems, the stabilization of P_n· radicals by the methyl group hinders R· dissociation (φ approaches 0), leading to significant deviations from theoretical molecular weights in the early stages, which are gradually reduced through short-chain regeneration. This study provides experimental and theoretical insights into the anomalous molecular weight prediction behavior in RAFT polymerization, offering valuable guidance for the precise control of RAFT systems.

Key words: RAFT polymerization, methyl acrylate, methyl methacrylate, mechanism, radical

摘要：

丙烯酸甲酯（MA）与甲基丙烯酸甲酯（MMA）虽具有相似的单体结构，但在可逆加成-断裂链转移（RAFT）控制的聚合反应中常表现出显著的行为差异。通过对比4种不同链转移剂（CTA），重点揭示了MA与MMA在RAFT聚合中关于分子量控制及多分散指数差异的内在机制。结合聚合动力学实验与密度泛函理论（DFT）计算，重点分析了中间体自由基的断裂倾向（φ）及CTA与增长自由基间的相互作用强度（控制指数C）。实验结果表明，中间体自由基的解离倾向及其与CTA的作用强度是影响聚合速率和分子量分布的关键因素。当φ趋近于1时（如MA/CTA 2-4体系），分子量与理论值吻合良好；而在MMA体系中，由于甲基对P_n·自由基的稳定作用导致R·解离受阻（φ趋近于0），初期分子量显著偏离理论值，后续通过短链再生逐步降低偏离。本研究为RAFT聚合中分子量预测的异常行为提供了实验与理论解释，对RAFT聚合的精确控制具有指导意义。

关键词: 可逆加成-断裂链转移聚合, 丙烯酸甲酯, 甲基丙烯酸甲酯, 机理, 自由基

CLC Number:

TQ 316.3

Yuhui WU, Jialong ZHANG, Yuanhe HOU, Zhen LIU. Study of radical intermediate cleavage on RAFT polymerization of methyl acrylate and methyl methacrylate[J]. CIESC Journal, 2025, 76(10): 5464-5474.

吴宇辉, 张家龙, 侯远赫, 刘振. 中间体自由基断裂对丙烯酸甲酯/甲基丙烯酸甲酯RAFT聚合的影响[J]. 化工学报, 2025, 76(10): 5464-5474.

Figures/Tables 10

Fig.1 Cleavage pathways of radical intermediates during RAFT chain transfer reaction

Fig.2 Structures of monomers (MA/MMA) and chain transfer agents (CTAs)

Table 1 Comparison of molecular weight for PMA/PMMA regulated by different CTAs

Entry	Monomer	RAFT agent	Conv^①/%	M_n,th^②	M_n,GPC^③	Ð^③
1	MA	CTA1	92	8171	37800	1.76
2	MA	CTA2	98	8811	9300	1.12
3	MA	CTA3	40	3636	3400	1.12
4	MA	CTA4	33	3086	2400	1.11
5	MMA	CTA1	99	10134	53100	2.36
6	MMA	CTA2	81	8455	7600	1.32
7	MMA	CTA3	32	3425	3000	1.25
8	MMA	CTA4	30	3250	2500	1.20

Fig.3 Molecular weights and polymerization kinetics for MA and MMA systems under CTA2 regulation

Table 2 Relative Gibbs free energies of species in pre-equilibrium stage of MA and MMA with CTA1—CTA4

Entry	Monomer	RAFT agent	ΔG_-α^①/(kJ·mol^-1)	ΔG_β^②/(kJ·mol^-1)	ΔG_{pre-equilibrium}^③/(kJ·mol^-1)	φ^④	C^⑤
1	MA	CTA 1	44.9	42.0	10.8	0.7384	9.81×10^-4
2	MA	CTA 2	57.6	44.6	-32.4	0.9902	6.88×10
3	MA	CTA 3	80.4	62.9	-25.8	0.9980	4.46×10²
4	MA	CTA 4	82.4	62.6	-30.1	0.9991	1.38×10³
5	MMA	CTA 1	31.8	50.5	40.3	0.0013	5.08×10^-3
6	MMA	CTA 2	46.4	61.0	2.9	0.0117	4.49×10
7	MMA	CTA 3	58.0	73.8	0.7	0.0058	2.64×10²
8	MMA	CTA 4	56.1	74.1	4.8	0.0004	7.20×10²

Fig.4 Schematic illustration of RAFT polymerization mechanisms governed by radical intermediate cleavage pathways

Fig.5 Primary orbital interactions in ETS-NOCV analysis(Transition state structure is divided into two fragments: CTA and attacking radical. Blue areas show decreased electron density while red areas show increased electron density)

Fig.6 Energy gap between SOMO of radical and LUMO of CTA

Fig.7 Orbital interactions of CTAs(CTA is divided into two fragments: Fragment 1 (R group and SC—S unit) and Fragment 2 (Z group). Blue labels indicate orbital serial numbers, and red percentage labels denote contribution of fragment orbitals to corresponding orbitals in complex. Fragment orbitals contributing less than 10% to orbital components of complex are not displayed. Solid bars represent occupied orbitals, while dashed bars represent unoccupied orbitals)

Fig.8 Energy decomposition analysis: (a)—(d) decomposition of relative Gibbs free energy (ΔG) into its contributing components; (e),(f) distortion-interaction analysis for ΔE of TS2′-MA-CTA2 and TS2′-MA-CTA3; (g),(h) MPI variation from CTA to transition state structure

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