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Professor Gao Haiyang's research group at Sun Yat-sen University made new progress in palladium-catalyzed alkoxystyrene coordination polymerization
2020-01-06 Source: Polymer Technology

Coordination polymerization is a synthetic method that can precisely regulate the structure of polymers. However, because polar groups often have σ-interactions with metal catalytic centers, it is generally difficult for polar vinyl monomers to coordinately insert through the π-coordination of double bonds. Aggregation faces huge challenges. 4-methoxystyrene is a polar styrenic monomer. Due to the strong electron-donating effect of methoxy groups, it usually polymerizes by free radical and ionic methods, especially cationic mechanisms. However, the molecular weight of the obtained polymer is generally not high (less than 30,000), and the molecular weight distribution is relatively wide. It is difficult to obtain a polymer with a narrow distribution through living polymerization.

The post-transition palladium metal catalyst shows good tolerance to polar monomers, and it can achieve coordination copolymerization of ethylene and polar monomers to prepare functional polyolefin materials. Professor Gao Haiyang's group from Sun Yat-sen University proposed an early strategy to increase the steric hindrance effect of the nickel-palladium catalyst framework, which can effectively enhance the catalyst's thermal stability, controllability and tolerance to polar monomers (Macromolecules 2014, 47 , 3325-3331, Chem. Eur. J. 2014, 20, 3225-3233, ACS Catal. 2015, 5, 122-128, Chem. Eur. J. 2016, 22, 14048-14055, J. Catal. 2019, 375, 113-123). Recently, they have designed and synthesized a nickel-palladium catalyst system with a highly sterically hindered dibenzobarene skeleton to achieve the active copolymerization of ethylene and polar monomers (Copolymers of ethylene and acrylate Macromolecules 2017, 50, 5661-5669, Macromolecules 2017, 50, 2675-2682 were copolymerized with methyl enoate, and Macromolecules 2018, 51, 9110-9121 were alternately copolymerized with CO and styrene).

On the basis of previous research work, Professor Gao Haiyang's group continued to study the homopolymerization of 4-methoxystyrene using an α-diimine palladium catalyst with a dibenzobarene skeleton. Because the styrene monomer is generally inserted into the palladium metal center through 2,1-, an allyl complex is formed, and a benzyl transfer reaction easily occurs quickly. Therefore, ordinary α-diimine palladium catalyzed styrene monomers are generally difficult to polymerize through coordination mechanisms. >100万,M w /M n <1.3)。 However, the highly hindered α-diimine palladium catalyst designed by Professor Gao Haiyang's research group can efficiently catalyze the polymerization of 4-methoxystyrene to obtain polymers with a narrow molecular weight distribution (M w > 1 million, M w / M n <1.3). The entire catalytic polymerization system showed the characteristics of fast initiation, fast growth, controlled chain transfer, and almost quantitative conversion of monomers (Figure 1). At the same time, the palladium catalyst can efficiently catalyze the polymerization of 4-ethoxystyrene, and can obtain polymers with higher molecular weight and narrower distribution in a short time. 倍以上),因而形成高分子量、窄分布的聚合产物。 Further kinetic studies show that the polymerization chain transfer reaction is carried out by a monomer-assisted β-H abstraction, and the chain growth rate constant is much larger than the chain transfer rate constant (104 times) Above), thus forming a polymer product of high molecular weight and narrow distribution.

Figure 1 Palladium-catalyzed polymerization of 4-alkoxystyrene

The structure analysis (NMR and MALDI-TOF) of the low-molecular-weight polymer obtained by controlled polymerization confirmed that the chain ends of the obtained polymer were mainly double bonds, so the monomers were coordinately inserted through the 1,2-mode ( figure 2). Through the controlled reaction of α-diimine palladium complex with 4-methoxystyrene, it was found that the methoxy group of 4-methoxystyrene does not interact with palladium metal, and the 4-methoxystyrene monomer mainly Inserted into Pd-Me in the manner of 2,1 to form allyl-coordinated Pd-MOS1 and Pd-MOS2 complexes. These two complexes are stable and cannot catalyze the polymerization of 4-methoxystyrene (image 3). Based on these experiments, it is proposed that the active species of the polymerization reaction is a complex formed by the 4-methoxystyrene monomer inserted into the palladium center in a small amount of 1,2-mode.

Figure 2 NMR and MALDI-TOF structure analysis of low molecular weight polymers

Figure 3 Isolation and characterization of palladium complex intermediates

High molecular weight poly (4-methoxystyrene) shows enhanced mechanical and thermal properties. Compared with 100,000-molecular-weight poly (4-methoxystyrene) obtained by cationic polymerization, the mechanical properties of high-molecular-weight poly (4-methoxystyrene) obtained by palladium polymerization are improved by 10%. Compared with commercial polystyrene, the introduction of methoxy group significantly improved its hydrophilic properties, and the contact angle of the polymer decreased from 98.6 to 61.6 . Therefore, the prepared high molecular weight poly (4-methoxystyrene) is a kind of high-performance polystyrene-based resin with great potential.

Figure 4 Mechanical, thermal, and hydrophilic properties of high molecular weight poly (4-methoxystyrene)

The above research results were published on Macromolecules doi.org/10.1021/acs.macromol.9b02274 under the title "Fast and Regioselective Polymerization of para-Alkoxystyrene by Palladium Catalysts for Precision Production of High-MolecularWeight Polystyrene Derivatives". Dr. Liao Guangfu is the first author of the paper, and Professor Gao Haiyang is the corresponding author. This work was supported by the National Natural Science Foundation of China (51873234, 21674130).

Paper link: http://pubs.acs.org/doi/10.1021/acs.macromol.9b02274

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