# Porous Organic Polymers-Supported Metallocene Catalysts for Ethylene/1-Hexene Copolymerization
## 2 Results and Discussion
### Chain Growth of Ethylene/1-Hexene Copolymer  
The \(\alpha\)-olefin comonomer effect was clearly observed during the process of ethylene/1-hexene copolymerization, as the copolymerization activities increased obviously in the presence of 1-hexene, compared with the ethylene homopolymerization results [32]. Gel permeation chromatographic (GPC) analysis revealed that the molecular weights of ethylene/1-hexene copolymer decreased from about 200,000 to 110,000 compared to ethylene homopolymer, and similar results were also reported for ethylene/\(\alpha\)-olefin copolymers with other single-site catalyst systems [34; 35]. The introduction of 1-hexene affects the catalytic activities and molecular chain length in different ways. Although the 1-hexene has stronger electron-donating ability than ethylene (which would benefit the coordination of 1-hexene to the active species and stabilizes the cationic active sites), the insertion rate of 1-hexene is slower than that of ethylene due to its steric hindrance [33]. The final results of the copolymerization showed that the increased activities and decreased molecular chain length might be caused by the increasing chain propagation rate and chain transfer rate combined, which could also be explained by the \(\alpha\)-olefin comonomer effect. There is another important factor that can significantly influence the molecular chain length of polyolefin--the pore size structure. Generally, the average molecular weights of PE from polymer-supported catalysts are lower than that of inorganic-supported catalysts [36; 37], because the porous polymer support is not as firm as mesoporous silica, and the soft polymer support will further result in quick chain termination. However, much higher average molecular weights of PE homopolymer were obtained from the POPs-supported metallocene catalysts, with pore size mainly scattering in the micro- and the narrow meso-pore (1-4 nm) range [32]. Similar results were also obtained, such that the average molecular weight of ethylene/1-hexene copolymers from the POPs-supported metallocene catalysts were higher than from silica-supported metallocene catalysts (Mw: 9.7 \(\times\) 10\({}^{4}\)) when introducing 1-hexene in process of ethylene polymerization. The pore structure of the POP supports could be used to tune the molecular weight distribution or polydispersity, as the processability of polyolefin materials [38] was highly related to the molecular weight distribution of the prepared polyolefin. It is practical to improve the processability of metallocene-based polyolefin materials such as LLDPE (1-hexene or 1-octene as comonomer) by using multiple metallocene complexes or multinuclear metallocene catalysts and a multiple-reactor process to increase molecular weight distribution. We have reported the weight-average molecular weights (Mw) of homo-polyethylene from the POPs-supported metallocene catalysts in the range of 90,000 to 400,000, and their polydispersities of 2-3 are typically narrower than that of PE produced by silica-supported metallocene catalyst due to their ability to provide much closer analogue to the environment prevailing in homogeneous polymerization. By tuning the distribution of the active species scattered in the micro- and the narrow meso-pore (1-4 nm), the chain length of the polyolefin can be tailored effectively during the polymerization process. That is, the smaller the pore size of the POP support, the longer the chain length of PE that can be obtained due to the different diffusion rates of different sizes of monomer (ethylene), comonomer (\(\alpha\)-olefin), hydrogen, and chain transfer agent (scavenger TIBA/TEAL), etc. Therefore, by tuning the pore size and pore size distribution of the POP supports, the molecular weight distribution of the prepared polyolefin can be tuned.