Information on propylene polymerization

Patent title: Propylene polymerization reactor Patent number: CN2472811

There is a book in the series \"Petrochemical Engineering Series – Engineering of Propylene Derivatives\" that provides a relatively comprehensive overview. If you need it, I can give you the address: http://zhangyong6404.home.imhb.cn/; go there to download it from the storage area

Is propylene polymerization a polymerization of the radical mechanism type? What does it look like exactly?

It is free radical polymerization; most polymerizations are of this type

Propylene polymerization 1.1 Overview of olefin coordination polymerization Since the discovery of Z–N catalysts in the early 1950s and their successful use to produce polypropylene with a high degree of stereoregularity, continuous efforts by several generations of scientists have led to significant advancements in both catalyst technology and polymerization processes. These advancements have played a crucial role in the development of polymer science and the polyolefin industry. Along with the innovation of catalysts in different periods, scientists’ understanding of the nature of catalyst active centers has also been deepening continuously. While reviewing some traditional theories and related models, this chapter also introduces the latest research findings. 1.1.1 Coordination between propylene molecules and transition metals: With the development of polyolefin catalysts, heterogeneous catalysts not only exhibit high activity but also high stereoselectivity. However, the structure of the active centers in these heterogeneous catalysts as well as certain polymerization mechanisms remain not fully understood. With the continuous improvement in our understanding of homogeneous metallocene catalysts and the growing knowledge regarding the structure and properties of their active centers, it is possible to apply the enantioselective mechanism of homogeneous metallocenes to heterogeneous catalysts. This not only helps to explain the polymerization mechanisms of many heterogeneous catalysts as well as the experimental results obtained, but it also is consistent with the polymerization mechanisms proposed for early heterogeneous catalysts. At the same time, it is possible to study the Mt–C bond, regioselectivity, the structure and properties of the active center, the enantioselective mechanism, and the role of electron donors in MgCl2-supported catalysts. According to literature reports, although there are different opinions regarding the nature of the active center, one thing is certain: in the polymerization of propylene using Z–N catalysts, the propylene monomer gradually inserts itself between the metal-carbon bonds of the active center. The reaction equation is as follows: Mt–R + nC=C → Mt–(C–C)n–R. It is generally believed that before the olefin monomer can undergo insertion reaction between the Mt–C bonds, it must first coordinate with the metal atom of the active center. Its main experimental evidence includes: ① The polypropylene molecular chains prepared by polymerization using a TiCl4-Al(C6H5)3 catalyst contain C6H5 end groups. ②13C NMR analysis was performed on the iPP prepared using a δ-TiCl3 catalyst or its MgCl2-supported catalyst, along with 13C-containing Al(CH3)3 as a co-catalyst. The molecular chains of iPP contained isobutyl chain ends, and the mechanism for their formation is as follows: Mt—13CH3+C3H6→Mt—CH2—CH(CH3)—13CH3. After quenching the polymerization reaction with 14CO, radioactive detection experiments were carried out, and radioactivity was found in the polymer. Since CO does not react with Al-C, this indicates that Mt-C is activated. ④Even in the absence of alkyl metal compounds, many catalysts remain active for olefin polymerization. 1.1.2 Insertion reactions of α-olefins into metal-carbon bonds In the polymerization of α-olefins, whether using heterogeneous catalysts or homogeneous metallocene catalysts, there are two ways in which the α-olefin monomer can insert itself between the Mt-C bonds at the active center: 1,2 insertion and 2,1 insertion, as shown below: →Mt—CH2—CH(CH3)—P (1,2 insertion or primary insertion) Mt—P + CH2=CH—CH3 →Mt—CH(CH3)—CH2—P (2,1 insertion or secondary insertion) 1,2 insertion occurs during the isotactic polymerization of olefins. Polymerization of propylene catalyzed by the homogeneous catalyst Me2C(Cp,Flu)ZrCl2/MAO also involves 1,2 insertion. In contrast, when propylene polymerization is catalyzed at low temperatures using a VCl4/Et2AlCl catalyst, 2,1 insertion predominates. The probability of head-to-head or tail-to-tail bonding of α-olefin monomer units within polymer chains is very low, and it is difficult to observe such bonding even in IR and NMR spectra. However, when propylene polymerization was catalyzed by a δ-TiCl3-Et2Al catalyst at high hydrogen concentrations, analysis of the polymer structure revealed that the polymer chain ends contained n-butyl groups. Similarly, when propylene polymerization was catalyzed using a fourth-generation magnesium chloride-supported catalyst containing internal and external electron donors, n-butyl chain ends could also be observed in the polypropylene molecular chains in the presence of the chain transfer agent hydrogen. The main reason for the formation of polypropylene with n-butyl chain ends is that the termination reaction resulting from H2 chain transfer occurs after 2,1 insertion. The reactions are as follows: Ti—CH(CH3)—CH2—CH2—CH(CH3)—P + H2 → Ti—H + CH3(CH2)3—CH(CH3)—P; Ti—CH2—CH(CH3)—CH2—CH(CH3)—P + H2 → Ti—H + CH3—CH(CH3)—CH2—CH(CH3)—P. The content of isotactic polypropylene with n-butyl end groups varies depending on the electron-donating agent (alkoxysilane) used, ranging from 12% to 28%. In the xylene-extracted solubles, the head-to-head irregular units in the polypropylene chains account for about 1%, a value similar to that in the polypropylene chains synthesized using a δ-TiCl3-Et2Al catalyst. It can be concluded that, in the propylene polymerization catalyzed by multiphase catalysts, the 2,1 insertion of propylene monomer can occur at the isotactic active sites, which in turn hinders further insertion of the propylene monomer. Isopropylene synthesized using metallocene catalysts also contains a small amount of irregular structural units (about 1%). The formation of these irregular structures is due to 2,1- and 1,3-insertion of propylene monomers into the Mt–C bond, and the mechanism for their formation is as follows. —The contents and relative ratios of the two irregular structural units, (CH2)2 and (CH2)4, in CH2—CH(CH3)—CH(CH3)—CH2—CH2—CH(CH3)— (2,1 insertion) —CH2—CH(CH3)—CH2—CH2—CH2—CH2—CH(CH3)— (1,3 insertion), are primarily determined by the Π-ligand, polymerization temperature, and monomer concentration. Generally, when the catalyst has high activity, 2,1 insertion is predominant. Zambelli et al. suggest that the formation of 1,3-insertion irregular units is due to secondary Zr alkylated units generated by 2,1 insertion isomerizing into primary Zr alkylated units before the insertion of the next olefin monomer. The polymerization of propylene catalyzed by Me2Si(benz--in-denyl)2ZrCl2-MAO yielded only 2,1 irregular structural units, indicating that the rate of insertion of the propylene monomer into the secondary Zr alkylated unit at the active site is greater than the rate of isomerization. Compared with the polypropylene synthesized using the above-mentioned multiphase catalyst, polypropylene synthesized with a homogeneous metallocene catalyst has a lower melting point under the same condition of equal content of the regular pentamer unit mmmmm in the polypropylene; the main reason for this is the presence of the aforementioned irregular structural units in the polymer chains. 1.1.3 Stereoselectivity of insertion reactions: During the growth of polymer chains, the mechanism of stereospecific polymerization can be further elucidated by studying the simultaneous addition of active metal ions and chain-end carbon atoms to the double bonds of new monomers. The isotactic polymerization of α-ene deuterated propylene monomers catalyzed by various multiphase and homogeneous catalysts yields erythro-diallostereotic polymers, whereas the isotactic polymerization of trans-deuterated propylene monomers yields threo-diallostereotic polymers. The structure of its formation is shown in Figure 3-1. 1.1.4 Stereoselective insertion of monomers: Since the two sides of an α-olefin monomer are different (the R and S enantiomeric faces), as shown in Figure 3-2, the α-olefin monomer is a chiral compound. After the polymerization reaction, the configuration (R or S) of the tertiary carbon atoms in the polymer chains depends on the way in which the R and S enantiomers of the monomer interact, the manner in which the monomer is incorporated, and the stereospecific addition mode of the monomer (cis or trans). If the catalyst exhibits high stereoselectivity, the insertion of monomers occurs via cis stereosynthetic addition, and when the same enantiomer is inserted multiple times, the chiral active center responsible for polymer chain growth retains the same configuration, resulting in an isotactic polymer ； When the monomer is inserted multiple times using crossed enantiomers, the two configurations of the chiral active center responsible for polymer chain growth alternate with each other, resulting in syndiotactic polymers ； When monomers are inserted multiple times in random enantiomeric forms, the configuration of the chiral active center responsible for polymer chain growth is irregular, resulting in a random polymer. Figure 3-1: Stereoaddition mode of the double bond in α-olefin monomers between Mt and P. Monomer coordination mode (1,2 coordination); Monomer coordination mode (2,1 coordination). Figure 3-2: Possible enantioselectivity. For catalyst systems that possess at least one chiral active center, they exhibit selectivity toward the two enantiomers of chiral α-olefin monomers. For the living polymerization chain, if the α-olefin monomer inserts at the 1,2 position, the chiral carbon is at the β-position of the living polymerization chain ； If the α-olefin monomer undergoes 2,1 insertion, the chiral carbon is at the α-position of the active polymerization chain. When the stereoselective mechanism is controlled by the terminal chiral induction effect, it is called end-group control. Another possible scenario is when the chiral active center is asymmetric, and its stereoselective mechanism is referred to as control by the enantiomerically active center. During chain growth, the different microstructures of the polymer resulting from dislocations allow it to be determined what stereoselective mechanism is involved in monomer insertion.