Polymerization catalyst is a catalyst that plays a catalytic role in the formation of high molecular weight polymers from low molecular weight monomers through chain growth. The common feature of polymerization catalysts is the use of transition metals. For the vast majority of transition metals, they often have unpaired single electrons in their d orbitals, which is an important reason for their catalytic effect. There are many kinds of polymerization catalysts.
- Radical polymerization catalyst: This kind of catalyst includes azobisisobutyronitrile (AIBN), benzoyl peroxide (BPO) and so on. They produce free radicals at about 50-80℃, which are suitable for bulk polymerization, solution polymerization and suspension polymerization of vinyl monomers such as styrene, methyl methacrylate, acrylonitrile and vinyl acetate.
Figure 1. The structural formula of benzoyl peroxide.
- Anionic polymerization catalyst: The catalytic activity order of anionic compounds is M, RM>RMgX>ROM, strong base>NR3, weak base (R = alkyl, M=Li, Na, etc.). It is usually used as a catalyst for the polymerization of styrene, methyl methacrylate, acrylonitrile, butadiene, 2-methyl styrene and so on. In the polymerization of methyl methacrylate, stereotactic polymerization can take place using RLi as a catalyst. In addition, living polymerization can take place when styrene is polymerized by naphthalene anion radical formed by the reaction of Na with naphthalene.
- Cationic polymerization catalysts: These kinds of catalysts include protonic acids such as H2SO4, H3PO4, BF3·OEt2, AlCl3, TiCl4 and other non-protonic acids. When using non-protonic acid, cocatalysts such as water and alcohol should be added. These catalysts are suitable for the polymerization of vinyl ether, styrene, p-methoxy styrene, aldehyde, isobutene, cyclic ether, lactam and so on.
Figure 2. Preparation of Poly (vinyl ether) by Cationic Polymerization Catalyst.
- Redox catalysts: These catalysts include carbonyl coordination metal-CCl4 mixtures, peroxides (H2O2, benzoyl peroxide, etc.) and mixtures of bivalent iron compounds. Free radical initiated polymerization by redox reaction, which is a kind of free radical polymerization catalyst, can initiate polymerization at room temperature.
- Coordination polymerization catalysts: Ziegler uses Zr(acac)4 and AlEt3 to catalyze ethylene polymerization to obtain a large number of powdered polyethylene solids. Further experiments on zirconium homologue elements show that the coordination of TiCl4 and AlEt3 is more effective, and TiCl4 is cheap and easy to obtain, so it becomes a new type of catalyst for low-temperature ethylene polymerization, which is called Ziegler catalyst. These catalysts include TiCl4-AlR3 (alkyl aluminum), TiCl3-AlR3, etc., which are used for low-pressure polymerization of ethylene and propylene. Using Mg compound as a carrier or adding phenyl carboxylate and other electron donors, very active catalysts can be obtained for the preparation of low-density polyethylene, ethylene and butene copolymer and polypropylene under low pressure. The composite catalysts made of Co, Ni and organometallic compounds (AlR3, etc.) can make diolefins take place in 1-cis-polymerization. Ring expansion polymerization of cycloolefins can take place by using Mo, W, etc.
Figure 3. Ziegler-Natta-type catalysts
- Polycondensation catalyst: When polyamides and polyesters are prepared by polycondensation, the reaction can be accelerated by adding metal compounds such as quaternary ammonium salt and alkaline earth metal salt [Ca(OAc)2, PbO].
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