Many vinyl monomers with electron-releasing groups such as alkoxy, phenyl, or alkyl are readily polymerized in the presence of very small amounts of a catalyst of the type used in Friedel-Crafts reactions. Examples of effective catalysts are: AlCl 3 , AlBr 3 , BF 3 , TiCl 4 , SnCl 4 . Sometimes strong protonic acids such as H 2 SO 4 , HClO 4 or H 3 PO 4 are also used. 1,4 Friedel-Craft catalysts are examples of Lewis acids with strong electron acceptor capability. 10 They usually require an auxiliary catalyst, i.e., a Lewis base such as water, alcohol, or acetic acid, which forms a complex with the catalyst that stabilizes the counterion and prevents recombination. 2.9:
A typical Lewis acid is boron trifluoride BF 3 , which when reacted with small amounts of water forms an electrophile that can initiate chain growth:
The initiator complex can exist in the reaction mixture in three different forms: (a) as ionized molecules, (b) as ion pairs, or (c) as free (solvent) ions. 5 Aggregation and solubility significantly affect the polymerization kinetics, which depends on the solvation energy of ions (which favors dissociation) and the attractive forces between ions (which favor their association). 6 In general, for dissociated ion pairs, the reaction rate is higher than for solid (solvent-bridge) ion pairs. 5 This explains why the kinetics and rate of polymerization are strongly influenced by the chemical nature of the solvent and initiator complex and not only by the type of monomer, temperature and amount of initiator. In most cationic polymerization systems, the anionsare (much) larger than cations, so that the charge density is relatively small. Then the solubility of the anions can be neglected as a first approximation. On the other hand, carbocation solubility can be relatively strong. 6,8
Similar to free radical polymerization, the initiator adds to a monomer that forms the center of chain growth. In the case of a Lewis acid-proton donor complex such as boron trifluoride monohydride, a proton from the complex is transferred to the double bond of the vinyl monomer, producing a reactive carbocation:
where R’ is an electron-releasing substituent and R’ is hydrogen or another electron-releasing substituent such as an alkoxy or alkyl group.
The proton of a strongly protonic (Brønsted) acid can also initiate cationic polymerization. Its effectiveness depends on the nucleophilicity of the conjugate base, i.e. low nucleophilicity of the counterion promotes polymerization. 5 In the case of perchloric acid (HClO 4 ), the starting steps are read
In the diffusion step, each carbocation reacts with a vinyl monomer to form a new carbocation:
This step is repeated until all the monomers are consumed or until the polymers are at the end stage. This is the case when the counter ion (gegen) removes a proton from an unsaturated terminal unit. The active chain end can also transfer a proton to a monomer, leaving a new cation that can initiate chain growth: 1,9
Pac Products Persian Poly Aluminum Chloride (PAC)
In the case of vinyl ethers, a proton can also be transferred to the counterion, which also terminates the carbocation propagation. 9
Cationic polymerization is usually carried out at high and (very) low temperatures at high rates. 1 For this reason, a uniform and constant reaction condition cannot be maintained during polymerization. For example, isobutylene is polymerized to high molecular weight polybutylene in a fraction of a second at -100°C in the presence of a strong Lewis acid. 1 To prevent excessive temperature rise in the reaction vessel, an internal coolant is usually added to the mixture, which removes the heat by evaporating some of the liquid.
Both reaction rate and molecular weight decrease with increasing temperature. For this reason, low temperatures are usually preferred. In fact, the molecular weights obtained at room temperature are often much lower than those obtained by free radical polymerization. The reaction speed also depends on the type of side groups. In general, the more stable the carbocation end group, the faster the diffusion rate. Steric hindrance also affects the rate of polymerization. For example, for alkyl vinyl ethers polymerized at about -80 degrees Celsius, the reaction rate decreases in the order: 5
Methyl > n-butyl > ethyl > i-propyl > t-butyl
For styrene compounds the situation is more complicated. For para-substituents, the rate is increased by the inductive effect of the electron-donating (+I) substituent: OCH 3 > CH 3 > H, while for ortho-substituents, the substituents delay diffusion due to strong steric hindrance regardless of the type of substituents. . .
References and notes
- Paul J. Florey, Principles of Polymer Chemistry, 1st edition, 1953, Cornell University
- M. Pitsikalis, ion polymerization. In: Reference Module in Chemistry, Molecular Sciences and Chemical Engineering, 2013
- S. Aoshima and S. Kanaoka, Chem. Rev. , 109, 5245-5287 (2009)
- Bronsted acid is defined as proton donor and Bronsted base as proton acceptor:
On the other hand, a Lewis acid is defined as an electron pair acceptor and a base as an electron pair donor. Therefore, a proton is only one species that can act as a Lewis acid:
- A. Ravve, Principles of Polymer Chemistry, 2nd Ed., Kluwer Academic, New York 2012
- PH Plesch, JC Austin, J. Poly. Scientific: Section A: Poly. chemistry . 46, 4265 – 4284 (2008)
- JP Kennedy, J. play. Scientific: Section A: Poly. chemistry. , 37, 2285 – 2293 (1999)
- Cationic polymerization is usually carried out in solvents of low to moderate polarity, such as hexane or chloroalkanes, which favor solvent-bridged ion pairs.
- MD Lechner, K. Gehrke, EH Nordmeier, Makromolekulare Chemie, Birkaeuser, Berlin 1993
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- Increasing the charge density of the central metal atom, for example, TiCl 4 > TiCl 2 .
- Increasing atomic number of metal in each group: Ti > Al > B. Sn > Si
- By decreasing the size of the ligands (for small central ion): BF 3 > BCl 3 > BBr 3 , TiCl 4 > TiBr 4, the strength of Lewis acid metal ion depends on its properties and electronic structure and generally increases with
For example, the strength of the following Lewis acids is ranked in order: BF 3 > AlBr 3 > TiBr 4 > BBr 3 > SnCl 4 . 5