The Catalyst Review October 2019 - 9

SPECIAL FEATURE
than benzoyl peroxide because the explosion risk is much lower. However, these compounds are still considered explosives. The
decomposition rate of azo compounds, like those of peroxides, are temperature dependent, that is, the rate increases with increasing
temperature. The rate also depends on the surrounding medium (solvent).
Anionic polymerization is a form of chain-growth polymerization that encompasses the
Figure 3. Anionic polymerization initiation and
propagation.
polymerization of vinyl or diene monomers with strong electronegative groups. This type
of polymerization is often used to produce synthetic polydiene rubbers, solution styrenebutadiene rubbers (S-SBR), and thermoplastic styrenic elastomers. All monomers with
strong electronegative substituents polymerize readily in the presence of carbanions.
Some electron-withdrawing substituents, that stabilize the negative charge through
charge delocalization and hence permit anionic polymerization, include -CN, -COOR,
-C6H5, and -CH=CH2, to name only a few. Therefore, monomers such as styrenes, dienes,
acrylates and methacrylates, aldehydes, epoxides, acrylonitriles and cyanoacrylates
readily undergo anionic polymerization reactions. The electron donors (or initiators)
are either electron transfer agents or strong anions. The transfer of an electron from
a donor molecule to the vinyl monomer leads to the formation of an anion radical,
Source: PSLC 2019
the so-called carbanion. Typical electron donors (Lewis bases or nucleophiles) are alkali
metals, such as lithium or sodium. Other strong nucleophilic initiators include covalent or ionic metal amides, alkoxides, hydroxides,
amines, phosphines, cyanides, and Grignard reagents. The initiator most often used in anionic polymerization is butyl lithium. Figure
3 illustrates the formation of the carbanion and the polymerization initiation step with a vinyl monomer.
Some vinyl monomers readily polymerize also in the presence of very small amounts of catalyst of the type used in Friedel-Crafts
reactions through a cationic polymerization mechanism. Examples of effective cationic initiators are AlCl3, AlBr3, BF3, TiCl4, SnCl4, and
in some cases strong acids such as H2SO4. All these catalysts are examples of Lewis acids with strong electron-acceptor capability.
They usually require a co-catalyst, namely a Lewis base such
Figure 4. Cationic polymerization initiation and propagation.
as water, acetic acid or alcohol: AlCl3+ H2O↔H+ AlCl3OH-.
Monomers that polymerize in the presence of these catalysts
include isobutylene, styrene, alpha-methylstyrene, butadiene,
vinyl alkyl ethers and many other monomers having electrondonating substituents that enhance the electron-sharing ability
of the double bond of the vinyl monomers. They all can be
readily polymerized to high-molecular weight polymers. Figure
4 illustrates the mechanism of the cationic polymerization
Source: PSLC 2019
initiation. The cationic polymerization usually proceeds at high
rates both at high and very low temperatures. For example, isobutylene at -1000C in the presence of a strong Lewis acid polymerizes
to high molecular weight polybutylene within a fraction of a second. Both the rate of reaction and the molecular weight decreases
with increasing temperature; for this reason, low temperatures are usually preferred. In fact, molecular weights obtained at room
temperature are often much lower than those achieved by free-radical polymerization (PSLC 2019). After having discussed the main
features of polymerization initiators, find in the following some recent innovative technology developments related to associated
polymer productions.
Low-density polyethylene (LDPE) is a flexible, semi-crystalline polymer that is distinguished by a highly branched tree-like random
molecular structure. This branching structure, and particularly its long chain branching, gives the polymer unique easy processing
behavior and physical properties that distinguish it from linear low-density polyethylene, and which make the polymer attractive to
the market. In the tubular reactor process, LDPE polymerization takes place in a long tube (>1000 m) at high pressure. Peroxide is
introduced at different zones to optimize productivity; also, the reaction temperature profile is controlled by the dose and reactivity
of the initiators. Selecting the proper organic peroxide package is a critical parameter to consider for polymerization because it
affects directly the polymer properties and the productivity. This helps to start, or "light-off" the polymerization, but also maximize
productivity as the temperature and pressure reach the maximum in each reactor zone. The criteria for selecting peroxides for
various zones is described by the peroxide decomposition rate (DR), which is determined using the half-life time index (Arkema
2019): Short half-life peroxides (high DR) provide fast light-off, but lower peak temperature, while Long half-life peroxides provide
high peak temperature, but are slower to light-off. Polymerization is propagating macroradicals building polymer chains to generate
short or long branched structure. The reaction takes place at temperatures between 100 to 300 °C and at high pressure (1200-3000
bar). At these pressures, ethylene liquefies and acts as a solvent under supercritical conditions. Molecular weight and density are
regulated by controlling the temperature and pressure, and by the addition of a chain transfer agent (e.g., propane).

The Catalyst Review

October 2019

The Catalyst Review October 2019

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