Overall Introduction
Polypropylene, a synthetic resin built up by the polymerization of propylene. As a thermoplastic polymer, it is produced via chain-growth polymerization. Chain-growth polymerization is a process in which the high molecular weight polymer is formed early in the polymerization process and the polymer yield (the percentage conversion of the monomer into the polymer) gradually increases with time.[6]
Figure 1: mechanism of chain growth polymerization Figure 2: common processing route of polypropylene
Synthesis of PP
Polymerization of polypropylene is a coordination polymerization of monomer propylene. The monomer propylene undergoes polymerization in the presence of catalyst such as Ziegler-Natta catalyst (or) metallocene catalyst.[7] The conditions of polymerization such as pressure, temperature, and concentrations of the reactants are given by the species of the polymer which is to be produced.
Based on the type of catalyst that is used for the polymerization, the tacticity of the propylene could be selected and the properties of the polypropylene can be determined. Using Ziegler Natta catalyst, there are two major processes for synthesizing of polypropylene.[7]
Heterogeneous Ziegler-Natta polymerization
The monomer propylene would be polymerized at high temperatures and high pressures. The polymerization could be effective only in the presence of inert solvent carried at moderate pressure in presence of stereospecific catalyst like TiCl4. In the Zeigler Natta polymerization of polypropylene, TiCl4 is the catalyst and Al(Et)3 is the co-catalyst. The polymer formed by the method of Ziegler Natta polymerization could have high molecular weights.
Figure 3: reactions of heterogeneous Ziegler-Natta polymerization
Homogeneous Zeigler Natta polymerization
Atactic polypropylene are formed when metallocene are reacted with methylaluminoxane. Here, the metals could be titanium, zirconium, and hafnium. Methylaluminoxane in short is known as MAO.
Figure 4: reaction of homogeneous Zeigler Natta polymerization
Critical material—propylene
As the raw material for the synthesis of polypropylene, propylene is a gaseous compound obtained by the thermal cracking of ethane, propane, butane, and the naphtha fraction of petroleum.[8] The chemical structure of the propylene molecule is CH2=CHCH3. Under the action of polymerization catalysts, however, the double bond can be broken and thousands of propylene molecules linked together to form a chainlike polymer (a large, multiple-unit molecule).
Figure 5: structure of polypropylene
Biochemical route for green propylene (typically fermentation)
The biochemical route is being commercialized by the Brazilian chemical company Braskem who is the leader in the manufacturing of bio-polyethylene. Enzymes are used in the fermentation process to convert sugars into ethanol and butene. Metathesis of ethylene and butene produces propylene monomer.
Figure 7: biochemical route
Synthetic routes to Bio-PP
There are multiple pathways to obtain bio-based propylene, which is the starting monomer for Bio-PP.[9] The polymerization is the same as above.
Various bio-based synthetic routes to green propylene：
Figure 6: routes to green propylene
Thermochemical route for green propylene (involving gasification)
The thermochemical route employs a wide variety of bio-based feedstocks including grass, agricultural wastes and corn, which are carbon rich and can be gasified to produce syngas. Bio-diesel and vegatable oils can also be used in the production of green propylene.
Figure 8: thermochemical route