How does propene molecules form a polymer

In conjunction with an organoaluminium compound, it acts as a catalyst for the polymerization of alkenes such as ethene and propene. The specific orientation of the zirconium compound means that each propene molecule, for example, as it adds on to the growing polymer chain is in the same orientation and an isotactic polymer is produced.

This is the only way of making syndiotactic poly propene commercially. As with the Ziegler-Natta catalysts, the bulk or gas phase described above can be used. Alternatively the slurry process is used. Poly propenes made in this way, mPP, are used in particular to make non-woven fibres and heat-seal films. There are two main types of co-polymer.

The simplest are the random co-polymers , produced by polymerizing together ethene and propene. Figure 5 Illustrating an alternating co-polymer formed from propene and a small amount of ethene.

The crystallinity and melting point are reduced and the products are more flexible and are optically much clearer. Major uses for these random co-polymers are for medical products pouches, vials and other containers and packaging for example, bottles, CD and video boxes. Many other co-polymers of ethene and propene, with higher alkenes such as hexene, are being developed which will produce polymers similar to LLDPE but which have better mechanical and optical properties. The second type of co-polymers is the so-called 'block' co-polymers.

These are made by following the poly propene homopolymerization with a further, separate stage, in which ethene and propene are co-polymerized in the gas phase. Thus these two processes are in series Figure 6. Figure 6 lllustrating the homopolymer and the block co-polymer formed from propene and ethene. The products of these two processes form a composite in which nodules of the block co-polymer are distributed with the homopolymer Figure 7. Figure 7 The propene-ethene block co-polymer nodules dissipate impact energy and prevent cracking.

It has rubber-like properties and is tougher and less brittle than the random co-polymer. Consequently, the composite is particularly useful in making crates, pipes, furniture and toys, where toughness is required.

The ethene and propene molecules polymerize to form very long molecular chains, with several thousand monomer molecules in a chain. Because polypropene is a thermoplastic it can be recycled. After the polypropene is collected for recycling it is cleaned then cut or shredded into small pieces.

These small pieces can be heated and moulded, extruded, or blown into new shapes. Identification of Polypropene Test Result Density Drop a small piece of polymer into water with a drop of added detergent. Floats Combustion: Flame Colour Use tongs to hold a piece of polymer in a flame. Burns with a yellow flame with a blue base Combustion: Smell after combustion Smells like candle wax Scratch Test Try to scratch the polymer with your fingernail. Can't be scratched by a fingernail Thermoplastic Heat a wire to just below red hot and press it into the polymer.

Hot wire penetrates the polymer. Polypropene has many uses in our society. Some of these uses have been summarised in the table below:. Uses for Polypropene Use Reason "living hinges" eg, plastic flip-top containers resistant to fatigue water pipes resistant to corrosion food containers 7 eg, yoghurt tubs moderately high melting point disposable bottles 8 eg, pill bottles resistant to corrosion carpets, rugs, mats colorfast and durable ropes strong and low density floats on water electrical insulation on cables electrical insulator packaging, storage boxes durable, resistant to corrosion medical sutures strong, resistant to chemical attack polymer banknotes 9 durable, resistant to chemical attack.

IUPAC nomenclature began in when an international assembly of Chemists met in Geneva, Switzerland, to try to come up with a rational system for naming organic molecules. The structure-based name is poly 1-methylethane-1,2-diyl. The traditional name is polypropylene. The source-based name is poly propene. An acceptable alternative to this is polypropene because there can be no ambiguity about the structure of the polymer if the round brackets are omitted from the name.

Source-based nomenclature began when we didn't know much about the structure of polymers generally so the name of a polymer was made by adding "poly" to the start of the name of the monomer. In the last 50 years or so enormous advances have been made in the determination of polymer structures and so the nomenclature of polymers is shifting towards a preference for structure-based names rather than source-based names.

Although this is the usual way to draw the structural formula for propene, for the purposes of showing how the molecule acts as a monomer and can form a polymer it should be drawn in a different way. Examples of other monomers are given below.

Given the structure of the monomer the structural formula of the addition polymer can be drawn. Three propene monomers. The molecules link together by partially breaking their double bonds. A section of a poly propene molecule. The following equations represent the overall reaction sequence:. Ethene does not polymerize by the cationic mechanism because it does not have sufficiently electron-donating groups to permit easy formation of the intermediate growing-chain cation.

The usual catalysts for cationic polymerization of 2-methylpropene are sulfuric acid, hydrogen fluoride, or a complex of boron trifluoride and water. Under nearly anhydrous conditions a very long chain polymer called polyisobutylene is formed.

Polyisobutylene fractions of particular molecular weights are very tacky and are used as adhesives for pressure-sealing tapes. The mechanism is like that of the polymerization of 2-methylpropene under nearly anhydrous conditions, except that chain termination occurs after only one 2-methylpropene molecule has been added:.

The short chain length is due to the high water concentration ; the intermediate carbocation loses a proton to water before it can react with another alkene molecule. The proton can be lost in two different ways, and a mixture of alkene isomers is obtained.

The alkene mixture is known as "diisobutylene" and has a number of commercial uses. Hydrogenation yields 2,2,4-trimethylpentane often erroneously called "isooctane" , which is used as the standard " antiknock rating" fuel for internal-combustion gasoline engines:. The initiation step involves formation of radicals, and chain propagation entails stepwise addition of radicals to ethene molecules. Chain termination can occur by any reaction resulting in combination or disproportionation of free radicals.

The polyethene produced in this way has from to ethene units in the hydrocarbon chain. The polymer possesses a number of desirable properties as a plastic and is used widely for electrical insulation, packaging films, piping, and a variety of molded articles.