9.2.A - Synthetic Polymers


A polymer is a large molecule (macromolecule) composed of repeating structural units. These sub-units are called monomers and are typically connected by covalent chemical bonds. Although the term polymer is sometimes taken to refer to plastics, it actually encompasses a large class of compounds comprising both natural and synthetic materials with a wide variety of properties.

Because of the extraordinary range of properties of polymeric materials, they play an essential and ubiquitous role in everyday life. This role ranges from familiar synthetic plastics and elastomers to natural biopolymers such as nucleic acids, proteins and cellulose that are essential for life.

The list of synthetic polymers includes synthetic rubber, Bakelite, neoprene, nylon, PVC, polystyrene, polyethylene, polypropylene, polyacrylonitrile, PVB, silicone, and many more. Most commonly, the continuously linked backbone of a polymer used for the preparation of plastics consists mainly of carbon atoms. A simple example is polyethylene whose repeating unit is based on the ethylene monomer.

In this topic students:

  • Describe the structure, properties, uses, chemistry and production of the addition polymers polyethylene, polyvinyl chloride (PVC) and polystyrene.
  • Describe the structure, properties, identification and reactions of ethene
  • Explain how ethene serves as a useful monomer from which many commercially important polymers are made
  • Describe the process of sourcing ethene for polymer production
  • Write equations for some of the the chemical reactions of ethene


Polymers

Polymers are substances whose molecules have high molar masses and are composed of a large number of repeating units. Synthetic polymers are produced commercially on a very large scale and have a wide range of properties and uses. The materials commonly called plastics are all synthetic polymers.

Polymers are formed by chemical reactions in which a large number of molecules called monomers are joined sequentially, forming a chain. In many polymers, only one type of monomer is used. In others, two or three different types of monomer may be combined. Polymers are classified by the characteristics of the reactions by which they are formed. If all atoms in the monomers are incorporated into the polymer, the polymer is called an addition polymer. If some of the atoms of the monomers are released into small molecules, such as water, the polymer is called a condensation polymer.

Most synthetic addition polymers are made from monomers containing a double bond between carbon atoms. Such monomers are called olefins, and most commercial addition polymers are polyolefins. Condensation polymers are made from monomers that have two different groups of atoms which can join together to form, for example, ester or amide links. Polyesters are an important class of synthetic polymers, as are polyamides (nylon). We will look at condensation polymers in the next section of work, 9.2.B - Biological Polymers. While synthetic condensation polymers do exist (e.g. polyester), the syllabus only requires that we deal with cellulose with its classification as both a condensation polymer and a naturally ocurring biological polymer.

 

Addition Polymerisation

Addition polymerisation is a type of polymerisation involving monomers with a double bond (alkenes) reacting to form polymers that have only single bonds. Monomer units are added across the double bond of each monomer with the product containing the same number and type of atoms as the sum total of the reacting monomers. In other words, there is only one product and that's the polymer.

Addition polymerisation can occur in two different ways and we will examine each of these in turn below.

Using an Initiator

The decomposition of benzoyl
peroxide which is commonly used
as an initiator.

In this three-step process, an initiator is used to create a free radical on the first monomer and then the chain grows in length until it is terminated.

1. Initiation

In the initiation step, a free radical species is generated. This means that the valence energy level of one of the atoms has only one electron instead of an electron pair.

For practical purposes, a free radical is a reactive molecule on the prowl for another electron to complete the valence energy level in one of its atoms. Free radicals are commonly generated by breaking chemical bonds in such a way that the two electrons involved in the bond end up on the two separate fragments of the original molecule. Benzoyl peroxide is a common initiator used in addition polymerisation. Heat or light breaks the -O-O- bond in the centre and gives two free radical fragments.

The initiator attacking the first monomer and paving
the way for propagation.

Each free radical fragment then attacks a monomer molecule. In the diagram on the left, ethene is acting as the monomer. The initiator, I·, takes an electron from the double bond (green arrow) and forms a single covalent bond. Another single covalent bond forms between the two carbons and the spare electron is relegated to the other carbon atom to form another free radical. Each polymer molecule will, therefore, end up with an initiator at the start of its carbon chain and another carbon free radical at the growing end of the chain.

The free radicals from the initiator tend not to react with themselves. This is because their concentration is low compared to the amount of ethene present and that they are also highly reactive. The probability of a free radical reacting with an ethene monomer is much higher than the probability of it reacting with another free radical. While a few radicals will actually react with other free radicals, most of them react with ethene monomers and begin the process of propagation.

2. Propagation

Propogation of the polymer as successive monomer units are added.

Propagation is the name given to the growth of the polymer chain as monomers are added one-by-one. Each time a monomer unit is added across the double bond, another free radical is formed at the end of the chain on the newly added monomer unit. Propogation continues until the free radical on the growing carbon chain is resolved by the addition of an electron in the next termination step.

3. Termination

Termination involving the linkage of two propagating polymers.

Termination generally occurs when the free radical on one polymer chain meets the free radical on another polymer chain. They join and resolve the free radicals forming one long polymer molecule.

The process of chain growth terminates when when free radicals of variable chain length combine to form non-activated species (a polymer with no free radical). Because of the large variation in chain length in a reacting system, polymer units will have a variety of carbon chain lengths. Said another way, the somewhat random nature of termination produces polymer molecules with a range of molecular masses.

The addition of inhibitors or lowering the temperature and pressure can also stop polymerisation so the length of polymer chains can be regulated to some degree. Generally such polymers will be made up of molecules with a variety of molecular weights but most molecules will have molecular weights with similar values.

A graph showing the variation of molecular weights
for a polymer produced by free radical
polymerisation.

Because of the random nature of the reaction taking place, the termination the propagating polymer can only be loosely controlled. The characteristic feature of the chain-propagation step is the reaction of a radical with with a molecule to give a new radical. In the polymerisation of ethene, chain lengthening reactions occur at a very high rate, often as fast as thousands of additions per second, depending on the experimental conditions.

Knowing when the polymer chains have grown to the desired length from an understanding of the reaction rate is important, because this is when termination needs to occur. Restricting the amount of ethene available to the growing polymer chains will promote termination but the molecular weight of the polymer molecules in the sample will still show considerable variation. Free radical polymerisation produces polymer molecules of varying molecular weight as shown in the graph above right. Catalytic polymerisation (next section) is another method of producing addition polymers whereby termination can be more tightly controlled.


The free radical polymerisation process for ethene. Not that in this animation, the chain is terminated by an
initiator rather than another polymer chain as described above.

Branching

Formation of branches during propagation. The
new chain will grow at the site of the free
radical in the last diagram.

Polymers madeby free radical polymerisation described above tend to have a branched structure due to a phenomenon known as back biting. Short chains are formed by back biting reactions and they tend to create branches which are only about four carbon units long. This happens when the growing polymer chain reacts with itself, pulling a hydrogen off the backbone chain and generating a radical site there that can then add more monomer units.

During propagation, the chains turn back on themselves and the free radical at the end is able to remove a hydrogen from a carbon atom somewhere further along the chain. This terminates the chain at the end but creates a new free radical somewhere else where the hydrogen was removed. This then allows the chain to propogate from that point creating a branch in the structure.

This type of polymerisation using an initiator tends to create a polymer with different properties to those created using a catalyst (see next section). Polyethylene made this way with a branching structure produces a form of the polymer known as low density polyethylene (LDPE). High density polyethylene (HDPE) is made using a catalyst and it is to this process that we turn next.

Using a Catalyst

The backbiting mechanism that causes branching in polymer chains is an unavoidable side effect of free radical polymerisation. This branching will greatly affect the properties of polyethylene. Whereas linear regions of the polymer chain can pack closely together to form highly ordered, crystalline regions in the polymer, the branched regions with their structural irregularities cannot participate in this crystallisation. Since closer packing and crystallisation give this material its high density and strength, chain branching means lower density and more flexibility. Since both density and strength are closely related to the use of commercial polymers, the development of a polymerisation process that could control branching and chain length was of great interest to chemists in the first half of the twentieth century. This led to the development of transition metal catalysts which allowed for the polymerisation of ethene without the need for free radicals.

Formation of a Ziegler-Nata catalyst and chain propagation
for catalytic polymerisation of ethene. The alkyl group
(shaded) on the alkyl aluminium compound in the catalyst
becomes the first group of the polymer. Ethene monomers
are inserted between this alkyl group and the titanium atom.

In the 1950s, Karl Ziegler from Germany and Giulio Natta from Italy developed an alternative method for the polyerisation of alkenes which won them the Nobel Prize in Chemistry in 1963. Ziegler-Natta catalysts were originally composed of titanium tetrachloride and an alkyl aluminium compound. Once formed, ethene units are repeatedly inserted into the titanium-carbon bond of the alkyl titanium compound to yield polyethylene.

Over 27 billion kilograms of polyethylene are produced worldwide each year using Ziegler-Natta catalysts and the polyethylene produced this way is called high density polyethylene (HDPE). It has greater mechanical strength than LDPE (about six times stronger) due to greatly reduced chain branching and higher crystallinity in the molecules.

Even greater improvements in the properties of HDPE have been realised recently through special processing techniques. In the molten state, HDPE chains have randomly coiled conformations similar to that of cooked spaghetti. Chemical engineers have developed extrusion techniques that force the individual polymer chains to align with one another and produce perfectly linear and highly packed crystalline structures in the solid. HDPE processed in this way is stiffer than steel and has approximately four times its tensile strength.

 

Polyethylene

Polyethylene is perhaps the simplest polymer, composed of chains of repeating -CH2- units. It is produced by the addition polymerisation of ethylene, CH2=CH2 (ethene). The equation showing the formation of the polymer using condensed structural formulae is:

nCH2=CH2 → -(CH2-CH2)-n


Note that the double bond is only present in the monomer and that the letter n represents any large number.

The properties of polyethylene depend on the manner in which ethylene is polymerised. When catalysed by Ziegler-Natta catalyst at moderate pressure (15 to 30 atm), the product is high density polyethylene, HDPE. Under these conditions, the polymer chains grow to very great length with molar masses in the order of many hundred thousand grams per mole. HDPE is hard, tough, and resilient. Most HDPE is used in the manufacture of containers, such as milk bottles and laundry detergent bottles.

When ethylene is polymerised at high pressure (1000 to 2000 atm) and elevated temperatures (190 - 210°C) by free radical polymerisation, the product is low density polyethylene, LDPE. This form of polyethylene has molar masses of 20,000 to 40,000 grams per mole. LDPE is relatively soft, flexible and and transparent and most of it is used in the production of plastic films such as those used in sandwich bags.

Physical Properties

Polyethylene is a thermoplastic polymer consisting of long hydrocarbon chains. Depending on the crystallinity and molecular weight, a melting point may or may not be observable. The temperature at which these occur varies strongly with the type of polyethylene. For common commercial grades of high-density polyethylene, the melting point is typically in the range 120 to 130°C. The melting point for average, commercial, low-density polyethylene is typically 105 to 115°C.

Most LDPE and HDPE grades have excellent chemical resistance, meaning that it is not attacked by strong acids or strong bases. It is also resistant to gentle oxidants and reducing agents. Polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of paraffin. Crystalline (solid) samples do not dissolve in any solvent at room temperature which makes it waterpoof, among other things. Polyethylene usually can be dissolved at elevated temperatures in some polar hydrocarbons such as toluene or xylene, or in chlorinated solvents such as trichloroethane or trichlorobenzene.

 

Polyvinyl chloride (PVC)

Polyvinyl chloride, commonly abbreviated PVC, is the third-most widely-produced plastic, after polyethylene and polypropylene. PVC is widely used in construction because it is durable, cheap, and easily worked. It can be made softer and more flexible by the addition of plasticisers, the most widely used being phthalates. In this form, it is used in clothing and upholstery, electrical cable insulation, inflatable products and many applications in which it replaces rubber.

PVC can be produced by free radical polymerisation of chloroethene (vinyl chloride) and the general equations for its formation are shown below using structural formulae and condensed structural formulae. The average molecular mass of commercial grade PVC molecules range from about 45,000 to 64,000 g mol-1 which means that the polymer chains contain between about 700 - 1000 monomers.

nCH2=CHCl → -(CH2-CHCl)-n

Properties

PVC has high hardness and is very strong which is why it is often used in construction. It gets stronger as the molecular weight of the polymer molecule increases but decreases as the temperature of the polymer increases. The heat stability of PVC is very poor because when the temperature reaches 140°C PVC starts to decompose. Its melting temperature is between 100 - 260°C. The melting point increases with increasing chain length due to enhanced dispersion forces.

Most PVC has an excellent chemical resistance, meaning that it is not attacked by strong acids or strong bases or other compounds. It is mostly insoluble and waterproof except. Pure polyvinyl chloride without any plasticizer is a white, brittle solid that is insoluble and waterproof, but slightly soluble intetrahydrofuran.

 

Polystyrene

Polystyrene is another polymer made from the free radical polymerisation of the monomer ethenyl benzene (styrene). Polystyrene is one of the most widely used plastics, the scale being several billion kilograms per year. Polystyrene has a solid (glassy) state at room temperature but flows if heated above its glass transition temperature of about 100°C (for molding or extrusion) and becomes solid again when cooled.

Pure solid polystyrene is a colourless, hard plastic with limited flexibility. It can be cast into moulds with fine detail. Polystyrene can be transparent or can be made to take on various colors. Solid polystyrene is used, for example, in disposable cutlery, plastic models, CD and DVD cases and smoke detector housings.

When a gas is blown through the molten plastic a foam product is produced. Products made from foamed polystyrene are nearly ubiquitous, for example packing materials, insulation, and foam drink cups.

Polystyrene can be produced as shown below using structural formulae and condensed structural formulae. About a few thousand monomers typically comprise a chain of polystyrene, giving a molecular weight of 100,000–400,000 g mol-1.

nCH2=CH(C6H5) → -CH2-CH(C6H5)-n

Properties

Polystyrene's properties are determined by short-range van der Waals attractions (dispersion and dipole-dipole forces) between polymers chains. Since the molecules are long hydrocarbon chains that consist of thousands of atoms, the total attractive force between the molecules is large. Extruded polystyrene is about as strong as an unalloyed aluminium, but much more flexible and much lighter.

Polystyrene is chemically resistance, waterproof, an electrical insulator and transparent to light.

Ethene: Properties, Reactions and Sources

Properties

Ethene is a member of the homologous series of alkenes. Alkenes are carbon compounds that contain at least one double bond. The high electron density in the double bond makes ethene particularly reactive and a useful starting material for commercially significant polymers. Ethene contains two carbon atoms with a double bond between them and four hydrogen atoms. An ethene molecule can be represented in different ways as shown.

Molecular Formula
Structural Formula
Condensed Structural Formula
Ball and
Stick Model
C2H4 CH2=CH2

 

Reactions of Alkenes

Alkanes are chemically unreactive and the substitution of a hydrogen atom with a halogen atom (such as bromine or chlorine) can only be achieved under extreme conditions. These reactions are called substitution reactions because a hydrogen atom is removed and another atom is substituted.

Alkenes are much more reactive than alkanes and will easily take up a hydrogen or halogen atom. These reactions are called addition reactions because atoms are added to the reactant across the double bond. If ethene is mixed with brown bromine water, the brown solution will become colourless as the reaction proceeds and the oily product, 1-2-dibromoethane is formed.

All alkenes will undergo similar reactions where the bromine water is added across the double bond. In this case (above) ethene is a gas at room temperature so it would need to be bubbled through a solution of bromine water. The bromine water would become colourless over time as it reacts with the ethene gas. In the laboratory we do this experiment with hexane and hexene as they are both liquids at room temperature. A solution of bromine water can easily be added to test tubes containing hexane and hexene.

Because of the double bond, alkenes are able to participate in any number of addition reactions. Some of these are shown in the table below using ethene as the alkene.

Reaction Name
Adding
Equation
Product Name
Catalyst
hydrogenation
hydrogen
ethane
Pt or Ni
halogenation
halogen
dihaloethane
None
hydro-halogenation
hydrohalogen
haloethane
None
hydration
water
ethanol
Dilute H+

 

Source of Ethene

We have seen that ethene is the building block for the monomers from which polyethylene, PVC and polystyrene are made. A ready supply of ethene is essential in order to sustain polymer production and it is to the source of ethene that we turn our attention to next.

Petroleum (or crude oil) is the fossilised organic remains of small marine plants and animals that settled to the sea floor millions of years ago. Crude petroleum consists of a mixture of compounds, mainly hydrocarbons, but also of smaller amounts of organic molecules containing oxygen, sulfur, nitrogen and some metals.

A typical distillation column and cracking unit used in an oil refinery
showing the fractions obtained at each temperature.

The first step in the refining of petroleum is to separate the oil into fractions on the basis of their boiling points. Fractions of typical crude petroleum can be seen in the diagram on the right. The petrol obtained in this separation is known as straight-run gasoline and is of too low a quality to be used directly in today's automobiles. The naphtha yields, kerosene and solvents for paints, lacquers and varnishes. Furnace and gas oils are burned in oil heaters and diesel engines or are used to make more petrol (see later). The residual fractions furnish a great variety of common products, ranging from waxes, mineral oils and paraffin to bitumen for roads.

When petroleum undergoes fractional distillation, some fractions, particularly petrol, are in demand and of high economic value. Those fractions with more than 10 carbon atoms are of low value and are in low demand. These unwanted fractions can be 'cracked' to produce shorter chained hydrocarbons that have a higher economic value.

By heating the high-boiling fractions (C-10 and above) with a catalyst to between 400 - 500°C, the larger molecules crack into smaller hydrocarbons. This process can be achieved using only heat and steam and in this case, is called thermal cracking. When a zeolite catalyst is also used it is called catalytic cracking. The products of cracking can include petrol, diesel and ethene. A typical reaction is shown below.

C14H30 → C12H26 + CH2=CH2

fuel oil → diesel + ethene

While ethene is a biproduct of producing hydrocarbons of high demand and economic value, it itself is used extensively in the manufacture of many useful materials, especially polymers.