Polymers for the ESAT Chemistry Paper

Addition Polymerisation and Polyalkenes

Addition polymerisation is a fundamental reaction of alkenes where many small molecules, known as monomers, react together to form a very long chain called a polymer.

The key characteristics of addition polymerisation include:

1. The monomers must be unsaturated, meaning they contain at least one $C=C$ double bond.
2. During the reaction, one of the bonds in the $C=C$ double bond breaks.
3. New $C-C$ single bonds are formed between adjacent monomer molecules, linking them end to end.
4. The resulting polymer is a saturated molecule, consisting of single bonds only.
5. A polymer chain typically incorporates thousands or tens of thousands of monomer units.

This process is an addition reaction because the monomers simply add together without the loss of any atoms. The name of the resulting polymer is produced by adding the prefix 'poly-' to the name of the monomer, usually in brackets if the name is complex. For example, the polymerisation of ethene produces poly(ethene).

In the equation above, $n$ represents a very large number of monomers. The structure shown in square brackets, $-[-CH_2-CH_2-]-$, is the repeating unit. Although the reaction conditions vary, they typically require high pressures and specific catalysts.

Exercise 111: Polymerisation of Propene

To write the equation for the polymerisation of propene ($CH_2=CHCH_3$), we identify the $C=C$ bond. During the reaction, this bond opens to connect to the next unit. The methyl group ($CH_3$) remains attached to one of the carbons in the chain. The equation is: $n\ CH_2=CH(CH_3) \rightarrow [-CH_2-CH(CH_3)-]_n$.

Exercise 112: Identifying Monomers and Repeating Units

Consider the following section of a polyalkene:

a) The repeating unit is the smallest part of the polymer chain that, when repeated, produces the entire structure. By looking at the chain, we see the pattern repeats every two carbon atoms. The repeat unit is: $-CH(CH_3)-CH(Cl)-$.

b) To find the monomer, we take the repeating unit and replace the single bond between the chain carbons with a double bond. The monomer is $CH(CH_3)=CHCl$, which is 1-chloroprop-1-ene.

Condensation Polymerisation

In condensation polymerisation, monomers join to form a polymer with the simultaneous elimination of a small molecule, such as water ($H_2O$) or hydrogen chloride ($HCl$). This differs from addition polymerisation, which produces no by-products.

Key features of condensation polymerisation include:

1. Each monomer must have at least two functional groups to allow the chain to grow from both ends.
2. It often involves two different types of monomers, though it can involve one monomer type if that monomer contains two different functional groups.
3. The linkage is formed by a condensation reaction between these functional groups.

Polyesters

Polyesters are formed when monomers are linked by ester groups. This usually involves a reaction between a carboxylic acid group ($-COOH$) and an alcohol group ($-OH$). When these react, a molecule of water is removed, and an ester linkage ($-COO-$) is formed.

A common example is Terylene, which is used in clothing fibres. It is made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid.

The formation of the ester linkage is shown here:

Because both monomers have functional groups at both ends, the process continues, building a long chain. The simplest repeating unit for Terylene is:

Exercise 113: Drawing Polyester Structures

For monomers hexanedioic acid, $HOOC(CH_2)_4COOH$, and butane-2,3-diol, $HOCH(CH_3)CH(CH_3)OH$, the repeating unit is formed by removing the $-OH$ from the acid and the $-H$ from the alcohol.

Repeating unit: $-O-C(O)-(CH_2)_4-C(O)-O-CH(CH_3)-CH(CH_3)-$.

Polyamides

Polyamides are formed via amide linkages. These result from the reaction between a carboxylic acid group ($-COOH$) and an amine group ($-NH_2$), with the elimination of water.

Nylon-6,6 is a well-known polyamide made from hexane-1,6-dioic acid and 1,6-diaminohexane.

The reaction creates the amide linkage as follows:

The repeating unit for nylon-6,6 is:

Exercise 114: Determining Polyamide Monomers

Given this polymer section:

To find the monomers, we 'break' the amide linkage ($C-N$ bond). We add an $-OH$ back to the carbonyl ($C=O$) to reform the carboxylic acid and an $-H$ back to the nitrogen to reform the amine. The monomers are propanedioic acid ($HOOC-CH_2-COOH$) and 1,2-diaminoethane ($H_2N-CH_2-CH_2-NH_2$).

Proteins and Amino Acids

Proteins are natural polyamides. They consist of long chains of amino acids joined by amide linkages, which in biological contexts are specifically called peptide bonds.

An amino acid contains three key groups attached to a central carbon atom: an amino group ($-NH_2$), a carboxyl group ($-COOH$), and a variable organic 'R-group'. A hydrogen atom occupies the fourth bonding position.

In glycine, the simplest amino acid, the R-group is a hydrogen atom:

Exercise 115: Tripeptide Formation

A tripeptide forms when three amino acids link. For the sequence alanine-glycine-valine (Ala-Gly-Val), we join the carboxyl group of alanine to the amino group of glycine, and the carboxyl group of glycine to the amino group of valine.

The structure will have the pattern $H_2N-CH(CH_3)-CO-NH-CH_2-CO-NH-CH(CH(CH_3)_2)-COOH$.

Biodegradability

Biodegradable polymers are those that microorganisms or enzymes can break down into natural products like $CO_2$, $N_2$, and $H_2O$. These polymers typically contain ester or amide linkages, which are susceptible to chemical or enzymatic attack. An example is poly(lactic acid) or PLA.

Non-biodegradable polymers do not break down easily. They usually consist of long carbon-hydrogen chains (like polyalkenes) held together by strong, inert $C-C$ and $C-H$ bonds that microorganisms cannot easily process.