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Polymerization
1. Polymerization is the formation of long carbon chains that occurs through a kind of chain
reaction. It is a kind of addition reaction is one in which two or more molecules join together to
give a single product.
2. Vocab
a. Polymer – a large molecule consisting of repeating units.
b. Repeating unit – recurring unit in a polymer
c. Monomer – smallest molecule from which the polymer is made
i. Naming Polymers: put poly- in front of the name of the monomer
3. There are many polymerization mechanisms. You need to know:
a. Addition of alkenes:
i. Polyethylene
ii. polyvinylchloride
b. Condensation of carboxylic acids and amines:
i. nylons (amides),
ii. peptides (amides)
c. Condensation of carboxylic acids and alcohols
i. polyesters
Example 1: Addition of Alkenes
1. 3 steps in addition of alkenes
d. initiation
i. need a radical to start the reaction: Ra
ii. radical: atom or compound w/ unpaired electron. It’s very reactive b/c it does
not have the octet rule satisfied.
iii. To form the radical heat is applied to a molecule with a O-O bond like peroxide
or oxygen gas.
1. breaking the O-O bond will create two radicals
e. Propagation
i. Reaction of monomer unit, hundreds of times
f. Termination
i. Two chains come together so that radicals are consumed
2. Polyethylene
a. Two kinds of polyethylene
ii. HDPE = gallon milk cartons (more rigid)
iii. LDPE = plastic bags, squeeze bottles (more flexible)
b. Reaction: nCH2=CH2 [-CH2-CH2-]n
c. Monomer: ethylene, CH2=CH2
d. Conditions:
iv. Temp: 200 C
v. Pressure: 2000 atm
vi. Initiator: O2
Initiation
Ra + 2 CH2=CH2  2 Ra-CH2-CH2
Propagation
Ra-CH2-CH2 + H2C=CH2  Ra-CH2-CH2-CH2-CH2
This step keeps repeating, making longer chains: Ra(CH 2-CH2)n-CH2-CH2
Termination
2 Ra(CH2-CH2)n-CH2-CH2 + CH2-CH2 -n(CH2-CH2)Ra  Ra(CH2-CH2)n-CH2-CH2-Ra
H
H
H
H
H
H
n
n
H
H
H
H
H
H
3. Polyvinylchloride
a. Vinyl is a common name for ethylene
b. Reactions: nCH2=CHCl  [-CH2-CHCl-]n
c. Monomer: vinyl chloride or chloroethylene, CH2=CHCl
Cl
H
Cl
H
Cl
H
n
n
H
H
H
H
H
H
Example 2: Condensation of carboxylic acid and amine
1. The basic condensation reaction occurs as below:
OH
+
N
+
HN
O
O
H2O
O
CH3
C
O
CH3
O
H
+
N
CH3
H
CH3
O-
C
+
H
+
N
H
CH3
CH3
H2O
CH3
O
CH3
C
N
CH3
1. Polymerization occurs when a molecule has two functional groups
a. Polymers formed are copolymers because they are made of two different monomers.
b. Polymers are formed in a step-growth method rather than a chain-growth method.
i. In other words, molecules with 2 functional groups can grow from both ends
instead of just one end as in polyethylene.
c. Formation of nylon –
i. Hexanedioic acid (2 carboxylic acid groups) and 1,6 diaminohexane (2 amine
groups)
O
HO
OH
+
NH2
H2N
O
O
NH
+
NH
2n H2O
O
n
d. Formation of polypeptide
i. Polypeptide is a chain of amino acids, each amino acid has one carboxylic acid
and one amine group
ii. Note that the polymerization here occurs because there are two different groups
on the same molecule
iii. Polypeptides are not, technically, polymers since they don’t have repeating units
(R group is different)
iv. Peptide bond between alanine and cysteine:
O
OH
+
HO
O
O
HS
NH
OH
NH2
O
NH2
NH2
+
H2O
+
H2O
SH
polypeptide:
O
R
OH
+
HO
O
O
HO
NH2
R
R
O
NH
R
NH2
NH2
adding another amino
acid to COOH group
O
R
OH
O
HN
O
R
NH
R
NH2
Example 3: Condensation of carboxylic acid and alcohol
1. Polyester is a step-growth polymer like nylon – can grow from both ends
2. Formation of polyester
a. Benzene-1,4-dicarboxylic acid (2 carboxylic acid group) and ethane-1,2-diol (2 amine
groups)
O
O
OH
+
HO
OH
HO
O
O
O
O
+
O
n
2n H2O
3. Basic Reaction mechanism for formation of ester from carboxylic acid and alcohol
1.
2.
O
+
R
H
O
H Cl
+
OH
R
3.
OH
OH
R
O
+
H
O
H
+
R
H
OH
O
R
O
+
H
H O
+
H
H
+
O
H
R
O
OH
5.
R
HO
4.
OH
R
R
+
+
H