Lab 1: Synthesis
Polymer Synthesis of Polystyrene and Nylon-6, 6
1. Introduction
Polymers are macromolecules built from smaller molecular subunits, called
monomers. Synthetic polymers can be classified into two main types according to the
mechanism by which they synthetically grow from monomer to polymer: chain-growth
polymers and step-growth polymers. This classification scheme is an update from
historical nomenclature, in which polymers were classified by whether there existed a
byproduct of the polymerization reaction (condensation polymerization) or not (addition
polymerization).
In a step-growth reaction, the growing polymer chains (of any
molecular length) may react with each other to form longer polymer chains. The
monomer or dimer may react in just the same way as a polymer containing hundreds of
monomer units. In chain-growth polymerization, however, only monomers may react
with growing polymer chains. That is, two growing polymer chains cannot join together
as in the case during step-growth polymerization. Figure 1 below illustrates the two types
of polymerizations and Table 1 summarizes the differences between the two.
Figure 1.1 A scheme of process of (a) step polymerization, and (b) chain polymerization.1
Chemistry Class Homepage, “Introduction to Polymer Chemistry”:
http://www.chem.rochester.edu/~chem421/stepchn.htm
1
1
Table 1.1 Step-Growth versus Chain Growth Polymerization.
Step-Growth Polymerization
Chain-Growth Polymerization
Monomer or polymer of any molecular weight
can react
Growth occurs only by addition of
monomer to active chain end.
Monomer disappears early.
Monomer is present throughout, but
its concentration decreases.
MW increases very slowly at first, and then Polymer MW increases gradually
increases rapidly at high conversion.
throughout.
Long reaction times increase MW, but yield of MW and yield depend on mechanism
polymer hardly changes.
details.
Only monomer and polymer are
All molecular species are present throughout.
present during reaction.
Usually (but not always) polymer
Usually (but not always) polymer repeat unit repeat unit has the same atoms as had
has fewer atoms than had the monomer.
the monomer.
In this lab, two sets of syntheses will be performed by each group. First, chaingrowth polymerization of styrene monomer using benzoyl peroxide initiator will be
performed. Under heat, thermal decomposition of the benzoyl peroxide (other possible
initiating sources are listed in Table 2) creates the initiating radicals. 2 The effect of
varying the amount of initiator will be studied. The other synthesis will be a step-growth
polymerization of Nylon-6, 6 via an interfacial polymerization.
1.1 Chain-Growth Polymerization
The characteristics of chain-growth polymerizations are: 1) the repeating units are
added one at a time to the chain, 2) monomer concentration decreases steadily throughout
the reaction, 3) high molecular weight polymers are formed quickly and the molecular
weight of the polymers changes gradually throughout the reaction, 4) long reaction time
2
. A. Ravve, “Principles of Polymer Chemistry”, Plenum Press, 1995.
2
produces high yields, but affects the molecular weight only slightly. While there are
many types of specific chain-growth polymerization mechanisms, four frequently used
types are: free radical, anionic, cationic and coordination.3
Table 1.2 Initiating Sources of Free Radical Polymerization2
Radicals Initiating Sources
Examples
1) Thermal decomposition with azo -2,2-azobisisobutyronitrile(AIBN)
and peroxy group
-dibenzoyl peroxide
2) Bimolecular initiating system
redox initiation of Fe3+ and Ce4+
3) Boron alkyls and metal alkyl -dialkylzinc
initiators
-dialkylcadmium
-triethylaluminum
4) Photochemical initiator
ultraviolet (UV) light
5) Radioactive sources and electron gamma rays, beta rays, alpha particles
beams
6) Ultrasonic radiation
Free radical polymerization consists of three steps: Initiation, Propagation and
Termination.3 At the initiation stage of chain-growth free radical polymerization of
styrene monomers, the initiator, I, forms free radicals species:
, which then couples
with one electron of the carbon-carbon double bonds of the styrene monomer to form
another radical (see scheme 1.1). These bonds are particularly susceptible to radical
attack because of their relatively low stability4 and resonance stabilization of the resulting
radical throughout the phenyl group. In the next stage, propagation, the newly formed
radical reacts with another monomer to form another radical, resulting in a polymer that
is one unit longer (see scheme 1.2). The process is then repeated to form a chain of
monomers (a polymer). The propagation step is terminated when two radicals combine
together to form a neutral species, known as coupling or combination (see scheme 1.3a);
3
4
. Fred W. Billmeyer, JR., “Textbook of Polymer Science”, A Wiley-Interscience Publication, 1984.
. Harold Hart, Lelie E. Craine, “Organic Chemistry”, Houghton Mifflin Company.
3
or disproportionation, in which hydrogen transfer results in the formation of two
molecules: one saturated and one unsaturated end group (see scheme 1.3b).
Initiation Step
I
2R
H
2R + CH2 CH
RCH2C
X
X
--- (Scheme 1.1)
Propagation Step
H
R
CH2CH
n
H
CH2C + CH2 CH
X
R
CH2CH2 CH2C
n+1
X
X
X ---(Scheme 1.2)
Termination Step
Combination or Coupling
CH2C
X
X
X
X
CCH2
CH2C
CCH2
+
H
H
H
H
---(Scheme 1.3a)
Disproportionation
---(Scheme 1.3b)
1.2 Step-Growth Polymerization
In this reaction, monomers need not necessarily add sequentially, but instead
small polymer chains may couple into larger chains.2 For example, nylon 6,6 is an
aliphatic polyamide that is synthesized using A-A/B-B step-growth condensationpolymerization.
The
two
monomers
involved
in
this
polymerization
are
hexamethylenediamine and adipoyl acid (Figure 1.2), each of which is bifuctional (i.e.,
two ends of each are reactive), and each end contains the same functionality (i.e., A or B
functional groups).5
4
The first step in the reaction between a molecule of hexamethylenediamine and a
molecule of adipoyl chloride (see Scheme 1.4 below). The hydrogen atom of the amine
group belonging to hexamethylenediamine forms a hydrochloric acid (HCl) molecule
with the chloride from the acid functional group. The remaining adipoyl chloride
molecule and the hexamethylenediamine molecule will join together to form a larger
molecule (polymer). The molecule formed has an acid group at one end and an amine
group at the other (Figure 1.3) and is a nylon 6,6 unit.5
Amine
Functional
Acid
Functional
Groups
Groups
hexamethylenediamine
Adipoyl chloride
Cl
Cl
Figure 1.2 Monomers used in synthesis of nylon 6,6.6
Scheme 1.4. Reaction mechanism.6
5
6
Fried, J.R., “Polymer Science and Technology”, Prentice Hall PTR, New Jersey, 1995.
“Spinnin the elements: Wallace Carothers and the Nylon Legacy. The Science of Nylon: Making nylon”:
http://www.chemheritage.org/EducationalServices/nylon/chem/make.html
5
Figure 1.3 Nylon 6,6 unit.6
The end groups can then react in a similar manner with other acid/amine groups present
on adipoyl chloride and hexamethylenediamine. After completion of many steps of this
process, the nylon 6,6 polymer will form (Figure 1.4). 6
Figure 1.4 Nylon 6,6 polymer. 6
1.3 Synthetic Polymerization of Polystyrene in Industry
Industrially, polystyrene polymers are synthesized in a similar way that will be
performed in this lab: benzoyl peroxide is used as an initiator in the polymerization
process of styrene monomers.7 Commercial polystyrenes are a family of thermoplastic
materials that possesses a wide range of properties suitable for many diverse applications.
In the process economics of producing polystyrene, these properties depend on the recipe
for the polymerization of styrene, the additives or impurities present in the polymer, and
the fabrication methods that give the polymer its final form.
7
Louisiana State University, Department of Chemistry (inorganic lab): www.stanley.chem.lsu.edu
6
Some other examples of commercially produced polymers that are synthesized
using free radical polymerization include poly (vinyl chloride) (PVC) and poly (methyl
methacrylate) (PMMA)8.
Table 1.3. Various commercial products of polymers with chain-growth and step-growth
polymerization.
Chemicals
Composition
Commercial Name
(Company)
Polystyrene(PS)
Poly (vinyl chloride) (PVC)
Poly
(methyl
PS/PB
Styron(Dow Chemical)
PS/SAN
Lacqram(ATO)
PVC/EVA
Levepren(Bayer)
methacrylate) PMMA
Acrypet(Mitsubishi Rayon)
(PMMA)
Polyamide(Nylon 6,6)
Nylon 6,6
SolarMax(DuPont)
2. Experimental Procedure
Important !!
1. Gloves, goggles, and lab aprons MUST be worn when handling hazardous chemicals.
2. Always work in the hood when working with volatile liquid chemicals and when
performing an exothermic reaction.
3. Do not touch the hot plate with bare hands.
2.1 Polystyrene Polymerization
2.1.1 Materials
1. Styrene monomer (manufactured by Aldrich). The boiling point of the styrene
monomer is 145 - 146 °C.
2. Benzoyl peroxide initiator (manufactured by Aldrich). Benzoyl peroxide decomposes
with hemolytic cleavage of its oxygen-oxygen bond at a temperature between 80 and
90°C.
8
University of Southern Mississippi, “Macrogalleria homepage”: www.psrc.usm.edu/macrog/vinyl.htm
7
2.1.2 Experimental Procedure
1. Weigh three portions of benzoyl peroxide initiator on a glassine weight paper: 100
mg, 250 mg, and 400 mg. The benzoyl peroxide initiator is white spherical solid
particles.
(Note: Benzoyl peroxide is explosive. Use only the glassine weight paper provided in
the lab. Benzoyl peroxide initiator would burn when in contact with any other types of
weight paper. Avoid any source of heat when working with this material.)
2. Transfer the weighed initiator into separate 50 ml glass jars and label them: polymer 1
(100 mg), polymer 2 (250 mg), polymer 3 (400 mg).
(Note: Use only wet paper towels to clean up any wasted benzoyl peroxide. Wet the
used paper towel completely before disposing it into the waste basket to avoid fire.)
3. Measure three portions of 15 ml of styrene monomer using a 25 ml graduated
cylinder. Styrene monomer is a transparent and slightly viscous liquid.
(Note: Styrene monomer is harmful when inhaled. Handle this material in the hood.)
4. Pour the measured styrene monomer into each of the labeled glass jars that contain
the benzoyl peroxide initiator prepared earlier, and for each beaker, follow the same
steps (step 5 to 14).
5. Stir the mixture gently with a glass rod until all the benzoyl peroxide initiators have
dissolved.
6. Place the beaker on a hot plate, which is heated to between 80C and 90C (consult
your TA for a recommended hotplate setting to reach this temperature), which is the
temperature at which benzoyl peroxide starts to form free radicals.
7. Stir the mixture continuously with a glass rod while heating. Take care to avoid sharp
edges of the glass rod.
8. After about 10 minutes, bubbles begin to appear as white smoke is released from the
solution. This indicates that carbon dioxide gas is being produced. In this case, carbon
dioxide gas is the side product of the initiator decomposition.
9. Take note of the changes in viscosity and volume of the polymerization during the
reaction.
10. To check if the reaction has come to completion, occasionally pull the glass rod out of
the beaker. This would cause the solution mixture to be stretched to form a fiber.
8
Allow the fiber to solidify for a few seconds, and then try to break the solidified fiber
with your fingers (make sure you are wearing gloves!!!)
11. If the fiber breaks in a brittle manner, remove the beaker from the hot plate and cast
the polymer solution gently, but quickly (solidification of the polymer takes place
quickly at room temperature) onto a Petri dish to form a thin layer of film. If the fiber
does not break appear brittle, keep stirring and checking.
12. Leave the casted polymer solution in the hood for a few minutes to solidify
completely.
13. Break the solidified polystyrene product using spatula, and keep them in separate
labeled vials for future characterization purposes.
14. Use tetrahydrofuran (THF) solvent to dissolve the leftover polystyrene polymer that
has solidified on the side walls of the jar. Dispose of this solution and excess THF in
the organic waste container
15. Prepare a small volume of solution (~5 ml total) containing approximately (5-10 mg
polymer)/(1 ml THF) for each polymer. Combine all three prepared THF/polymer
solutions together in to one vial, label, and set aside for future labs.
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2.2 Nylon-6, 6 Interfacial Polymerization
2.2.1 Materials
1. Hexamethlenediamine (1,6-hexanediamine), manufactured by Aldrich.
2. Adipoyl chloride, manufactured by Aldrich.
3. Sodium Hydroxide pellets.
4. Solvent: tap water and cyclohexane.
2.2.2 Experimental Procedure
1. Prepare 25 mL of a 0.35 M aqueous solution of hexamethylenediamine (1,6hexanediamine). For example, you would need 1.0 gram of 1,6-hexanediamine in 25
mL of water to make a 0.35 M solution. To do this, weigh out the solid 1,6hexanediamine on paper using a balance.
2. Add water. Use a glass stirring rod to mix the solid until it is completely dissolved in
water. Measure 10 mL of this solution in a graduated cylinder and set aside.
(Note: every time you pour out a solution, touch one glass container to the other to
prevent solution spills and to also ensure that all of the solution is removed from one
container to the other, if desired.)
3. Prepare 20 mL of a 5% by volume solution of adipoyl chloride (e.g. 1 ml of adipoyl
chloride in 19 ml of solvent). Use cyclohexane as the solvent and mix gently for 30
seconds. Measure out 10 mL of this solution in a graduated cylinder and set aside.
Since this solution has an organic solvent, place a cap over the graduated cylinder.
4. Add water to sodium hydroxide pellets to make 25 mL of a 20% by weight solution.
You will need to use the balance to weigh the pellets.
(Caution: the sodium hydroxide will cause skin and eye irritation. Be sure wear
gloves while handling and not to touch your eyes or skin after handling. Also, the
reaction is exothermic and will produce heat at a temperature over 80 oC.)
5. Transfer the 10 mL of 1,6-hexanediamine into a 50 mL beaker and add 10 drops of
the sodium hydroxide solution to it.
6. Tilt the beaker with the 1,6-hexanediamine and the sodium hydroxide solution in it at
a 45 degree angle and SLOWLY add the 10 mL of the adipoyl chloride solution to it.
Two layers will be present since these solutions are not miscible in each other and the
polymer will immediately form at the liquid-liquid interface (Figure 1.5).
10
(Note: HCL acid vapor is harmful when inhaled.)
7. Using a copper wire with a hook at the end,
slowly go around the inside wall of the beaker
to free it of the polymer and in a circular
motion, approach the center of the beaker.
Gently pull up on the polyamide at the center
Figure 1.5: Formation of nylon-6,6
rope by interfacial polymerization.
9
of the polymer’s mass and a continuous rope
will form out of the solution. Slowly pull the
rope until a few feet of the polymer is out of the solution (if you pull it too fast the
rope will break), and then cut the rope off. Lay this rope on a paper towel to dry.
8. Quickly stir the remaining solution until a bulk polymer forms inside the beaker.
9. Rinse both the bulk polymer and the polymer rope in water three times. Use a paper
towel to get dab excess water out of the fiber and then use a piece of filter paper to
pull more water out of the fiber. Wrap the polyamide in filter paper and place it
aside.9
10. Dispose of all organic waste in the “organic waste” container. Dispose of all waterbased solution in the container marked “base waste”. Clean all glassware with water
and put on the rack to dry. Throw away all paper waste. Also clean the balance.
3. Assignments
3.1 Describe the experimental observation of the final products in the polymerization of
styrene in terms of
(a) The appearance
(b) The difference in viscosity
3.2 Describe the experimental observation on synthesis of nylon 6,6 in terms of
(a) The appearance of the products
9
Engle, Pavia, Kriz, and Lampman, “Intorduction to organic Laboratory Techniques: A Microscale
Approach”, Suanders College Publishing, 1990.
11
(b) The difference between the nylon rope and the nylon bulk
3.3 How is the amount of benzoyl peroxide initiator expected to affect the molecular
weight of the polystyrene?
3.4 Why does the volume of styrene/benzoyl peroxide mixture slightly decrease during
the polymerization reaction?
3.5 Explain how the two different termination steps, combination/coupling and
disproportionation, affect the molecular weight of polystyrene.
3.6 Nylon was synthesized in an interfacial polymerization in this lab. How and why
would the molecular weight be affected if this step-growth reaction were run in a bulk
polymerization, similar to that of the synthesis of polystyrene?
3.7 What do the two numbers correspond to in the name “Nylon-6,6?” What would the
chemical structure of the repeat unit be in “Nylon-2,2?”
3.8
Why is
a
solution
of
sodium
hydroxide
added
to
the
solution
of
hexamethylenediamine?
12