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Chapter 14: Problem 2

Compute repeat unit molecular weights for the following: (a) polytetrafluoroethylene (b) poly(methyl methacrylate), (c) nylon 6,6 and (d) poly(ethylene terephthalate)

Short Answer

Expert verified
Answer: The repeat unit molecular weights for each polymer are: (a) Polytetrafluoroethylene (PTFE): 100.02 g/mol (b) Poly(methyl methacrylate) (PMMA): 100.12 g/mol (c) Nylon 6,6: 226.32 g/mol (d) Poly(ethylene terephthalate) (PET): 192.18 g/mol

Step by step solution

01

Identify the repeat unit for each polymer

For each polymer, we can identify the molecules that repeat throughout the structure. These are: (a) Polytetrafluoroethylene (PTFE): -CF2-CF2- (b) Poly(methyl methacrylate) (PMMA): -C(CH3)-C(O)OCH3-CH2- (c) Nylon 6,6: -NH-(CH2)4-CONH-(CH2)6-CO- (d) Poly(ethylene terephthalate) (PET): -O-CH2-CH2-O-CO-(C6H4)-CO-
02

Compute the molecular weight of the repeat units

Using the repeat unit structures identified in Step 1, we can calculate the molecular weight of each repeat unit by summing up the molecular weights of all their constituent atoms. (a) Polytetrafluoroethylene (PTFE): -CF2-CF2- Molecular weight = 2x(12.01 + 2x19.00) = 100.02 g/mol (b) Poly(methyl methacrylate) (PMMA): -C(CH3)-C(O)OCH3-CH2- Molecular weight = 12.01 + 3x1.01 + 12.01 + 12.01 + 16.00 + 1.01 + 12.01 + 3x1.01 = 100.12 g/mol (c) Nylon 6,6: -NH-(CH2)4-CONH-(CH2)6-CO- Molecular weight = 14.01 + 1.01 + 4x(12.01 + 2x1.01) + 12.01 + 16.00 + 14.01 + 1.01 + 6x(12.01 + 2x1.01) + 12.01 + 16.00 = 226.32 g/mol (d) Poly(ethylene terephthalate) (PET): -O-CH2-CH2-O-CO-(C6H4)-CO- Molecular weight = 16.00 + 12.01 + 2x1.01 + 12.01 + 2x1.01 + 16.00 + 12.01 + 16.00 + 6x(12.01 + 1.01) + 12.01 + 16.00 = 192.18 g/mol Here are the repeat unit molecular weights for each polymer: (a) Polytetrafluoroethylene (PTFE): 100.02 g/mol (b) Poly(methyl methacrylate) (PMMA): 100.12 g/mol (c) Nylon 6,6: 226.32 g/mol (d) Poly(ethylene terephthalate) (PET): 192.18 g/mol

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Polytetrafluoroethylene
Polytetrafluoroethylene (PTFE), commonly known by the brand name Teflon, is a type of polymer known for its exceptional chemical resistance and use at high temperatures. Its repeat unit, comprising two carbon atoms each bonded to two fluorine atoms (CF2-CF2), contributes to the polymer's non-reactive nature.

Calculating the molecular weight for PTFE involves adding the atomic masses of the constituents in the repeat unit. Given that the atomic mass of carbon is approximately 12.01 g/mol and that of fluorine is around 19.00 g/mol, the molecular weight for a single repeat unit of PTFE is easily computed as 100.02 g/mol. This weight is critical for various applications, including understanding the material's physical properties such as its density and mechanical strength.
Poly(methyl methacrylate)
Poly(methyl methacrylate) (PMMA), also known as acrylic or Plexiglas, is a transparent thermoplastic often used as a lightweight or shatter-resistant alternative to glass. The PMMA repeat unit structure includes a methyl group (CH3), an ester group (COOCH3), and a backbone composed of carbon and hydrogen atoms.

The molecular weight calculation for PMMA takes into account the molecular weights of carbon (12.01 g/mol), hydrogen (1.01 g/mol), and oxygen (16.00 g/mol). Summing these values for all atoms in the repeat unit, we get a molecular weight of 100.12 g/mol. Understanding this value helps in analyzing the polymer's processing conditions and its performance in finished products.
Nylon 6,6
Nylon 6,6 is a type of polyamide known for its robustness and is widely used in fabrics, carpets, and industrial applications. A distinctive feature of nylon 6,6 is its repeat unit which contains an amide group (NH-CO), critical for hydrogen bonding that provides the material's high tensile strength.

To determine the molecular weight of a nylon 6,6 repeat unit, sum the masses of the constitutive atoms: carbon (12.01 g/mol), hydrogen (1.01 g/mol), oxygen (16.00 g/mol), and nitrogen (14.01 g/mol). The final calculation results in a value of 226.32 g/mol. This molecular weight is fundamental for determining the density, melting point, and how the material will respond under various environmental conditions.
Poly(ethylene terephthalate)
Poly(ethylene terephthalate) (PET) is widely known for its use in beverage, food, and other liquid containers. PET is appreciated for its stiffness, transparency, and recyclability. The repeat unit of PET includes an ethylene group (CH2-CH2), linked terephthalate (a benzene ring with two carboxyl groups).

The molecular weight of PET's repeat unit is obtained by summing the individual weights of carbon, hydrogen, and oxygen in the structure, leading to a calculated molecular weight of 192.18 g/mol. This information is key when exploring the polymer's characteristics, such as its barrier properties and how it can be manipulated during the injection molding or extrusion process.

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Most popular questions from this chapter

Crosslinked copolymers consisting of 35 wt \(\%\) ethylene and 65 wt\% propylene may have elastic properties similar to those for natural rubber. For a copolymer of this composition, determine the fraction of both repeat unit types.

Sketch cis and trans structures for (a) butadiene, and (b) chloroprene.

Explain briefly why the tendency of a polymer to crystallize decreases with increasing molecular weight.

High-density polyethylene may be chlorinated by inducing the random substitution of chlorine atoms for hydrogen. (a) Determine the concentration of \(\mathrm{Cl}\) (in wt\%) that must be added if this substitution occurs for \(8 \%\) of all the original hydrogen atoms. (b) In what ways does this chlorinated polyethylene differ from poly(vinyl chloride)?

Is it possible to have a poly(vinyl chloride) homopolymer with the following molecular weight data, and a degree of polymerization of \(1120 ?\) Why or why not? $$\begin{array}{ccc} \hline \text {Molecular Weight} & & \\ \text {Range }(\mathrm{g} / \mathrm{mol}) & \boldsymbol{w}_{\boldsymbol{i}} & \boldsymbol{x}_{\boldsymbol{i}} \\ \hline 8,000-20,000 & 0.02 & 0.05 \\ 20,000-32,000 & 0.08 & 0.15 \\ 32,000-44,000 & 0.17 & 0.21 \\ 44,000-56,000 & 0.29 & 0.28 \\ 56,000-68,000 & 0.23 & 0.18 \\ 68,000-80,000 & 0.16 & 0.10 \\ 80,000-92,000 & 0.05 & 0.03 \\ \hline \end{array}$$
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