(a) Compute the repeat unit molecular weight of polypropylene.
(b) Compute the number-average molecu- lar weight for a polypropylene for which the degree of polymerization is 15,000.
The correct answer and explanation is :
(a) The repeat unit molecular weight of polypropylene (C₃H₆) is 42.08 g/mol.
(b) The number-average molecular weight ((M_n)) for a polypropylene with a degree of polymerization ((DP)) of 15,000 is:
[
M_n = DP \times \text{Repeat Unit Molecular Weight}
]
[
M_n = 15,000 \times 42.08 = 631,170 \text{ g/mol}
]
Explanation:
Polypropylene (PP) is a polymer made up of repeating propylene (C₃H₆) units. The molecular weight of a polymer depends on its degree of polymerization, which is the number of repeat units in the polymer chain.
- Repeat Unit Molecular Weight Calculation
The molecular weight of the repeat unit in polypropylene is determined by adding the atomic weights of the elements in the propylene monomer (C₃H₆):
- Carbon (C): (3 \times 12.01 = 36.03) g/mol
- Hydrogen (H): (6 \times 1.008 = 6.048) g/mol
- Total: 42.08 g/mol
- Number-Average Molecular Weight Calculation
The number-average molecular weight ((M_n)) is given by: [
M_n = DP \times M_{\text{repeat unit}}
] Given (DP = 15,000), we multiply by 42.08 g/mol: [
M_n = 15,000 \times 42.08 = 631,170 \text{ g/mol}
] This value represents the average molecular weight of the polymer chains in the sample.
The molecular weight is crucial in determining the physical properties of polypropylene, such as tensile strength, melting point, and flexibility. Higher molecular weights generally lead to improved mechanical properties and increased viscosity, making the polymer suitable for various applications like packaging, automotive components, and textiles.
