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Chapter 14: Problem 11

Sketch portions of a linear polypropylene molecule that are (a) syndiotactic (b) atactic, and (c) isotactic.

Short Answer

Expert verified
Question: Describe the differences between syndiotactic, atactic, and isotactic polypropylene chains and provide a sketch for each configuration. Answer: The differences between these polypropylene chain configurations are determined by the orientation of their methyl groups: 1. Syndiotactic: Methyl groups are positioned on alternating sides of the polymer chain. The sketch of the syndiotactic chain looks like: ``` H H R H H R | | | | | | H-C-C-C--C-C-C-H | | | | | | H H H H H H ``` 2. Atactic: Methyl groups are positioned randomly on either side of the polymer chain. One possible representation for the atactic chain is: ``` H H R H H R | | | | | | H-C-C-C--C-C-C-H | | | | | | H H H H H H H H R | | | H-C-C-C-H | | | H H H ``` 3. Isotactic: Methyl groups are positioned on the same side of the polymer chain. The sketch of the isotactic chain looks like: ``` H H R H H R | | | | | | H-C-C-C--C-C-C-H | | | | | | H H H H H H ``` In these sketches, the varying orientation of the methyl groups (R) demonstrates the differences between the syndiotactic, atactic, and isotactic configurations of the polypropylene chains.

Step by step solution

01

Sketch the basic structure for propylene unit

First, let's sketch a basic propylene unit that will be repeated in our polypropylene chain. The propylene unit looks like this (where C is carbon and H is hydrogen): ``` H H H | | | H - C - C - C - H | | | H H R ``` Here, R represents the methyl group.
02

Sketch the syndiotactic polypropylene chain

To draw a syndiotactic polypropylene chain, we need to ensure that the methyl groups alternate between sides of the carbon backbone. The sketch of the chain should look like: ``` H H R H H R | | | | | | H-C-C-C--C-C-C-H | | | | | | H H H H H H ``` In this example, syndiotactic configuration is shown by the alternating orientation of the methyl groups (R).
03

Sketch the atactic polypropylene chain

For an atactic polypropylene chain, the methyl groups will have a random arrangement. One possible representation is: ``` H H R H H R | | | | | | H-C-C-C--C-C-C-H | | | | | | H H H H H H H H R | | | H-C-C-C-H | | | H H H ``` The atactic configuration is demonstrated by the random orientation of the methyl groups.
04

Sketch the isotactic polypropylene chain

Finally, for an isotactic polypropylene chain, all the methyl groups should be positioned on the same side of the carbon backbone: ``` H H R H H R | | | | | | H-C-C-C--C-C-C-H | | | | | | H H H H H H ``` Here, the isotactic configuration is shown by the uniform orientation of the methyl groups on the same side of the polymer chain.

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Key Concepts

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

Polymer Chain Structure
Polymer chain structures are fundamental to understanding materials like polypropylene, which have a variety of applications ranging from packaging to automotive parts. A polymer is a large molecule composed of many repeated subunits, known as monomers. The structure of these chains can significantly affect the material's properties, such as strength, flexibility, and melting point.

For polypropylene, the monomer unit is propylene. When propylene monomers are linked together, they form a polypropylene polymer chain. Each carbon atom in the chain has two hydrogen atoms attached and a third bond connects to a methyl group (–CH₃). The different spatial arrangements of the methyl group along the polymer chain result in different types of polypropylene: isotactic, syndiotactic, and atactic.

The unique properties of polypropylene arise from the way these chains interact with each other. For instance, the crystallinity of the polymer can be altered by the chain configuration, which then influences the transparency, melting point, and durability. Understanding the polymer chain structure is essential for tailoring materials to specific applications.
Syndiotactic Polypropylene
Syndiotactic polypropylene is one type of polymer configuration where the methyl groups (–CH₃) are positioned alternately on opposite sides of the polymer backbone. Visualize this as a zigzag line, with the methyl groups alternating from left to right as you move along the chain.

Such a configuration leads to a structure that is somewhat crystalline, which provides syndiotactic polypropylene with good clarity and a moderate melting point. Syndiotactic polypropylene is less common than the other two types but offers unique characteristics that make it suitable for certain applications that require clarity combined with resistance to chemicals and solvents.
Atactic Polypropylene
Atactic polypropylene stands out due to its random positioning of the methyl groups. There is no regular pattern, and as a result, this type of polypropylene has a highly amorphous structure. It's akin to having a box of balls of various colors thrown in without any order — unpredictable and unorganized.

The amorphous nature of atactic polypropylene means that it is less crystalline and therefore softer and more pliable. It typically has a lower melting point and is not as strong as the other forms. However, its randomness gives it an elastic quality, which is beneficial for products like sealants and adhesives where flexibility is an advantage.
Isotactic Polypropylene
Isotactic polypropylene is characterized by all the methyl groups being located on the same side of the polymer chain. Picture a straight line where all the branches extend in the same direction. This leads to a highly crystalline and well-ordered structure, contributing to this type's increased strength and higher melting point.

The orderly fashion of isotactic polypropylene allows the chains to pack closely together, resulting in a material that is tougher and has better resistance to heat. It is the most common type of polypropylene in the market because of its balance between stiffness and toughness, which makes it ideal for a wide range of applications, from plastic containers to automotive parts and textiles.

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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.

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

(a) Compute the repeat unit molecular weight of polypropylene. (b) Compute the number-average molecular weight for a polypropylene for which the degree of polymerization is 15,000

For each of the following pairs of polymers, do the following: (1) state whether or not it is possible to determine whether one polymer is more likely to crystallize than the other; (2) if it is possible, note which is the more likely and then cite reason(s) for your choice; and (3) if it is not possible to decide, then state why. (a) Linear and atactic poly(vinyl chloride); linear and isotactic polypropylene. (b) Linear and syndiotactic polypropylene; crosslinked cis-isoprene. (c) Network phenol-formaldehyde; linear and isotactic polystyrene. (d) Block poly(acrylonitrile-isoprene) copolymer; graft poly(chloroprene- isobutylene) copolymer.

The density of totally crystalline nylon 6,6 at room temperature is \(1.213 \mathrm{g} / \mathrm{cm}^{3} .\) Also, at room temperature the unit cell for this material is triclinic with lattice parameters $$\begin{array}{ll} a=0.497 \mathrm{nm} & \alpha=48.4^{\circ} \\ b=0.547 \mathrm{nm} & \beta=76.6^{\circ} \\ c=1.729 \mathrm{nm} & \gamma=62.5^{\circ} \end{array}$$ If the volume of a triclinic unit cell, \(V_{\mathrm{tri}}\), is a function of these lattice parameters as $$\begin{aligned} &V_{\mathrm{tri}}=a b c \sqrt{1-\cos ^{2} \alpha-\cos ^{2} \beta-\cos ^{2} \gamma+}\\\ &2 \cos \alpha \cos \beta \cos \gamma \end{aligned}$$ determine the number of repeat units per unit cell.
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