Problem 16 - Introductory Chemistry Atoms First

Question

This polymer is called silicone and has a noncarbon, "inorganic" backbone of alternating silicon and oxygen atoms: (a) Put a set of parentheses around one monomer unit. (b) Another name for this polymer is polydimethylsiloxane. Explain why it has this name.

Step-by-Step Solution

Answer

(a) One monomer unit of silicone can be represented as: \((\text{CH}_3\text{SiO})\). (b) The name "polydimethylsiloxane" comes from: "Poly" referring to a polymer, "dimethyl" for the two methyl (CH3) groups bonded to each silicon atom, and "siloxane" for the alternating silicon and oxygen atoms forming a siloxane bond (-Si-O-).

Step 1: Identify silicone's monomer unit

To identify the monomer unit for silicone, look at its given structure, focusing on the pattern of alternating silicon (Si) and oxygen (O) atoms, with two methyl groups (CH3) bonded to each Si atom. Observe the repeating units that make up the structure and place parentheses around a single unit.

Step 2: Name: Polydimethylsiloxane

The name "polydimethylsiloxane" is derived from different parts of the polymer's structure: 1. "Poly" indicates that the molecule is a polymer, composed of a repetition of monomer units. 2. "Dimethyl" accounts for the two methyl (CH3) groups bonded to each silicon atom in the structure. 3. "Siloxane" is a term that originates from the combination of "silicon" and "oxygen" atoms (alternating Si and O) in the polymer backbone, forming a siloxane bond: -Si-O-. Put together, the name "polydimethylsiloxane" accurately describes the structure of this polymer.

Key Concepts

Monomer Unit

In polymers, a monomer unit is the smallest structural unit that repeats to form the larger polymer chain. For silicone polymers, this monomer consists specifically of silicon and oxygen atoms. The neat arrangement involves alternating these atoms, producing a distinctive pattern.

To picture a monomer unit in such a polymer, imagine a single segment from the long chain. In silicone, particularly, each silicon atom bonds to two methyl groups (CH₃). This forms a structure that looks like this:

• Silicon atom (Si)
• Methyl group (CH₃) bonded to silicon
• Oxygen atom (O)
• Another methyl group (CH₃) bonded to silicon

When you put parentheses around this unit in diagrams, you're highlighting this repeating pattern. It's what makes the polymer a true "chain" - a fundamental feature of silicone polymers.

Polydimethylsiloxane

Polydimethylsiloxane, often abbreviated as PDMS, is a specific type of silicone polymer. Its name offers insights into its unique chemical structure. Let's break it down:

• "Poly": This prefix highlights that the substance is a polymer – many monomer units are connected together into a long chain.
• "Dimethyl": This part addresses the two methyl groups (CH₃) bonded to each silicon atom, explaining the "di-" prefix in dimethyl.
• "Siloxane": A blend of "silicon" and "oxygen." This denotation refers to the alternating sequence of these elements in the polymer backbone.

When these components combine, they describe the structure of the material without needing to visualize its structural formula.

PDMS is renowned for its flexibility and unique physicochemical properties, derived from its specific arrangement of silicon, oxygen, and organic groups.

Silicon-Oxygen Backbone

The silicon-oxygen backbone is a distinctive feature of silicone polymers. Unlike carbon-based polymers, silicones have their main chain composed of alternating silicon and oxygen atoms.

The alternating nature of silicon (Si) and oxygen (O) provides silicones with unique attributes, such as:

• Flexibility: The Si-O bond length and nature contribute to high flexibility.
• Thermal Stability: Silicones withstand extreme temperatures due to the strength of the Si-O bond.
• Chemical Resistance: This backbone is more inert than carbon-based polymer backbones, granting resistance to many chemical reactions.

These properties are significant in applications ranging from medical devices to sealants and lubricants. The backdrop of oxygen with silicon in the backbone, replacing the traditional carbon, is why silicones are also referred to as inorganic polymers.