【Chemistry】 Summary Notes on Polymer Compounds (Natural and Synthetic)
Hello everyone! When you hear the term "polymer compounds," it might sound a bit intimidating. But in reality, our lives are overflowing with polymers—from the proteins that make up our bodies, to the clothes we wear every day (fibers), and even plastic bags and PET bottles (plastics).
In this chapter, we will learn the rules behind "how small molecules link together to form giant molecules." It might feel like there is a lot to memorize at first, but once you spot the patterns, this topic will become a reliable source of points on the Common Test. Let's do our best together!
1. The Basics of Polymer Compounds
First, let's learn the common rules of the polymer world.
Monomer: A small molecule that serves as the building block for a polymer.
Polymer: A giant molecule formed by linking many monomers together.
Polymerization: The reaction in which monomers are linked.
Degree of Polymerization (\( n \)): The number representing how many monomers are linked together.
【Understanding through Analogy!】
Think of a polymer as a "long chain of LEGO bricks." A single LEGO brick is a "monomer," and a long chain made of 1,000 of them linked together is a "polymer."
■ Key Points for Calculating Molecular Weight
The molecular weight \( M \) of a polymer can be expressed using the molecular weight of the monomer \( m \) and the degree of polymerization \( n \) as follows:
\( M = m \times n \)
*On the Common Test, calculation problems asking you to find this \( n \) appear frequently!
Point: Since the chains in a polymer vary in length, the molecular weight is treated as an "average molecular weight."
---2. Natural Polymer Compounds (1): Sugars (Carbohydrates)
These are polymers produced by plants and animals. Let's start with "sugars."
① Monosaccharides: Sugars that cannot be hydrolyzed further
Representative examples: Glucose, fructose, galactose
Molecular formula: \( \text{C}_6\text{H}_{12}\text{O}_6 \)
Important Note: Glucose exists in \( \alpha \) form and \( \beta \) form. If the \( \text{-OH} \) group on the 1st carbon is pointing "down," it is the \( \alpha \) form; if it is pointing "up," it is the \( \beta \) form.
② Disaccharides: Two monosaccharides linked together
Representative examples:
・Maltose (malt sugar): Glucose + Glucose
・Sucrose (table sugar): Glucose + Fructose
・Lactose (milk sugar): Glucose + Galactose
Caution: Sucrose (the main component of table sugar) does not show reducing properties! Other disaccharides have reducing properties (they reduce Fehling's solution).
③ Polysaccharides: Many monosaccharides linked together
・Starch: A polymer of \( \alpha \)-glucose. Energy storage for plants.
・Cellulose: A polymer of \( \beta \)-glucose. Found in plant cell walls (raw material for cotton and paper).
Fun Fact: Starch includes amylose, which has no branches, and amylopectin, which is branched. The chewy texture of mochi (glutinous rice) is due to its high amylopectin content!
【Summary: Sugar Checklist】
・Does it have reducing properties? (Yes, except for sucrose!)
・What is the color of the iodine-starch reaction? (Amylose: blue, amylopectin: reddish-purple, starch: blue-purple)
3. Natural Polymer Compounds (2): Amino Acids and Proteins
Our muscles, hair, enzymes, and more are all made of proteins.
① Amino Acids
Each molecule contains both an amino group (\( \text{-NH}_2 \)) and a carboxyl group (\( \text{-COOH} \)).
Natural proteins are composed primarily of \( \alpha \)-amino acids.
② Peptide Bonds
When amino acids link together by releasing a water molecule \( \text{H}_2\text{O} \) from the \( \text{-NH}_2 \) of one and the \( \text{-COOH} \) of another, it is called a condensation reaction, and the resulting bond is called a peptide bond (\( \text{-CONH-} \)).
③ Properties of Proteins
・Denaturation: The structural breakdown of a protein due to heat, acid, or base, causing a change in its properties (like how a raw egg hardens when cooked!).
・Detection Reactions (Important!):
1. Biuret reaction: Turns reddish-purple (if there are two or more peptide bonds).
2. Xanthoprotein reaction: Turns yellow with concentrated nitric acid, and orange-yellow with ammonia (for amino acids with a benzene ring).
3. Ninhydrin reaction: Turns reddish-purple to blue-purple (for those with an amino group).
Common Mistake: "All proteins return to their original state if heated." -> Wrong! Once a protein is denatured, it generally cannot be returned to its original state (it is irreversible).
---4. Synthetic Polymer Compounds
These are polymers created by humans from sources like petroleum. There are two main ways to make them.
① Addition Polymerization: Double bonds open and link up
A reaction where monomers with a double bond (\( \text{C=C} \)) link together one after another.
・Polyethylene (PE): Plastic bags, containers.
・Polyvinyl chloride (PVC): Water pipes, erasers.
・Polystyrene (PS): Styrofoam.
② Condensation Polymerization: Water (or similar) is removed as they link
・Polyamide (Nylon 66): Adipic acid + Hexamethylenediamine. The world's first synthetic fiber.
・Polyester (Polyethylene terephthalate - PET): Terephthalic acid + Ethylene glycol. Raw material for PET bottles.
③ Classification by Plastic Properties
・Thermoplastic resins: Become soft when heated (like chocolate). E.g., Polyethylene.
・Thermosetting resins: Become hard when heated (like cookies). E.g., Phenol resins.
Point: If you get confused about which is which, remember that the "set" or "hard" in the name indicates the type that hardens into a solid when heated!
---5. Synthetic Rubber
Rubber is also a type of polymer compound.
・Raw rubber (Natural rubber): A polymer of isoprene.
・Cross-linking: Adding sulfur to raw rubber and heating it increases its elasticity. This process is called vulcanization.
Fun Fact: Tires are black because carbon (carbon black) is mixed in to reinforce the rubber.
---★ Common Test Prep: Don't forget these!
1. Get used to calculation problems: Make sure to practice the calculation for "finding the degree of polymerization by adding atomic weights."
2. Match structural formulas with names: Memorize the shapes of representative monomers like terephthalic acid and adipic acid.
3. Memorize reaction colors: The iodine-starch reaction, Fehling's reaction, and protein detection reactions are easy points.
At first, the structural formulas might look complicated and make you feel overwhelmed, but after looking at them repeatedly, you'll start to recognize them like old friends—like saying, "Oh, it's adipic acid again!"
Don't rush, take it one step at a time. I'm rooting for you!