Polymers - Basic Introduction

This paragraph introduces the concept of polymers as large macromolecules made up of many smaller units called monomers. It explains the prefixes 'mono' and 'poly' and illustrates how monomers combine during polymerization to form a polymer. The paragraph emphasizes that the repeating units in a polymer, rather than the end groups, determine its physical properties. The size of a polymer can range from hundreds to tens of thousands of monomer units. The discussion also touches on different types of polymerization, such as free radical, ionic, and cationic, and how they can affect the identity of the end groups of the polymer.

The second paragraph provides examples of natural polymers found in biology, such as DNA (whose monomer is nucleotides), proteins (with amino acids as monomers), and polysaccharides like starch and cellulose (with glucose as the monomer). It also introduces ethylene, or ethene, as a common monomer in organic chemistry that polymerizes to form polyethylene, an alkane with the 'poly' prefix indicating its monomer is an alkene. Substituted ethylene molecules are also discussed, with an example of how to draw the repeating unit for a substituted ethylene derivative, such as ethyl chloride, which polymerizes to form PVC (polyvinyl chloride).

This paragraph delves into the radical polymerization mechanism, starting with the initiator, typically an organic peroxide that generates radicals when subjected to UV light or heat. The radicals react with ethyl chloride, leading to the formation of a new radical that can propagate the chain by reacting with more ethyl chloride molecules. The process is illustrated step by step, showing how the chain grows and the repeating unit of PVC emerges. Termination of the reaction is also briefly mentioned, where radicals can combine to end the polymerization.

The fourth paragraph discusses cationic polymerization, which uses electrophiles to initiate the reaction. An example of this is the use of boron trifluoride in the presence of water to generate an electrophile. The reaction is demonstrated using methyl vinyl ether as the monomer, which acts as a nucleophile and reacts with the electrophile to form a carbocation. The carbocation then reacts with another monomer, and the process is repeated to build the polymer chain. The paragraph explains how certain R groups can stabilize carbocations and thus favor cationic polymerization.

The fifth paragraph covers anionic polymerization, which uses nucleophiles like sodium amide to initiate the reaction. The nucleophile attacks the CH2 group of styrene, a substituted ethylene monomer with a phenyl group, resulting in the formation of a new negative charge that can stabilize on the benzene ring through resonance. The process is shown to continue with the addition of more styrene molecules, building the polymer chain and revealing the repeating unit. The paragraph also explains how certain R groups can influence the preference for anionic over cationic polymerization.

The final paragraph concludes the video by summarizing the different forms of polymerization discussed, including radical, cationic, and anionic polymerization. It mentions that there are other types of polymerization, such as condensation polymerization, which may be covered in a future video. The paragraph serves as a recap of the key points made throughout the script, reinforcing the understanding of polymerization mechanisms and their applications.