The primary difference between step growth polymers and addition polymers lies in how the polymer chains grow.
Based on the provided information:
- In a step-growth reaction, the growing chains can react with each other to form longer chains. This holds true for chains of all lengths, meaning monomers, oligomers (short chains), or even long polymer chains can react with one another.
- But in an addition polymerization, only the monomers can react with growing chains. Two growing chains can't join together the way they can in a step-growth polymerization.
Understanding Chain Growth Mechanisms
Let's break down this fundamental difference:
Step-Growth Polymerization
In step-growth polymerization, monomers react to form dimers, dimers react to form tetramers, or a monomer reacts with a dimer to form a trimer, and so on. The key characteristic, as highlighted by the reference, is that any two molecules containing the appropriate functional groups can react together, regardless of their size.
- Chain Building: The reaction proceeds step-wise throughout the reaction mixture.
- Reacting Species: Monomers, oligomers, and polymer chains can all react with each other.
- Growth Rate: Molecular weight increases relatively slowly initially but builds significantly as the reaction nears completion.
Addition Polymerization
Addition polymerization, also known as chain-growth polymerization, involves the rapid addition of monomer molecules to an active site on a growing polymer chain. This process typically involves an initiator starting a chain, followed by monomers quickly adding one after another.
- Chain Building: The reaction occurs only at the active end of a growing chain.
- Reacting Species: According to the reference, only monomers can react with growing chains. Growing chains do not react with each other.
- Growth Rate: Once initiated, chains grow very quickly, meaning high molecular weight polymer is formed early in the reaction, even while significant amounts of monomer are still present.
Key Distinction Summary
The core difference, as provided, is the ability of polymer chains to react with one another:
- Step-Growth: Growing chains can react with each other.
- Addition: Growing chains cannot react with each other; only monomers add to the growing chain.
This fundamental difference in reaction mechanism leads to variations in how molecular weight builds, the types of functional groups involved, and the presence or absence of byproducts (step-growth often produces small molecule byproducts like water, while addition generally does not). However, the critical distinction in reaction pathway, focusing on which species can link together, is the defining feature described.
