What characterizes a co-polymer?

A co-polymer is characterized by being made up of two or more different types of repeating units, commonly referred to as "mers," that are combined along the same polymer chain. This distinction allows for a greater variety of physical and chemical properties compared to homopolymers, which consist of only one type of mer. The combination of different mers can lead to tailored materials with specific characteristics suited for various applications, such as improved flexibility, strength, or resistance to environmental factors.

In contrast, a polymer formed from a single type of mer represents a homopolymer and does not fit the definition of a co-polymer. High crystalline structure refers to the arrangement of molecular chains in a polymer, which can apply to both types of polymers but is not a defining characteristic of co-polymers specifically. Thermosetting polymers are a category based on their behavior under heat application; they harden permanently and do not melt, but again, this does not specifically pertain to the classification of a co-polymer. Thus, the first choice accurately captures the essence of what a co-polymer is.