The flexibility of addition polymers depends on several factors related to their molecular structure. Firstly, the length of the polymer chains plays a significant role. Longer chains can slide past each other more easily, imparting more flexibility to the polymer. Secondly, the presence of side groups affects flexibility. Bulky or rigid side groups can hinder the movement of the polymer chains, making the material stiffer. In contrast, small and flexible side groups allow for easier chain movement, resulting in a more flexible polymer. The third factor is the type of intermolecular forces present in the polymer. Polymers with strong intermolecular forces, like hydrogen bonding or polar interactions, tend to be less flexible due to the tight hold these forces have on the chains. Conversely, polymers with weaker van der Waals forces exhibit greater flexibility. The arrangement of the polymer chains also influences flexibility; highly ordered, crystalline regions in a polymer reduce flexibility, while amorphous regions (where chains are arranged randomly) increase it. Finally, cross-linking within the polymer can drastically reduce flexibility. Cross-linked polymers, where the chains are chemically bonded to each other at various points, have a rigid, networked structure, limiting the movement of the chains and thus reducing flexibility.

The presence of different functional groups in monomers significantly influences the properties of the resulting polymers. Functional groups are specific groups of atoms within molecules that have characteristic chemical behaviours. In polymers, these groups can affect properties like solubility, melting point, chemical reactivity, and mechanical strength. For example, a polymer derived from a monomer with polar functional groups, such as -OH or -COOH, tends to be more hydrophilic and may exhibit higher solubility in water. These polymers can also form hydrogen bonds, which can increase their strength and melting point. On the other hand, nonpolar groups like -CH₃ or -C₆H₅ (methyl and phenyl groups, respectively) can make the polymer more hydrophobic and soluble in nonpolar solvents. Additionally, certain functional groups can introduce cross-linking opportunities within the polymer, leading to a three-dimensional network structure that greatly enhances the polymer's strength and heat resistance. For instance, monomers with multiple reactive sites can form cross-linked polymers, as seen in vulcanized rubber, which is much tougher and more durable than its non-cross-linked counterpart. In essence, the functional groups in monomers are key determinants of the final polymer's physical and chemical characteristics.

The degree of polymerisation, which refers to the number of repeating units in a polymer chain, has a profound impact on the properties of the polymer. Generally, as the degree of polymerisation increases, the molecular weight of the polymer also increases, which in turn affects its mechanical properties. For instance, polymers with a high degree of polymerisation typically exhibit greater tensile strength and toughness. This is because longer chains have more entanglements, providing better load distribution and resistance to breakage under stress. Additionally, the melting point and thermal stability of the polymer tend to increase with higher molecular weight. However, solubility tends to decrease with increasing molecular weight, as the polymer becomes too large to dissolve easily in solvents. The degree of polymerisation also influences the polymer's viscosity in its molten state – higher molecular weight polymers are more viscous, which can affect processing techniques like injection moulding or extrusion. It's also noteworthy that polymers with very high degrees of polymerisation can become difficult to process due to their high viscosity and melting points. Therefore, controlling the degree of polymerisation is crucial in tailoring the properties of a polymer for specific applications.

Addition polymers can be recycled, but the process presents several challenges. The primary challenge is the sorting of polymers, as different types of plastics need to be separated due to their varying properties and melting points. Contamination is another significant issue; even small amounts of different polymers or other materials can degrade the quality of the recycled plastic. Additionally, the recycling process often leads to some degree of polymer degradation, which can affect the mechanical properties of the recycled material. For example, during thermal processing, polymers may undergo chain scission, leading to shorter polymer chains and resulting in a less durable material. To mitigate this, additives and stabilisers are often used during recycling. However, repeated recycling can progressively diminish the quality of the plastic. Despite these challenges, recycling of addition polymers remains a crucial endeavour for environmental sustainability, as it reduces waste and conserves resources. Advances in chemical recycling methods, where polymers are broken down to their monomers and repolymerised, hold promise for more efficient recycling with less degradation of material properties.

The structure of the monomer plays a crucial role in determining the physical and chemical properties of the resulting polymer. The size, shape, and functional groups present in the monomer can greatly influence the polymer's characteristics. For example, if the monomer contains bulky side groups, the resulting polymer will have a more irregular structure, potentially leading to lower density and higher flexibility. The presence of polar functional groups in the monomer can increase the polymer's polarity, affecting its solubility and interaction with other substances. Furthermore, the type of bonding (single, double, or triple bonds) and the arrangement of atoms within the monomer can determine the strength, elasticity, and thermal stability of the polymer. For instance, monomers with double bonds undergoing addition polymerisation usually result in saturated polymers, which tend to be more rigid and have higher melting points compared to those derived from single-bonded monomers. In summary, the monomer's structure dictates the polymer's molecular architecture, which in turn influences its physical and chemical behavior.