Polymers

=Introduction:=

Polymers are everywhere. Look down at your Vitamin Water. Yep. There's a polymer. Look at your car. There's another one. Even look at clothes you are wearing. More polymers. But what exactly are they? Well first picture a tiny little Lego, all alone in the world. But the Lego meets some friends, and they all join hands in one big line to play Red Rover on the playground. The little line of Red Rover playing Legos is like a polymer. Each Lego is a single unit of the polymer, or monomer, and when they all join together, they create a polymer, or long line of monomers joined together. There are two general types of polymers: Those created synthetically, and those occurring naturally. Within each subcatagory, there are either homopolymers, polymers consisting of a single monomer, or copolymers, which are polymers that consist of multiple monomers.

=Addition Polymers=

The simplest form of a polymer is an addition polymer. Its easiest to explain with an example, so that's how I will proceed. Consider Ethylene, C2H4, structure shown in the graphic below. Suppose that the electrons forming the pi bond in the center of the molecule split apart. The two electrons contained in that bond will seek to repel each other as much as possible, so each will move to the end of a separate carbon. If there are other Ethylene molecules in the area, the free electrons from two neighboring Carbons will bond together and form an intermolecular covalent bond. Therefore, two monomers of the same type will have been connected creating a polymer. This process is shown below.


 * Therefore, from that example, it can be seen that addition polymers are polymers that occur through the coupling of monomers using their multiple bonds.** This chain of monomers may repeat itself for many hundreds or thousands of times.

=Condensation Polymers:=

Another type of reaction used to synthesize commercial polymers is condensation polymerization. In condensation polymerization, two molecules are joined into a monomer by elimination of a molecule, most likely water. Many proteins and organic polymers are condensation polymers. Notice how between each bond, two hydrogens and an oxygen have been eliminated, and also how in condensation polymers, the central chain of atoms is often not comprised of Carbon atoms. An example of condensation polymerization is shown below:



=Properties of Polymers=


 * Types of Polymers:**

Plastics- Materials that can be formed into different shapes, by application of heat and pressure Thermoplastics - Plastics that can be reshaped easily. Often used in recyclables Thermosetting plastics - Plastics that are shaped through irreversible chemical processes and cannot be reshaped. Elastomer - Polymer that when stretched or bent, returns to its original shape.


 * Chain Linearity:**

The simplest form of a polymer is a straight polymer, where there is a single chain at the center of the polymeric structure. Therefore the atoms are arranged around the central C-C chain in a tetrahedral fashion, making the atoms free to rotate around the C-C chain. Therefore, these polymers are very flexible, and often linear polymers fold and bend. The flexibility of a linear polymer is characterized by its persistence length. If a polymer has several chains or "branches" diverging from the main C-C chain, it is considered a branch polymer. Branched polymers can form special arrangements like stars, "combs" or grafts.


 * Chain Size:**

When expressing the size of a mere molecule, often molar mass is used. In polymers, where the molar mass of a single chain may be in the thousands of atomic units, it is often easier to use the degree of polymerization, or essentially the number of times a monomer is repeated within a polymer. Using the degree of polymerization, it is also easier to notate polymers using degree of polymerization. Instead of writing out the entire molecular formula, the formula of the monomer is written/drawn in parenthesis followed by a subscript n, where n is the degree of polymerization. For a graphical representation, see below.



In almost all polymers manufactured synthetically, it is almost impossible to produce the exact same chain length. So to express the average molar mass, the arithmetic mean of all the chain lengths is taken. This number is called the number average molecular weight. To express the average weight of a polymer, the following formula is used. The ratio of these two numbers is called the polydispersity index of the polymer or the "width" of the molecular weights of a certain polymer.


 * Crosslinking and Crystallinity:**

Imagine polymers are long strands of spaghetti. Sure, they are nice and pliable, but they don't really stick together well at all. Crosslinking is the process by which multiple chains of a polymeric material are connected to one another. If a substance has hydrogen bonding capability, different physical processes, such as freezing in an aqueous medium, can be used to place the chains in close enough proximity to one another that they bond. Also, chemical agents can be used that will react with parts of monomers from two or more chains and join them together. Also, processes such as radiation, or vulcanization can induce crosslinks within certain polymers. Increasing the amount of crosslinks increases the rigidity or crystallinity of the polymeric material. Uncrosslinked polymers are generally considered amorphous materials for the same reasons they flex easily. However, when crosslinking the polymer, crystalline regions can be induced. Increased crystallinity leads to increasing density, mechanical strength, and decreasing solubility.



A really simple way to understand the crystallinity of polymers is to think of the properties of rubber and glass. In a "rubbery" (uncrystalline) polymer, the polymer is generally squishy, pliable, and elastic. However, in a glassy polymer, the material is hard, prone to shattering, and has large tensile strength. So just think, glass is crystalline, rubber is uncrystalline. Ironically, glass is considered a relatively amorphous solid when compared to metallic and ionic substances, but not when compared to polymers.

Picture Credits (in order of appearance): http://www.sci.usq.edu.au/staff/robertsa/thing2.jpg [|http://www.elmhurst.edu/~chm/vchembook/401addpolymers.html] http://chemed.chem.purdue.edu/genchem/topicreview/bp/1polymer/graphics/13.gif http://www.zeusinc.com/newsletter/chemical_resistance.asp http://en.wikipedia.org/wiki/Weight_average_molecular_weight http://www.leedsteachinghospitals.com/sites/optometry/services/images/crosslinking_more.gif

Factual Sources: http://en.wikipedia.org/wiki/Polymer Bursten, Brown, & LeMay. __Chemistry: The Central Science, 9th Edition. My life.__