Question

How do you determine the active center of a monomer/molecule?

Answer 1

The study of polymer science begins with understanding the methods in which these materials are synthesized. Polymer synthesis is a complex procedure and can take place in a variety of ways. Addition polymerization describes the method where monomers are added one by one to an active site on the growing chain.

Addition Polymerization

The most common type of addition polymerization is free radical polymerization. A free radical is simply a molecule with an unpaired electron. The tendency for this free radical to gain an additional electron in order to form a pair makes it highly reactive so that it breaks the bond on another molecule by stealing an electron, learing that molecule with an unpaired election (which is another free radical). Free radicals are often created by the division of a molecule (known as an initiator) into two fragments along a single bond. The following diagram shows the formation of a radical from its initiator, in this case benzoyl peroxide.

The stability of a radical refers to the molecule's tendency to react with other compounds. An unstable radical will readily combine with many different molecules. However a stable radical will not easily interact with other chemical substances. The stability of free radicals can vary widely depending on the properties of the molecule. The active center is the location of the unpaired electron on the radical because this is where the reaction takes place. In free radical polymerization, the radical attacks one monomer, and the electron migrates to another part of the molecule. This newly formed radical attacks another monomer and the process is repeated. Thus the active center moves down the chain as the polymerization occurs.

There are three significant reactions that take place in addition polymerization: initiation (birth), propagation (growth), and termination (death). These separate steps are explained below.

Initiation Reaction

The first step in producing polymers by free radical polymerization is initiation. This step begins when an initiator decomposes into free radicals in the presence of monomers. The instability of carbon-carbon double bonds in the monomer makes them susceptible to reaction with the unpaired electrons in the radical. In this reaction, the active center of the radical "grabs" one of the electrons from the double bond of the monomer, leaving an unpaired electron to appear as a new active center at the end of the chain. Addition can occur at either end of the monomer. This process is illustrated in the following animation in which a chlorine atom possessing an unpaired electron (often indicated as cl-) initiates the reaction. As it collides with an ethylene molecule, it attracts one of the ethylene's pair of pi bonded electrons in forming a bond with one of the carbons. The other pi electron becomes the active center able to repeat this process with another ethylene molecule. The sigma bond between the carbons of the ethylene is not disturbed. (Note that a molecular orbital model is employed here in describing this process. See any introductory college chemistry text for further discussion)

In a typical synthesis, between 60% and 100% of the free radicals undergo an initiation reaction with a monomer. The remaining radicals may join with each other or with an impurity instead of with a monomer. "Self destruction" of free radicals is a major hindrance to the initiation reaction. By controlling the monomer to radical ratio, this problem can be reduced.

Propagation Reaction

After a synthesis reaction has been initiated, the propagation reaction takes over. In the propagation stage, the process of electron transfer and consequent motion of the active center down the chain proceeds. In this diagram, (chain) refers to a chain of connected monomers, and X refers to a substituent group (a molecular fragment) specific to the monomer. For example, if X were a methyl group, the monomer would be propylene and the polymer, polypropylene.

In free radical polymerization, the entire propagation reaction usually takes place within a fraction of a second. Thousands of monomers are added to the chain within this time. The entire process stops when the termination reaction occurs.

Termination Reaction

In theory, the propagation reaction could continue until the supply of monomers is exhausted. However, this outcome is very unlikely. Most often the growth of a polymer chain is halted by the termination reaction. Termination typically occurs in two ways:combination and disproportionation.

Combination occurs when the polymer's growth is stopped by free electrons from two growing chains that join and form a single chain. The following diagram depicts combination, with the symbol (R)representing the rest of the chain.

Disproportionation halts the propagation reaction when a free radical strips a hydrogen atom from an active chain. A carbon-carbon double bond takes the place of the missing hydrogen. Termination by disproportionation is shown in the diagram.

Disproportionation can also occur when the radical reacts with an impurity. This is why it is so important that polymerization be carried out under very clean conditions.

Living Polymerization

There exists a type of addition polymerization that does not undergo a termination reaction. This so-called "living polymerization" continues until the monomer supply has been exhausted. When this happens, the free radicals become less active due to interactions with solvent molecules. If more monomers are added to the solution, the polymerization will resume.

Uniform molecular weights (low polydispersity) are characteristic of living polymerization. Because the supply of monomers is controlled, the chain length can be manipulated to serve the needs of a specific application. This assumes that the initiator is 100% efficient.

Statistical Analysis of Polymers

When dealing with millions of molecules in a tiny droplet, statistical methods must be employed to make generalizations about the characteristics of the polymer. It can be assumed in polymer synthesis, each chain reacts independently.

Therefore, the bulk polymer is characterized by a wide distribution of molecular weights and chain lengths. The degree of polymerization (DP) refers to the number of repeat units in the chain, and gives a measure of molecular weight. Many important properties of the final result are determined primarily from the distribution of lengths and the degree of polymerization. The following simulation allows you to examine the distribution of chain lengths under varying conditions.

Statistical Polymer Growth

In order to characterize the distribution of polymer lengths in a sample, two parameters are defined: number average and weight average molecular weight. The number average is just the sum of individual molecular weights divided by the number of polymers. The weight average is proportional to the square of the molecular weight. Therefore, the weight average is always larger than the number average. The following graph shows a typical distribution of polymers including the weight and number average molecular weights.

The molecular weight of a polymer can also be represented by the viscosity average molecular weight. This form of the molecular weight is found as a function of the viscosity of the polymer in solution (viscosity determines the rate at which the solution flows - the slower a solution moves, the more viscous it is said to be - and the polymer molecular weight influences the viscosity). The following simulation allows you to calculate the viscosity of a polymer solution, and use the data you find to produce the viscosity average molecular weight.

Viscosity Measurements

The degree of polymerization has a dramatic effect on the mechanical properties of a polymer. As chain length increases, mechanical properties such as ductility, tensile strength, and hardness rise sharply and eventually level off. This is schematically illustrated by the blue curve in the figure below.

However, in polymer melts, for example, the flow viscosity at a given temperature rises rapidly with increasing DP for all polymers, as shown by the red curve in the diagram.