D07. Use ethylene as an example to describe the following processes related to addition polymerization: activation, the four stages of polymerization, and inhibition.

1. Induction (Activation and Initiation)

2. Propagation

3. Termination

4. Chain transfer

5. Inhibition

1. Induction

Activation

The activation step implies that something is introduced to start the polymerization process. Activation can start by exposing the ethylene to energy (e.g. x-rays and heat), or by incorporating chemicals that can form free radicals when they are activated. For example, by mixing two pastes, one containing a tertiary amine and one benzoyl peroxide, the tertiary amine will disturb the chemical bond in the benzoyl peroxide to such an extent that this bond breaks. When this bond breaks, the benzoyl peroxide forms two free radicals, which interact with the carbon double bonds of the ethylene. The tertiary amine on the other hand does not react with any molecule and forms chemical bonds; it is just present to activate the free radical formation. One can also avoid using a tertiary amine, and just use benzoyl peroxide and heat to polymerize the ethylene. When such a mixture is heated, the heat breaks the central bond of the benzoyl peroxide, and when this happens, free radicals form.

From the above we can conclude that chemicals (e.g. tertiary amines), heat, x-rays etc. act as activators.  

Initiation

During activation, a free radical is formed. The formation of the free radical and its instantaneous interaction with the first monomer molecules initiates the polymerization process.  

2. Propagation

When the free radical (I*) approaches a monomer molecules (in this case ethylene, H₂C = CH₂), the free electron of the free radical attracts one of the four electrons present in the carbon double bond. When that occurs, the free radical and the attracted electron forms a covalent bond, and at the same time, the third electron of the original carbon double bond starts acting as a new free radical (I - H₂C - CH₂*). From now on, the chain growth propagates.  

3. Termination

The chain growth reaction stops at a certain point. The more growing sites there are, the faster the chain termination will occur, which leads to shorter molecules. If the molecules are not long enough, the properties of the resin will be compromised.  

Question:

During the past few years, manufacturers of lasers have tried to introduce them as activators for resin polymerization. What are the pros and cons with such an approach? Consider chain length.

4. Chain transfer

Chain transfer can be described as the interaction between a growing chain and a monomer molecule and where the outcome is that the growing chain becomes a neutral molecule unable to participate further in chain propagation. The monomer molecule, on the other hand, picks up a hydrogen atom from the previously growing chain (in other words, an atom has been transferred from one molecule to another) and form a free radical able to start  interacting with other monomer molecules.  

5. Inhibition

Inhibitors added to improve storage and increase setting time

To avoid spontaneous polymerization during storage, manufacturers add different inhibitors to dental resins. Common inhibitors include hydroquinone. The function of hydroquinone is to react with any free radical that is formed. Thus, before the free radical is allowed to initiate a chain growth, the inhibitor "kills" the further growth. It is first when all hydroquinone has been consumed as chain propagation can occur. By increasing the hydroquinone concentration, the working time will be increased.

Oxygen inhibition

Oxygen inhibition works differently than hydroquinone. If chain propagation has started and oxygen molecules are present, the two oxygen atoms attach to the growing chain.

I-(CH₂-CH₂)_n*+O₂=> I-(CH₂-CH₂)_n-O-O*

The I-(CH₂-CH₂)_n-O-O* unit is not as reactive as the I-(CH₂-CH₂)_n* unit, which means that the I-(CH₂-CH₂)_n-O-O* unit will not grow as easily as other units, which means that the oxygen has inhibited further chain growth. Oxygen inhibition is important in dentistry, because oxygen inhibition can sometimes be seen as a greasy film on newly placed sealants and restorations. Oxygen inhibition is both beneficial and unfavorable. The beneficial aspect is that it makes it possible to build up light curable composites in layers and cure each increment before a new layer is added. Due to the delayed reaction of the oxygen inhibited surface of the cured layer, the new layer can then react with the oxygen inhibited layer and form chemical bonds. However, drawbacks with oxygen inhibition are increased release of poorly cured material to the adjacent tissues, softer resin (composite) surfaces which are more susceptible to wear and discoloration, and weaker bonding to a tooth surface if material shrink away from the oxygen inhibited layer during curing.