User:Robertkim1999/sandbox/depolymerizable polymers

Depolymerizable polymers are molecules made of repeating smaller units called monomers that are able to break down into these units in the presence of certain stimuli such as heat, light, and pH. Monomers can be thought of as individual links, and a polymer as a chain of these links. Temperature sensitive polymers in particular each have a unique ceiling temperature where the polymer is being broken down as fast as it is being built up. At temperatures higher than this, they begin to decompose.

As the consumer demand for electronics and other related disposable products increases, so does the need for recyclable materials. Depolymerizable polymers have many potential applications in biodegradable electronics, as well as structural and recyclable materials.

Ceiling Temperature Phenomenon
Depolymerizable polymers that are sensitive to temperature begin to break down at a certain temperature known as the ceiling temperature or Tc. At this point, the polymer is being built up from monomers as fast as it is being broken down into these units. Above the Tc, the rate of polymer deconstruction exceeds the rate of construction. At temperatures lower than Tc, the rate of polymer construction exceeds the rate of deconstruction. Each unique monomer has its own unique ceiling temperature. [2]

In 1943, Snow and Frey were the first researchers to discover the concept of a ceiling temperature by observing chemical reactions between sulfur dioxide and olefin monomers to form a polysulfone polymer.[1]

When they raised the temperature of the reaction, polysulfone was formed at a faster rate. At a certain temperature known as the ceiling temperature, polymerization (the formation of a polymer from monomers) stopped. They discovered that the rate of polymerization was equal to the rate of depolymerization at this temperature, forming an equilibrium. This is comparable to the equilibrium found at the freezing/melting point of water where water molecules are freezing as fast as they're melting.

Calculating Ceiling Temperature
We start with Gibb's free energy equation:

$$\Delta G_p = \Delta H_p - T \Delta S_p$$

where ∆Gp is equal to the change in Gibb's free energy during polymerization, ∆Hp is the change in enthalpy during polymerization, T is temperature in degrees Celsius, and ∆Sp is the change in entropy during the polymerization.

$$\Delta G = 0 $$ at  $$T_c$$

We know at the ceiling temperature, the rate of depolymerization is equal to the rate of polymerization. In other words, enthalpy is equal to entropy making the overall change in Gibb's free energy, G, zero.

$$T_c =$$ $$ \Delta Hp \over \Delta Sp$$

Solving for T gives us this equation. However, this only applies at standard pressure and monomer concentrations of 1 mole/L. These conditions may be unrealistic under real life circumstances so

$$T =$$ $$ \Delta H_p \over (\Delta S_p + Rln[M_e])$$

is a more general form of the ceiling temperature equation that includes [M]e, the monomer concentration at equilibrium at a certain temperature.

When graphed, a linear relationship appears for 1/T vs ln[M]e, giving us the mathematical model

$$ ln[M_e] =$$ $$ \left ( \frac{\Delta H_p}{R} \right ) \left ( \frac{1}{T} \right ) - \left ( \frac{\Delta S_p}{R} \right )   $$

The slope is represented by (∆Hp /R) and the y-intercept is -(∆Sp /R) This model allows scientist to make a fairly accurate estimate of a polymer's ceiling temperature.

Stabilization
What makes depolymerizable polymers special is their ability to break down into their respective monomers at relatively low temperatures. Due to this, scientists needed to develop a way to prevent these polymers from breaking down at unwanted temperatures in order to make them useful.

"End-capping" has proven to be the most successful method in halting depolymerization. The polymer end caps restrict the access of depolymerizing agents, stopping the polymer from breaking down and stabilizing it at very high temperatures.

An alternative to end-capping is constructing the polymer with small amounts of certain monomers that do not depolymerize. In the presence of a stimulus, the polymer will begin to unzip until reaching one of these "unzippable" monomers, effectively stopping depolymerization. The unzipping of a polymer can be compared to depolymerization with kinks in the zipper corresponding to monomers that do not depolymerize. These polymers will never be able to fully break down into its original monomers but will break down into smaller chains instead.[2]

Current Challenges
The biggest challenge for scientists researching depolymerizable polymers and their uses is to make these polymers structurally and chemically stable under all conditions except their respective stimuli. Realistically, conditions are always changing in our world and it's very difficult to engineer a material to break apart on command in the presence of a stimulus, yet be structurally sound under all other circumstances. [2]

Possible Applications

 * transient electronics
 * lithography
 * triggerable delivery vehicles (eg. pill capsules)


 * facilitation in the production and breakdown of recyclable materials
 * adaptive structural material (eg. regeneration, shape-shifting, patterning)

One application of depolymerizable polymers is in transient or biodegradable electronics. This includes any technology with components that disappear partially or completely at a certain rate or time.[4] Over the past several decades in the electronics industry, the research focus has been on durable polymeric materials. Today, however, fast growing consumer electronic products and environmental concerns about electronic wastes has increased the need for sustainable, recyclable materials. For example, a group of researchers employed light-sensitive poly(phthalaldehyde) as substrate materials for circuits. The destruction of the polymer substrate was then triggered by UV irradiation (~379 nm).[1]

Another use for depolymerizable polymers is in structural materials to be used in construction or consumer products. Being able to control when a polymer breaks up and when it builds itself back up introduces new features such as regenerative or shape-shifting capabilities for traditional structural materials. Poly(phthalaldehyde) is one example of a polymer that is capable of breaking down in the presence of ultrasonication, and spontaneously building itself back up afterwards.[1]