25: Polymersynthesis
- Page ID
- 148286
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Name: ______________________________
Section: _____________________________
Student ID#:__________________________
Introduction to Polymer Synthesis
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three- dimensional networks.
There are two prominent approaches to polymerization:
- Chain Growth
Chain Growth usually involves 3 different types of steps: Initiation, propagation, and termination.
- Label each type of reaction above as one of these types of steps.
- Step Growth:
The easiest way to visualize the mechanism of a step-growth polymerization is a group of people reaching out to hold their hands to form a human chain — each person has two hands (= reactive sites).
- Does Step Growth polymerization have initiation or termination steps?
Polymerization Processes
- Complete the following table comparing the two types of polymerization processes.
Characteristic Step Growth Chain Growth Growth Style? Rapid Loss of Monomer? Long chains formed quickly? Chains Active after
T ermination?Initiator Required? - Which line on the graph would best represent the growth rate of
- step growth polymerization?
- chain growth polymerization?
Molecular Weight Control and Polydispersivity
In the synthesis of polymers, one is usually interested in obtaining a product of very specific molecular weight, since the properties of the polymer will usually be highly dependent on molecular weight.
Degree of polymerization refers to the number of monomers linked together in a chain.
- In the above samples, what is the average degree of polymerization?
Left: Right:
Suppose each monomer has molecular weight of 100 g/mol.
- What would be the molecular weight of a dimer? A trimer?
- What is the average molecular weight of the polymers?
Left: Right:
- Why do we have to deal with average molecular weights instead of just molecular weights?
One of the samples has more uniform molecular weights. One has a wider distribution of molecular weights.
- Which one is which?
The width of distribution is referred to as polydispersity. In a sample with polydispersity = 1.0, all the polymers have very close to the same molecular weight. Many commercially available polymers have wider polydispersities (between 1.0 and 2.0).
Quenching Polymerization
Since the degree of polymerization is a function of reaction time, the desired molecular weight can be obtained by quenching the reaction at the appropriate time.
- In which type of growth would it be easier to stop the reaction at the desired high molecular weight: step growth or chain growth? Explain.
Polyesters can be made through step growth polymerization.
- Show how these monomers would lead to chain formation. Draw the polymer product of two of the acid chlorides and two of the diols.
- What would happen if the following alcohol were added instead of the diol above?
- How might this alcohol be useful in controlling molecular weight of a growing polymer?
Trans-esterification reactions can result in a drastic drop in molecular weight of a growing polyester chain.
- Show how, using butanol and a short polyester chain. Remember that an ester is also an electrophilic center.
