Chemistry 367L/392N Macromolecular Chemistry Step Growth Chain Growth Lecture 5

Chemistry 367L/392N Paul Flory Clears Things Up Polymer Structure is distinct from polymerization process

Chemistry 367L/392N Basic Types of Polymerization Mechanisms Chain-growth Step-growth Ring-opening

Chemistry 367L/392N Chain growth system The characteristic of a chain polymer is that polymer growth takes place by monomer reacting only with the reactive centers. Monomer does not react with monomer and the different-sized species such as dimer, trimer, and n-mer do not react with each other. The polymerization ceases when the active center is destroyed by termination reaction(s).

Chemistry 367L/392N A condensation takes place between two polyfunctional molecules to produce one larger polyfunctional molecule with the possible elimination of a small molecule such as water. The reaction continues until one of the reagents is used up. Step Growth system

Chemistry 367L/392N The chain growth vs. step growth Step Chain

Chemistry 367L/392N - Step-growth polymerization

Chemistry 367L/392N - Chain-growth polymerization

Chemistry 367L/392N Chain Growth Polymerization DP = N o /N = 12 / 7 = 1.7 (for 50%, b)

Chemistry 367L/392N Step Growth Polymerization DP = N o /N = 12 / 9 = 1.3 (for 50%, b)

Chemistry 367L/392N The relationship between M wt and conversion With no termination reactions The chain growth system

Chemistry 367L/392N The relationship between M wt and conversion With termination reactions The chain growth system

Chemistry 367L/392N The relationship between M wt and conversion Step growth system

Chemistry 367L/392N Distinguishing features of chain- and step-polymerization mechanisms

Chemistry 367L/392N Let’s look at this more closely…. Consider a flask of monomer ….If there are N o molecules in the flask at time = 0 and N remaining at time t then the DP at time t is the average degree of polymerization… must just be N 0 /N!

Chemistry 367L/392N The Carothers Equation If there are N o molecules at time = 0 and N remaining at time t then the amount reacted is N 0 -N and we can define p as the “conversion” or fraction reacted then as p= (N o – N ) / N o or N = N o ( 1 – P) If DP is the average degree of polymerization… N 0 /N ….substituting gives DP = 1 / (1 – P) and for P = 0.98 (98% conversion), DP = only 50! High Molecular weights are hard to get this way

Chemistry 367L/392N It all happens at the end!!! DP = 1 / (1 – P)

Chemistry 367L/392N A-A, B-B vs A-B ??? A……………………………….B A ………………….A B B

Chemistry 367L/392N Chain Growth Polymers Chain polymer growth takes place by monomer reacting only with the reactive centers. Monomer does not react with monomer and species such as dimer and trimer do not react with each other. The polymerization ceases when the active center is destroyed by termination reactions. Chain polymer growth takes place by monomer reacting only with the reactive centers. Monomer does not react with monomer and species such as dimer and trimer do not react with each other. The polymerization ceases when the active center is destroyed by termination reactions. Reactive intermediates in chain-growth polymerizations include radicals, carbanions, carbocations, and organometallic complexes

Chemistry 367L/392N Radical Chain-Growth Polymers Look at classical example of polymerizations of ethylene and substituted ethylenes Look at classical example of polymerizations of ethylene and substituted ethylenes R An alkene R n

Chemistry 367L/392N Radical Chain Growth Polymerization Among the initiators used for radical chain-growth polymerization are diacyl peroxides, which decompose as shown on mild heating

Chemistry 367L/392N Benzoyl peroxide is a popular pharmaceutical

Chemistry 367L/392N Radical Chain Growth Polymerization Another common class of initiators are azo compounds, which also decompose on mild heating or with absorption of UV light Another common class of initiators are azo compounds, which also decompose on mild heating or with absorption of UV light Azoisobutyronitrile (AIBN)  or h NN N C N CN N CN + 2 Alkyl radicals

Chemistry 367L/392N Initiation

Propagation

Termination Disproportionation Coupling +