Macromolecular Chemistry Lecture 5 Step Growth Chain Growth
Paul Flory Clears Things Up Polymer Structure is distinct from polymerization process Addition Polymerization H H Condensation Polymerization H H "Condensation" Polymer
Basic Types of Polymerization Mechanisms Step-growth Dimer Tetramer ctamer Hexamer... H C CH 2 CH 2 H H C CH 2 CH 2 H H 2 Chain-growth Ring-opening R - + - R H
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).
Step Growth system 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.
The chain growth vs. step growth Step Chain
- Step-growth polymerization
- Chain-growth polymerization
Chain Growth Polymerization 1 1 2 3 4 5 6 7 DP = N o /N = 12 / 7 = 1.7 (for 50%, b)
Step Growth Polymerization DP = N o /N = 12 / 9 = 1.3 (for 50%, b)
The chain growth system The relationship between M wt and conversion With no termination reactions
The chain growth system The relationship between M wt and conversion With termination reactions
Step growth system The relationship between M wt and conversion
Distinguishing features of chain- and step-polymerization polymerization mechanisms
Let s 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!
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
It all happens at the end!!! DP = 1 / (1 P)
A-A, A, B-B B vs A-B B??? Cl C H 2 C H 2 C A.A C Cl + H-CH 2 -CH 2 -H.A B...B Cl C H 2 C H 2 C C CH 2 -CH 2 -H A.B
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. Reactive intermediates in chain-growth polymerizations include radicals, carbanions, carbocations, and organometallic complexes
Radical Chain-Growth Polymers Look at classical example of polymerizations of ethylene and substituted ethylenes R An alkene R n n
Radical Chain Growth Polymerization Among the initiators used for radical chain-growth polymerization are diacyl peroxides, which decompose as shown on mild heating Dibenzoyl peroxide 2 2 + 2 C 2 A benzoyloxy radical A phenyl radical
Benzoyl peroxide is a popular pharmaceutical
Radical Chain Growth Polymerization Another common class of initiators are azo compounds, which also decompose on mild heating or with absorption of UV light N N N C C N Azoisobutyronitrile (AIBN) or hν 2 + N N N C Alkyl radicals
Initiation CH H 2 C CH 2 CH
Propagation CH 2 CH H 2 C CH CH 2 CH H 2 C CH
Termination Coupling Ph * n n Ph * R R R R 2 Ph * n R R 2 Ph * n + H 3C n Ph Disproportionation R R R