Lecture 27 More Polymers Step Chain April 25, 2018
Where: MEZ 1.306!! Final Exam When: Friday, May 11 th, 2:00 5:00 PM Do: Study lecture notes, homework, reading Practice: Hydrolysis, signatures and synthesis. Review: Spectroscopy and unknowns Please: Do a good job! See web site for old exams
MEZES Hall (MEZ) -
Basic Types of Polymerization Mechanisms Step-growth Dimer Tetramer Octamer Hexamer O... O HO C CH 2 CH 2 OH H O C CH 2 CH 2 O H H 2 O Chain-growth O Ring-opening RO - + O O O - RO O O H
Chain-Growth Polymers Chain-growth polymerization: a polymerization that involves sequential addition reactions, either to unsaturated monomers or to monomers possessing other reactive functional groups Reactive intermediates in chain-growth polymerizations include radicals, carbanions, carbocations, and organometallic complexes
Chain-Growth Polymers We will concentrate on chain-growth polymerizations of ethylene and substituted ethylenes R An alkene R n n
Radical Chain Growth Polymerization Initiation Thermolysis or phtolysis and reaction with monomer Propagation Chain end reaction with monomer Termination Chain Transfer
MALDI Mass Spectrum of Polystyrene
Molecular Weight All polymers are mixtures of individual polymer molecules of variable MWs Number average Mn: count the number of chains of a particular MW, multiply each number by the MW, sum these values, and divide by the total number of polymer chains MiNi Ni weight average Mw: record the weight of each chain of a particular length, sum these weights, and divide by the total weight of the sample Mw Mn WiMi Wi NiMi 2 NiMi
Polydispersity Ð Polydispersity index (PDI) Ð is Mw/Mn a measure of the breadth of the molecular weight
Step Growth: The Carothers Legacy Stuff is lost in this reaction
Kevlar is a polyaromatic amide (an aramid) used in tire cords and bullet proof vests O nhoc Polyamides O COH + nh 2 N NH 2 O O C CNH NH n + 2 nh 2 O cables of Kevlar are as strong as cables of steel, but only about 20% the weight. Kevlar fabric is used for bulletproof vests, jackets,
Step Growth Polymerization The forgoing were all examples of step growth polymerizations What Carothers called condensation polymerizations A-A, B-B vs A-B advantages Problems with achieving high Mol. Wt. But.there are tricks to be played Interfacial polymerization, etc.
Comparison of Step and Chain Step Growth Growth throughout the matrix between monomers, oligomers and polymers DP is low to moderate Monomer is consumed rapidly but Mw increases slowly No initiator needed and reaction same throughout process No termination step chain ends still reactive Rate decreases steadily as funcional groups are consumed Chain Growth Successive addition of monomer to a limited number of growing chain ends DP can be very high Initiation and propagation reactions are different Generally a chain termination step Polymerization rate increases initially remains relatively constant until monomer depleted
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 vs. step growth Step Chain
- Step-growth polymerization
- Chain-growth polymerization
Let s look at this 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 High Molecular weights are hard to get this way 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 N/N = N o /N( 1 P) or DP = 1 / (1 P) and for P = 0.98 (98% conversion), DP = only 50!
The step growth system It all happens at the end!!! DP = 1 / (1 P)
The chain growth system The relationship between DP and conversion With termination reactions steady state
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.
Distinguishing features of chain- and step-polymerization mechanisms
A-A, B-B vs A-B??? O O Cl C H 2 C H 2 C A.A C Cl + HO-CH 2 -CH 2 -OH B...B O O Cl C H 2 C H 2 C C O CH 2 -CH 2 -OH A.B
More Historical Figures Wilhelm Schlenk Michael Szwarc
Anionic polymerization Some History 1914, Schlenk reacts Na with butadiene and styrene 1929, Ziegler proposes a mechanism 1952 Higginson, styrene, KNH 2, kinetic study 1956 Szwarc, sodium naphthalene, Styrene, living polymerization conception 60's, 90 s, commercial products were available study on the living polymerization of polar monomers
Alkenes with electron Withdrawing Groups undergo anionic polymerization examples C H 2 CH C N C H 2 O CH N O C H 2 CH 3 C C O O CH 3 Acrylonitrile Nitroethene Methyl methacrylate
Classical Monomers CH 3 C H 2 CH C H 2 C styrene a-me-styrene
Reactivity of monomers Group A: H 2 C C(CN) 2 > H 2 C C COOC 2 H 5 > CN C H 2 CH NO 2 Group B: H 2 C CHCN > H 2 C C CH 3 > CN H 2 C CH C O CH 3 Group C: C H 2 CH C O O CH 3 > H 2 C C CH 3 C O O CH 3 CH 3 Group D: H 2 C CH CH CH 2 H 2 C CH H 2 C C
Cyano Acrylate initiator H 2 C CN C C Jöns Jacob Berzelius (1779-1848) O OC 2 H 5 + H 2 O POLYMER
Anionic Initiators and initiation (1) alkali metals.one electron reductions Lithium (Li) Sodium (Na) Potassium (K) as mirrors or fine dispersions. Na e H electron - + C CH. CH2 CH Na + 2 transfer Radical anion dimerize radical couple Na + - CH CH2 - CH 2 CH Na + Szwarc s favorite
Break Seal Glassware
Schlenk Tube Approach
b
Anionic Polymerization Apparatus
Szwarc s Experiment Living test Reddish Orange
The Living Polymerization DP t [ M ] [ I] M DP I
Anionic Polymerization of diblock copolymer M n = 64.2 kda M w = 65.7 kda PDI = 1.02 M n = 54.2 kda M w = 54.2 kda PDI = 1.00
Polymer Blends Polymers do not generally form blends or alloys. About 99% of binary blends are heterogeneous except for small regions of the phase diagram Ethylene and propylene are mutually soluble, but polyethylene and polypropylene are not.
Block co-polymers Covalent linkage of two or more polymers that are intrinsically incompatible. Synthesis requires special techniques.
A B Volume Fraction A
SBS Thermoplastic Elastomer Krayton 46
Orienting Block Copolymers Wafer Wafer Wafer Wafer Lamellae Cylinders 47
Directed self-assembly
How big is 10 nm??? 50 Micron Hair 5 Micron Bacterium
100 nm Virus 2-5 micron e. Coli Bacteria 50
10 nm wide line 0.5 nm diamet er atom