Macromolecular Chemistry

Transcription

1 Macromolecular Chemistry BHT Lecture 11

2 Light Scattering Experiment Measure I/I 0 = f(θ)

3 Standard Approach Measure scattering of an analyte relative to a well characterized very pure liquid Toluene is often used due to good scattering signal and values well characterized for a range ot temperatures and wavelengths. Ratio is tabulated in many reference books. CH 3

4 Some Messy but accessible Constants K = 2 2π 4 λ N 0 A n 0 dn dc λ o = laser wavelength N A = Avogadros number n o = Solvent RI dn/dc = differential RI increment 2 KC R θ = 1 M + 2A c 2 = I R A 0 θ 2 I S n n 2 S R S I A = lntensity of analyte (sample I solvent I) n o = Solvent RI I S = Intensity of standard (Toluene?) n S = RI of Standard R S = Rayleigh ratio of standard

5 Molecular Weight Example

6 Light Scattering For Rayleigh scatterers, KC R θ 1 = + 2A2c (y = b + M (y = b + mx) Therefore a Debye plot of KC/R θ versus c should give a straight line whose intercept at zero concentration will be 1/Mw and whose slope will be 2A2 2! Note that the moment of the distribution is Mw!

7 The Debye Plot KC/Rө (1/Mw) x 1/Mw x x KC R x θ = x 1 M + 2A c 2 Conc (g/ml)

8 Chain Transfer in Free Radical Polymerization A termination and re-initiation reaction

9 Chain Transfer Agents (CTAs( CTAs) Thiols are efficient examples

10 Chain Transfer to Polymer Creation of branches Back biting

11 Branching in Polyethylene Common Branch length is 4 or 5.why??? H H 3 C H 3 C

12 Chain Transfer to Initiator Example for acrylonitrile and BPO What effect does this process have on DP, PDI??

13 Chain Transfer Kinetics The chain transfer constant, C is defined as the ratio of the chain transfer and propagation rate constants; C = k tr /k p There is some C, which includes a term for transfer to a CTA, and one to monomer, solvent, polymer, etc. The higher the value of C the smaller the amount required to lower the molecular weight

14 Chain transfer constant = ktr k p C I = k tri k p C S = k trs k p C M = k trm k p B ush I * + + n I n H B us*

15 Measurement of Chain Transfer Constants The Mayo Equation: 1 1 = DP n DP 0 Transfer agent + Cs monomer DP n DP o C S = degree of polymerisation WITH transfer agent = degree of polymerisation WITHOUT transfer agent = Chain Transfer constant or coefficient

16 Generic Mayo plot 1/DP Cs 1/DP 0 [S]/[M]

17 CHAIN TRANSFER AGENTS Chain transfer activity dependent upon the monomer being polymerised and the structure of the CTA For 60 o C CTA C s x 10 4 Comment Benzene 0.02 Addition to propagating radical Toluene 0.1 Resonance stabilized Ethyl Benzene 0.7 Weakening of C-H Bond Acetone 4.1 CCl Weak C-C1 Bond CH 3 (CH 2 ) 3 SH 210,000 Weak S-H Bond

18 Chain Transfer constants at 60 C CTA Benzene Toluene Sryrene Vinyl acetate n-butanol CHCl n-butyl amine 7.0 CCl ,700 n-butyl Mercaptan 210, ,000

19 Effect of CTA on DP of 60 O C 1/DP ~ 1/(mol.wt) no CTA n-butyl mercaptan (C S = 210,000) [Transfer Agent] [Styrene] CCl 4 (C S = 110) Benzene (C S = 0.02)

20 Transfer to Polymer Intramolecular reaction/backbiting - short chain branches Intermolecular reaction - long chain branches C(Psty) = 10 * 10 4 C(PMMA)= * 10 4

21 Trapping Radicals OH O R + RH BHT O R O R

22 Trapping carbon centered radicals Carbon centered radicals stopped by addition to oxygen or carbon R. R O O O O Benzoquinone O O R O O Tautomerize R H R O O R H + HO O R R

23 Stable Radical Inhibitors O 2 N N N NO 2 O 2 N Diphenylpicrylhydrazyl, DPPH N O TEMPO O N N N N O Triphenylverdazyl Galvanoxyl

24 Some Familiar CTA s Ascorbic acid Vit C Butylated HydroxyAnisole BHA α-tocopherol Vit E

25 Butylated HydroxyTolueneoluene BHT radical will not initiate new chains

26 Interesting CTA s N O N O Phenyl-a-t-butylnitrone PBN TEMPO

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28 Radical Theory of Aging BHT Recent Review: Mechanisms of Ageing and Development Volume 125, Issues 10-11, October-November 2004