MIT 10 569 - Radical Polymerization, Homogeneous Reaction Rate Kinetics (5 pages)

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Radical Polymerization, Homogeneous Reaction Rate Kinetics



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Radical Polymerization, Homogeneous Reaction Rate Kinetics

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5
School:
Massachusetts Institute of Technology
Course:
10 569 - Synthesis of Polymers
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10 569 Synthesis of Polymers Prof Paula Hammond Lecture 11 Radical Polymerization Homogeneous Reaction Rate Kinetics How to determine MW in free radical polymerization Kinetic Chain Length of monomers added per effective free radical rate of chain growth rate of chain growth rate of chain initiation rate of chain termination Rp Ri Rp Rt k p M 1 2 fk d k t I 2 p n if termination is by disproportionation process p n 2 if termination is by coupling Generally if no chain transfer p n 2a where a 1 100 disproportionation 100 disproportionation M n M n pn molecular weight of vinyl monomer unit What happens more often Coupling usually greater than disproportionation Percent of coupling increases if steric factors prevent effective coupling CH3 CH3 C C CH3 CH3 or if hydrogens are more reactive Citation Professor Paula Hammond 10 569 Synthesis of Polymers Fall 2006 materials MIT OpenCourseWare http ocw mit edu index html Massachusetts Institute of Technology Date H C C C H Consider k p M 1 2 fk d k t I 2 1 fk p I 2 M R p k p kt Increase Rp by M I But increase M I Thus you want to increase M Chain Transfer 1 ktr Mn X Y Rtr d Y k tr M X Y dt Mn ktr transfer constant X Y Chain transfer can occur when there are solvent impurities But sometimes using chain transfer can be advantageous 2 Y M ka YM 3 YM M kp Chain transfer agent CTA Used to decrease MW in polymerization YMn kp ktr and kp ka Rp is the same pn slightly moderately depending on CTA kp ktr and kp ka Rp same p n dramatically kp ktr and ka kp Rp slightly and p n slightly kp ktr and ka kp Rp drastically and p n drastically Transfer Types 1 to monomer ktr m Mn M Mn M 2 to solvent or impurity Mn S Mn S ktr s 10 569 Synthesis of Polymers Fall 2006 Prof Paula Hammond Lecture 11 Page 2 of 5 Citation Professor Paula Hammond 10 569 Synthesis of Polymers Fall 2006 materials MIT OpenCourseWare http ocw mit edu index html Massachusetts Institute of Technology Date or CTA 3 to initiator Mn I Mn I ktr I All act to decrease p n assume coupling pn Rp Rp Rt Rt Rtr m Rtr s Rtr I k tr m M M k tr s M S k tr I M I 2 2 Use resistor analogy resistors in series C transfer constant relative rate const vs Rp Cm k tr m kp CS k tr S kp CI k tr I kp since Rp kp M M 1 pn Ri S C I Cm C S M I M 2R p Additive effect of each constant Rt 2R p 1 1 p 2 n o Often only have transfer to CTA or impurity 1 pn Ri S CS M 2R p 1 fk d k t I 2 k p M 1 2 10 569 Synthesis of Polymers Fall 2006 Prof Paula Hammond Lecture 11 Page 3 of 5 Citation Professor Paula Hammond 10 569 Synthesis of Polymers Fall 2006 materials MIT OpenCourseWare http ocw mit edu index html Massachusetts Institute of Technology Date For a given amount of initiator I and monomer M 1 C S S M p n p n o 1 1 pn Increase Increas e MW Cs 1 pn o S M Useful to control MW is free radical with high kp and or really low kt Cs values for different compounds orbitals alkanes weakest cyclic hydrocarbons benzenes aromatics H unstable negative charge H extraction unlikely Increasing radical stability weakest share e cloud with other C atoms CH2 resonance stabilization CH2 High Cs values weak C H bonds stabilized by conjugation O O N C weak C Cl C Br C I 10 569 Synthesis of Polymers Fall 2006 Prof Paula Hammond Lecture 11 Page 4 of 5 Citation Professor Paula Hammond 10 569 Synthesis of Polymers Fall 2006 materials MIT OpenCourseWare http ocw mit edu index html Massachusetts Institute of Technology Date R R C R Cl R R H C RCl H weak S S bonds S H weakest largest CS CTA chain transfer agents CS x 104 CS x 104 For styrene Vinyl acetate Benzene 0 023 1 2 Cyclohexane 0 031 7 0 0 42 17 0 n butyl alcohol 1 6 20 0 CHCl3 chloroform 3 4 150 0 Tri methyl amine 7 1 370 210 000 480 000 Heptane n butyl mercaptan SH 10 569 Synthesis of Polymers Fall 2006 Prof Paula Hammond Lecture 11 Page 5 of 5 Citation Professor Paula Hammond 10 569 Synthesis of Polymers Fall 2006 materials MIT OpenCourseWare http ocw mit edu index html Massachusetts Institute of Technology Date


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