10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 32: Stable Free Radical Polymerization, Atom Transfer Radical Polymerization, Controlled Free Radical Polymerization

Free Radical Polymerization

Free radical is most versatile type of chain growth → Most monomers are available → Can be done in • emulsion • suspension • bulk → Most robust method: less sensitive to • solvent • impurity • atmospheric conditions → Limitations • broad • lack of control over PDI • difficult to make well defined block copolymers → Methods to improve upon traditional free radical • Stable Free Radical Polymerization (SFRP) • Atom Transfer Radical Polymerization (ATRP)

“Controlled Free Radical Polymerization”

Stable Free Radical Polymerization

TEMPO → stable nitroxy free radical

Use with initiator that has a fast half life N O Benzoyl peroxide (BPO) half life <∼ 3 minutes

Requirements for SFRP • Stable free radical must NOT initiate polymerization • RSFRcomb > Rp

rate of free radical combination

• Need Ri >> Rp • Rt << Rp, RSFR

This system first described by Georges, Veregin, Kazmeier, Homer

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. Macromolecules, 26, 5316 (1993)

Results • PDI’s as low as 1.1 – 1.5 • Can form diblocks and triblocks with acrylates and co-monomers (styrene) regardless of addition order!! • Slow reaction times (30-70 hours) - rate can be increased with camphor sulfonic acid (∼6 hours) • High MWs achievable • PDI’s narrower for higher ratios of TEMPO:BPO ⇒ RSFRcomp • Initiation at high temps lowers PDI Ri (complete ∼ 15 min)

RkMMpp⎣⎦⎣⎦⎡⎤⎡⎤⋅ • Higher temps ⇒ = R 2 t kMt ⎣⎦⎡⎤⋅ • Reaction has 2 stages 1) TEMPO catalyzed decomposition of BPO 2) Propagation through reversible TEMPO activation/deactivation

Disadvantages - variability in PDI, MW - different TEMPO/initiator/monomer combinations can yield different results

Kinetics for SFRP

K L Pn T ←⋅+⋅ ⎯→⎯ Ln (trapping)

Propagating dissociated trapped “living” chain polymer chain TEMPO or P+T

L kLt. ⎣⎦⎡⎤n KL == kPLrev, ⎣⎦⎣⎡⎤⎡ n ⋅⋅T⎦⎤

⎣⎦⎡⎤Ln ⎣⎦⎡⎤Pn ⋅= ≅constant KTL ⎣⎦⎡⎤⋅

much lower than in traditional free radical

generally []≈⋅ %5 []TT o

RkTPtrap=⋅ L. t⎣⎦⎣⎦⎡⎤⎡⎤ n ⋅>>Rp termination rate is much lower

2 as R ∝ [Pnt ⋅] and ∝ []PR np ⋅

10.569, Synthesis of Polymers, Fall 2006 Lecture 32 Prof. Paula Hammond Page 2 of 8

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.

Probability of untrapped living chains meeting is very low (termination by combination is negligible).

Atom Transfer Radical Polymerization (ATRP)

Kinetics same as previously described.

Requirements for Trapping Process to Work

k . fL 1. KL >>= 0.1 ⇒ forward rate is much faster k ,rL 2. Rate of free radical trapping >> propagation rate

. 3. >> RR tp (if [M ] is low, usually true)

ATRP Mechanism

Initiation z+ z )1( ++ 1. LiMXR t +⋅⇔+− t XLiMR alkyl metal halide ligand oxidized complex complex

X = Cl, Br M = Cu, Ni, Pd, Ru, Fe

most common

k p 2. R +⋅ R' ⎯⎯→ P1 ⋅

Propagation

z+ keq z )1( ++ N LiMXP t ←+− ⎯→ N +⋅⎯ t XLiMP

k p PN +⋅ R' ⎯⎯→ PN +1 ⋅

Can use a broad range of catalytic systems.

Synthesis of Custom Polymers using Controlled Radical Polymerization

⇒ Grow a linear polymer from a dendrimer.

⇒ Create a graft copolymer.

10.569, Synthesis of Polymers, Fall 2006 Lecture 32 Prof. Paula Hammond Page 3 of 8

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.

⇒ Transfer from ionic polymerization to controlled radical to create block copolymers otherwise inaccessible. J. Am. Chem. Soc., Vol 118, No 45, 1996 Pg 11112

Dendrimer Block Copolymer

O O O O O O O O O O O O O O O O O O O O O O O O O O O O N + N NaH O O O O O Br O O O O HO O O O O O O O O O O O O O O O O O O O O O O O O O O

10.569, Synthesis of Polymers, Fall 2006 Lecture 32 Prof. Paula Hammond Page 4 of 8

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. Use of Aluminum Alkyls ⇒ low PDI’s, promising approach

THF N N iBu AlH N N 2 Al iBu iBu

iBu N N iBu + iBu Al Al iBu iBu N N iBu

O N

iBu iBu iBu N iBu N Al Al N iBu N iBu + O O N N

O MeO

iBu iBu N Pol Al iBu N MMA O O N MeO

10.569, Synthesis of Polymers, Fall 2006 Lecture 32 Prof. Paula Hammond Page 5 of 8

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.

Table 1. Initiating and Terminating Species for Living Radical Polymerizations

I⋅ X⋅

(C2H5)2NCS SCN(C2H5)2 1 1 iniferter S S (CH ) C 3 2 H3C CH3 2 CN 7 O N (C6H5)CO 3 H C CH O 3 3

4 C6H5⋅ 8 ON N

5 CH3⋅ 9 Al complexes O H3C CH3 Nitroxide Stable Free Radicals 6 N 10 O N

H C O 3 CH3 H3C CH3

11 O N

H C 3 CH3 CO2H

10.569, Synthesis of Polymers, Fall 2006 Lecture 32 Prof. Paula Hammond Page 6 of 8

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.

Scheme 2. Living Polymerization of Acrylates and Methacrylates

H3C CH3 H3C CH3 CH3 CH3 CH3 N O + H CCN N C CH 3 3 H3C O N CN CN CN H C 3 CH3 H C CH AIBN 3 3

H2C CR C O

CH3

H3C CH3 CH3 R R R H3C C CH2 C CH2 C CH2 C O N

CH3 C O C O C O CH CH3 CH3 CH3 H3C 3 R=H or alkyl n

Li

Si Si O O O n Si Si O

10.569, Synthesis of Polymers, Fall 2006 Lecture 32 Prof. Paula Hammond Page 7 of 8

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.

Here is an example involving an anionic polymerization that is then end capped with an initiator for ATRP, Pt (catalyst) toluene excess O H C 60oC O Br 3 OSi O Si O Si O Si O OSi O Si O Si O Si O + H Si R y-1 z-1 + x-1 y-1 z-1 x-1 Br Br H3C

R R R R Si Si Si PMDETA (ligand) Si O O CuBr Br OSi O Si O Si O Si O Br OSi O Si O Si O Si O + x-1 y-1 z-1 n x-1 y-1 z-1 110oC

the additional step of hydrosilation of the vinyl groups on the siloxane is not needed for a simple PDMS and would look like: excess O Si OSiO z-1 + x-1 Br Br

PMDETA (ligand) O CuBr Br Si O Si O + x-1 z-1 110oC

O Si OSiO Br x-1 z-1 n

The details for the catalyst and ligand are in the review articles listed below.

Patten, Timothy E., and Krzystof Matyjaszewski. “Atom Transfer Radical Polymerization and Synthesis of Polymeric Materials.” Advanced Materials 10, no. 12 (1998): 901-915.

Pyun, Jeffrey, Shijun Jia, Tomasz Kowalewski, and Krzystof Matyjaszewski. “Synthesis and Surface Attachment of ABC Triblock Copolymers Containing Glassy and Rubbery Segments.” Macromol. Chem. Phys. 205 (2004): 411-417.

10.569, Synthesis of Polymers, Fall 2006 Lecture 32 Prof. Paula Hammond Page 8 of 8

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.