Polymer Synthesis CHEM 421 Odian Book Chapter 3-15, 5-3

Polymer SynthesisCHEM 421

• Odian Book

Chapter 3-15, 5-3


Living Polymerization


Co Co


Neutral and Highly Reactive

or


Living Polymerizations

• Living polymerizations are chain growth polymerizations which proceed in the absence of irreversible chain transfer and termination steps.*

• Diagnostic Characteristic of Living Polymerizations– The reaction proceeds until all monomer is consumed. If more

monomer is introduced then the polymerization will continue

– The number average molecular weight, Mn, is a linear function of conversion.

– The number of propagating chains (active centers) is constant and independent of conversion.

– Mn can be controlled by the reaction stoichiometry.

– Sequential monomer addition results in the preparation of block copolymers.

– Resulting polymers will exhibit a narrow molecular weight distribution and the polymer must exhibit a Poisson distribution in molecular weight.**

* Szwarc, M., Nature, 1956, 178, 1168.

** Flory, P. J., J. Am. Chem. Soc., 1940, 62, 1561.



• No termination or chain-transfer side reaction during polymerization–Control of molecular weight

» Mn=

–Narrow molecular weight distribution

–Synthesis of block copolymers by sequential monomer addition

–Control of polymer chain microstructure

–End-group functionalization

grams of monomer

Moles of initiator


Initiation

• Nucleophilic Initiation of Vinyl Monomers

CH3CH2CH

CH3

Li H2C CH Bu CH2 CH

CH CH CH

Li

Li LiLi


Anionic Polymerization of Styrene

CH3CH2CH

CH3

Li H2C CH

cyclohexane, 25 oC

tight ion pair, red-orange solution

sec-BuLi styrene



CH3CH2CH

CH3

Li H2C CH

cyclohexane, 25 oC


sec-BuLi styrene

CH2 CH Li

Bu CH2 CH CH2 CH Lin-1

Bu



CH3CH2CH

CH3

Li H2C CH

cyclohexane, 25 oC


sec-BuLi styrene

Bu CH2 CH CH2 CH Li

THF

loose ion pair and f ree ions,blood-red solution

n-1




CH3CH2CH

CH3

Li H2C CH

cyclohexane, 25 oC


sec-BuLi styrene

Bu CH2 CH CH2 CH Li

THF

loose ion pair and f ree ions,blood-red solution

n-1

CH3OHLiCH3O


Bu CH2 CH CH2 CHn-1

H


Initiators: Organolithiums

• Soluble in hydrocarbons

• Direct nucleophilic attack

• No electron transfer

(RLi)N N= 6,4,2

MW= g of monomer

moles of initiator


The Poisson Distribution in Molecular Weight

• The constraints imposed on a living polymer require that the polymer exhibit a Poisson distribution in molecular weight.*

Conversion & Mw/Mn as a Function of Time

Time

Co

nve

rsio

n

Mw

/Mn

ConversionPDI

nn

w

DP

11

DP

DP


• Conversion should be linear with time in semi-logrithmic coordinates

• Deceleration indicates termination or deactivation of catalyst.

Ln[M]o/[M] vs. Time

Time

ln[M

] o/[M

] t

kp/kt = 1.05kp/kt = 10kp/kt = 100Living

Scale of “Livingness”

• When chain termination cannot be completely suppressed (kt > 0):

- deviation from living behavior becomes more pronounced with time

- The degree to which a reaction deviates at time, t, is proportional to the ratio of the rate of propagation to termination.


End Group Functionalization

Carboxylic acid terminated:

Primary alcohol terminated:

CH2 CH Li

CH2 CH Li

CH2 CH Li

MeOH

[H ]

CO2

1)

2) HCl

O

CH2 CH

CH2 CH2

CH2 CH

C

O

O

MeO Li

Li

CH2 CH2 OH


End Group Functionalization

CH2 CH Li

SiCl

Me

Cl

Me

Si

Me

Me

PS PS

CH2 CH LiSiCl

Me

Me

Me CH2 CH Si Me

Me

Me

CH2 CH Li

SiCl

Cl

Si

Me

Cl

Cl

Me

PSPS

PSPS


Monomers

• Generally olefin w/ EWG or delocalizing groups

Methyl acrylate (MA)Methyl methacrylate (MMA)t-Butyl methacrylate (tBuMA)

R

C

OR'

O acrylates (acrylic esters)

R

H CH3CH3 CH3CH3

R'

1,3-butadiene

isoprene

styrenes & their derivatives

C N

acrylonitrile

NO2


Ion Pair Aggregation

• Need to stabilize propagating anions

R R RRM M M M

covalent bond tight "contact", "intimate"ion pairs

loose"solvent separated"

ion pairs

f ree ionsvery reactive

unhindered

nonpolar solvents (cyclohexane) polar solvents (THF)

less reactive, more controlled lewis base solvated Li+ ions


Ion Pair Aggregation

R R RRM M M M

covalent bond tight "contact", "intimate"ion pairs

loose"solvent separated"

ion pairs

free ionsvery reactive

unhindered

• Need to stabilize propagating anions

R H R H

Ka=[H ][R ]

[RH]

pKa = - log Ka

As Ka gets larger (stronger acid)

pKa gets smaller

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Polymer Synthesis CHEM 421 Odian Book Chapter 3-15, 5-3