CHAPTER 9

CHAPTER 9

REACTIONS OF POLYMERS

→ the synthesis of new polymers by modification of exist- ing polymers using a variety of chemical reactions

1) the esterification of cellulose, crosslinking of polyisoprene, hydrolysis of poly(vinyl acetate), and chlorination of polyethylene.

2) polymer reactions using a polymer as a carrier or support for some component of a reaction system (polymeric brominating and Wittig reagents, the Merrifield solid-phase synthesis of polypeptides, and polymeric catalysts)

9-1 PRINCIPLES OF POLYMER REACTIVITY

Polymer reaction rates and conversions are usually lower than those for the corresponding low-molecular-weight homolog although higher rates are also found in some reactions

9-1a Yield

80% yield in the hydrolysis of methyl propanoate

→ pure propanoic acid is obtained in 80%

80% yield in the hydrolysis of poly(methyl acrylate)

→ not 80% yield of poly(acrylic acid) with 20% unreacted poly(methyl acrylate)

→ but copolymer products with on the average 80% acrylic acid and 20%

methyl acrylate units

9-1b Isolation of Functional Groups

In the the random reaction of a pair of neighboring functional

groups, the maximum conversion is limited due to the isolation of sin- gle functional groups between pairs of reacted functional groups.

Ex) acetal formation in poly(vinyl alcohol) by reaction with an alde- hyde.

EX) the dechlorination of poly(vinyl chloride)

maximum conversion : 83%

9-1c Concentration

1% solution of poly(vinyl acetate) molecular weight 10⁶

The overall concentration of acetate groups is about 0.11 M

Larger than 0.11 M of acetate groups; larger reaction rate

0 M of acetate groups : zero reaction rate

The observed overall reaction rate: an average of the rates inside and outside

the polymer coils

The overall rate may be the same, higher, or lower than the correspond- ing reaction with a low-molecular-weight homolog of the polymer

depending on the concentration of the small molecule reactant inside the polymer coils relative to its concentration outside.

9-1d Crystallinity

Reactions of the polymers having crystalline regions

→heterogeneous product

because only functional groups in the amorphous regions are available for reaction

Ex) chlorination of polyethylene, acetylation of cellulose, and aminolysis of poly(ethylene terephthalate)

9-1e Changes in Solubility

9-1f Crosslinking

For the crosslinked polymers,

the concentration of small molecule reactant inside the polymer domains can be lower than outside because of a low degree of swelling.

9-1g Steric Effects

Polymer reactivity can be sterically hindered,

1. functional group is close to the polymer chain

2. functional group is in a sterically hindered environment 3. Bulky small molecule reactants are used

examples

1. hydrolysis of acrylamide copolymers with monomer I

The rate decreases sharply with decreasing value of n, although the reaction rate is not affected by the molecular weight of the copolymer

2. Hydrogenation of cyclododecene using the polymeric rhodium catalyst II occurs at a rate 5 times slower than does cyclohexene

The low-molecular-weight homolog III shows no difference in catalytic ac- tivity

toward the two cycloalkenes.

3. n-C₄H₉I and n-C₁₈H₃₇I react at the same rate with pyridine, but n- C₄H₉I is

almost fourfold more reactive than n-C₁₈H₃₇I toward poly(4-vinylpyri- dine)

9-1h Electrostatic Effects

The conversion of uncharged functional groups to charged groups can decrease in reactivity with conversion.

Acceleration of reactivity is also observedrepells hydroxide ion

9-1i Neighboring-Group Effects

the saponification of poly(methyl methacrylate) proceeds with autoac- celeration when carried out with bases

9-1j Hydrophobic Interactions

the hydrolysis of various 3-nitro-4-acyloxybenzoic acid substrates (V) catalyzed by imidazole (VI) and poly[4(5)-vinylimidazole] (VII) in

ethanol–water mixtures

Catalysis by VII is more effective than by VII

VII for n = 11 is 30-fold larger than for n = 1, and almost 400-fold larger than for VI.

9-1k Other Considerations

1. Reaction between functional groups on different polymer molecules occurs

- when the two polymers are sufficiently similar (polymer chain inter- penetration is possible

- when polymer mixing is highly exothermic (acidic and basic polymers) 2. The conformation of polymer chains on the reactivity

- the expanded coils (not the tight coil) can increase the accessibility of polymer functional groups and the local concentration of a small-

molecule reactant.

3. The functional groups in a number of polymers are not of the same reactivity.

9-2a Alkyds

9-2 CROSSLINKING

→ unsaturated polyesters in which the unsaturation is located at chain ends instead of within the polymer chain

double bond resides in a fatty acid component

Unconjugated double bonds undergo crosslinking by the initial formation of an allylic hydroperoxide followed by decomposition of the hydroperox- ide.

9-2b Elastomers Based on 1,3-Dienes

Crosslinking is an absolute requirement for the applications as elastomers (rapidly and completely recovering from deformations).

“vulcanization” is used synonymously with ‘crosslinking’ in elastomer technology.

Crosslinking or vulcanization can be achieved by using sulfur, peroxides, other reagents, or ionizing radiation

9-2b-1 Sulfur Alone

Not the radical process but the ionic process

- Neither radical initiators nor inhibitors affect sulfur vulcanization

- But accelerated by organic acids, bases, and solvents of high dielectric con- stant.

Vulcanization by heating with sulfur alone is not efficient !

- 40–50 sulfur atoms incorporated into the polymer per crosslink.

9-2b-2 Accelerated Sulfur Vulcanization

long polysulfide

crosslinks (very large n) vicinal crosslinks intramolecular cyclic sulfide structures

Therefore commercial sulfur vulcanizations are carried out in the presence of various additives (accelerators) with a metal oxide and fatty acid (activator).

Then most of the crosslinks are monosulfide or disulfide with very little vicinal or cyclic sulfide units.

How ?

The most used accelerators are sulfenamide derivatives such as 2-mercap- tobenzothiazole

2-mercaptobenzothiazole

2,2’-dithiobisbenzothiazole

cleavage followed by oxidative coupling.

polysulfide

Zinc : increasing the efficiency and rate of crosslinking by chelat- ing

9-2b-3 Other Vulcanizations

by heating with p-dinitrosobenzene

9-2c Peroxide Crosslinking

→ compounding with a peroxide such as dicumyl peroxide or di-t-butyl peroxide and then heating the mixture.

→ The crosslinks formed via peroxides are more thermally stable than those formed via sulfur vulcanization.

→ peroxides more expensive than sulfur,

therefore peroxides used for those polymers that cannot be easily crosslinked by sulfur,such as polyethylene and other polyolefins, ethylene–propene (no diene) rubbers (EPM), and polysiloxanes.

mechanism

The crosslinking efficiency can be increased by incorporating small amounts of a comonomer containing vinyl groups.

Ex)

9-2d Other Crosslinking Processes

Crosslinking of polymers containing fluorinated monomers such as vinylidene fluoride, hexafluoropropene, perfluoro(methyl vinyl ether), and tetrafluoroethylene

→ dehydrohalogenation followed by addition of the diamine to the double bond with the metal oxide acting as an acid acceptor.

9-3 REACTIONS OF CELLULOSE

9-3a Dissolution of Cellulose

Fibers or films are pro- duced by spinning or casting cellulose sodium xanthate solution10%

sulfuric acid.

The acid hydrolyzed the cellulose xanthic acid (XXV), is unstable and decomposes (without isolation).

Then solid cellulose fiber (rayon) or film (cello- phane)

products are produced!

9-3b Esterification of Cellulose

mixed acetate–propionate and acetate–butyrate, and nitrate esters of cellulose are produced commercially

9-3c Etherification of Cellulose

9-4 REACTIONS OF POLY(VINYL ACETATE)

Poly(vinyl alcohol) is obtained by alcoholysis of poly(vinyl acetate) with methanol:

Reaction of poly(vinyl alcohol) with an aldehyde yields the corresponding poly(vinyl acetal):

The two most important acetals are the formal and butyral (R = H and C₃H₇, respectively)

9-5 HALOGENATION

9-5a Natural Rubber

Hydrochlorination at about 10 ^oC by electrophilic addition to give the Markownikoff product with chlorine on the tertiary carbon

9-5b Saturated Hydrocarbon Polymers

The chlorination of polyethylene, poly(vinyl chloride), and other satu- rated polymers by a free-radical chain process catalyzed by radical ini- tiators:

The reaction of polyethylene with chlorine in the presence of sulfur dioxide yields an elastomer containing both chloro and chlorosulfonyl groups

vulcanized with metal ox- ides such as lead or magne- sium oxide to form elas-

tomers

9-6 AROMATIC SUBSTITUTION

Aromatic electrophilic substitution to produce styrene polymers with ion-exchange properties

9-7 CYCLIZATION

Natural rubber and other 1,4-poly-1,3- dienes are cyclized by treatment with strong protonic acids or Lewis acids

carbon fibers from polyacrylonitrile (PAN) (acrylic fiber)

-successive thermal treatments— initially 200–300 ^oC in air followed by 1200–2000 ^oC in nitrogen

Further heating at above 2500C in nitrogen or argon for brief periods to yield carbon fibers with graphitelike morphology.

9-9 GRAFT COPOLYMERS

Grafting onto

Grafting from

Grafting through

9-9a Radical Graft Polymerization

9-9a-1 Vinyl Macromonomers

Preparation of vinyl macromonomers

1. Using HO-terminated polymer such as polysiloxane, polycaprolactone, or polytetrahydrofuran

2. Using vinyl chloroacetate as the initiator in ATRP

Then the vinyl macromonomers can be polymerized by any of the methods of radical polymerization to produce graft poly- mers.

9-9a-2 Chain Transfer and Copolymerization

the radical polymerization of a monomers in the polymer solutions produce a mixture of homopolymerization and graft polymerization Copolymerization

chain transfer

- High-impact polystyrene (HIPS): styrene polymerized in the presence of poly(1,3 butadiene)],

- ABS and MBS: styrene–acrylonitrile and methyl methacrylate–styrene, respectively, copolymerized in the presence of either poly(1,3-butadi- ene) or SBR

9-9a-3 Ionizing Radiation

The polymer is swollen by monomer but does not dissolve in the monomer.

Ex) poly(ethylene-graft-styrene can be produced from polyethylene/styrene mixture

9-9a-5 Living Radical Polymerization

Using ATRP

Using NMP A halogen-containing monomer such as vinyl chloroacetate or p- chloromethylstyrene is reacted with an HO containing

alkoxyamine to yield a vinyl alkoxyamine, then (co)polymerization of the vinyl macromonomer produces the graft copolymer.

Using RAFT

9-11 POLYMERS AS CARRIERS OR SUP- PORTS

1. polymer reagents

a polymer containing a functional group that acts as the reagent to bring about a chemical transformation on some small (i.e., low-

molecular-weight) molecule 2. polymer catalysts

Having a group that performs a catalytic function in some reaction—

usually a reaction between small molecules 3. polymer substrates

having an attached molecule on which some transformation is car- ried out using a small-molecule reagent

9-11a Synthesis a polymer reagent, catalyst, or substrate

Two approaches

1. Functionalization of polymer approach

the required functional group, for the reagent, catalyst, or substrate function, is attached to the polymer.

2. functionalization of monomer approach

an appropriate monomer with the desired functional group is synthe- sized and then polymerized or copolymerized.

9-11a-1 Functionalization of Polymer

polymer rhodium catalyst

polystyrenes containing OH, COOH, B(OH)₂, RSnCl₂, and P₂ groups can be prepared by reaction with ethylene oxide, CO₂, B(OR)₃, MgBr₂ fol- lowed by RSnCl₃, and ₂PCl, respectively.

polymer reagent

9-11a-2 Functionalization of Monomer

The approach of synthesizing a monomer containing the desired functional group followed by (co)polymerization

the sequence of reactions starting from histidine (XXXVII) to yield 4(5)-vinylimidazole (XXXVIII)

9-11b Advantages of Polymer Reagents, Cata- lysts, and Substrates

An insoluble polymer reagent, catalyst, or substrate can be easily sep- arated

from the other (i.e., the small molecule) components of a reaction sys- tem by filtration.

Small-molecule reagents, catalysts,or substrates that are highly reac- tive, toxic, or malodorous can be handled much more safely and eas- ily in the form of the corresponding polymers.

disadvantages

Functional polymers are more expensive than their smallmolecule analogs.

Filtration of a polymer reagent, catalyst, or substrate is often not easy.

Then, multistep sequential reactions and automated parallel combina- torial synthesis to produce libraries of compounds is possible

The epoxidation of an alkene by a polymer peracid (polymer reagent)

9-12 POLYMER REAGENTS

Preparation of polymer peracid

The application of polymer peracid

filtration

Regeneration of polymer peracid

Oxidation of alcohols to aldehydes or ketones : anion-ex- hange resin

,

Polymer reducing agents

obtained bycomplexing AlH₃ or BH₃ with poly(4- vinylpyridine)

a chiral polymer for an asymmetric synthesis

stereoselective reduction of acetophenone to (R)-1-phenylethanol in 76–97% enantiomeric excess using the chiral support

halogen addition to alkenes and -substitution on aldehy- des and ketones

Small molecular weight Pyridinium bromide is dangerous to handle.

halogenating reagents

Polymer Wittig reagents : the conversion of an aldehyde or ketone to an alkene

Polymer scavengers

8-hydroxyquinoline group for chelating nickel, cobalt, and copper ions

chiral groups are for resolving

racemic mixtures into the individual enantiomers

the copper(II) complex of this poly- mer (either the R- or S-enantiomer) resolves racemates of amino acids by the formation of a pair of diastere-

omeric complexes with the two enan- tiomers.

9-13 POLYMER CATALYSTS

hydrogenation by the polymer rhodium catalyst

ester hydrolysis by poly[4(5)-vinylimidazole]

9-14 POLYMER SUBSTRATES

(substrate)

insoluble

9-14a Solid-Phase Synthesis of Polypeptides

Synthesis of glycylalanine requires that the amine and car- boxyl

ends of glycine and alanine, respectively, be protected.

Boc (protecting group) formed by reacting the amino acid with 2-(t butoxy- carbonyloxyimino)-2-phenylacetonitrile.

Solid-phase synthesis of polypeptide