Chemical modification of unsaturated polymers

4 CHEMICAL M O D I F I C A T I O N OF U N S A T U R A T E D POLYMERS A. BRVDONAND G. G. CAMERON Department of Chemistry, Universityof Aberdeen, Old Aberdeen, Scotland Contents 1. Introduction 2. Modification of Polymers and Copolymers of Dienes 2.1. Structural Modification of the Polymer Chain 2.1.1. Hydrogenation 2.1.2. Halogenation, Hydrohalogenation, and Related Reactions 2.1.3. Addition of Thiols and Related Sulphur Compounds 2.1.4. Cycloaddition Reactions 2.1.5. Miscellaneous Modifications 2.2. Crosslinking of Polydienes 2.2.1. Crosslinking with Sulphur-containing Compounds 2.2,2. Crosslinking by Cycloaddition Reactions 2.2.3. Other Crosslinking Reactions 2.3. Graft Copolymerization of Unsaturated Polymers 2.3.1. Free-radical Methods 2.32. Ionic and Coupling Reactions References

209 211 213 213 214 216 219 223 227 227 230 232 237 238 242 245

1. Introduction The mid-nineteenth century saw the first successful attempts to modify polymers in a useful way. The nitration of cellulose ~1) was reported in 1833 and Goodyear's vulcanization process was patented in 1844. (2) Celluloid was in common use by the end of the century and could be said to be the first synthetic, or at least partly synthetic, plastic. Since then the scope of application of many polymers, both natural and synthetic, has been widened by suitable chemical modification. The prime aim of chemical modification has been to produce materials with new and improved properties, and many such materials are now produced in commercial quantities. The technical importance of many chemically modified polymers has stimulated research at a fundamental level. Modification reactions on preformed polymers can be divided conveniently into three categories: structural modification of the polymer chain, crosslinking, and chemical combination with different polymers. The boundaries between these classes 209

210

A. BRYDON AND G. G. CAMERON

are not entirely rigid since, for example, crosslinking can be achieved by combination or grafting with a second polymer. The precise objectives in chemically altering the structure of polymer chains are highly varied and depend upon the nature of the parent polymer and its intended applications. The degree or extent of modification is an important factor in achieving these objectives. Thus hydrocarbon polymers can be rendered more hydrophilic by attaching hydroxyl or carboxyl groups to the backbone. Provided the degree of reaction is not too high, the physical properties of the polymer are not greatly altered, but the acquisition of the polar side groups endows the polymer with increased compatibility with and adhesion to hydrophilic substrates such as paper and natural fibres. Crosslinking of linear macromolecules brings about profound changes in the properties of the bulk polymer. The melt viscosity and glass transition temperature increase with increasing crosslink density, while heavily crosslinked polymers are insoluble and infusible because of the three-dimensional networks they contain. One of the most important crosslinking reactions is the vulcanization of rubbers, where the aim is to impart strength and shear resistance along with reversible elastomeric properties. Chemical combination of structurally distinct polymers produces either a block copolymer AAAAABBBBB by suitably linking chain ends, or a graft copolymer AAAAA...AAAAA

I

r

B B B B B B

B B B B B B

where A and B are the monomer units from which the two polymer components are derived. In this article only the latter type of polymer combination, designated poly(A-g-B), is considered. Graft copolymers can be formed by creating on the polymer A a reactive site, such as a free radical, which can subsequently initiate the polymerization of B. Alternatively, a functional group may be placed on an A unit and then reacted with a polymer of B carrying an appropriate terminal group. For example, a hydroxyl group on polymer A can be reacted with an isocyanate-terminated B polymer to produce a graft via a urethane link. In many cases grafting reactions are not 100 ~ efficient, so that the final product comprises some free homopolymer along with the graft copolymer. Polymers containing olefinic bonds are particularly adaptable to chemical modifications of the three types mentioned above. Not only does the olefinic bond react directly with a multitude of other functional groups but it can activate adjacent groups,

CHEMICAL MODIFICATION OF UNSATURATED POLYMERS

21 l

thereby providing alternative sites of reaction. Unsaturated elastomers such as polybutadiene are therefore eminently suited for modification, and this situation is usefully exploited by the rubber industry. This article is concerned with reactions of such unsaturated polymers and copolymers, the intention being to illustrate the scope of modification processes with particular emphasis on newer and unusual reactions rather than to provide an exhaustive review of the literature. A number of reactions, such as cyclization and epoxidation, have therefore been omitted.

2. Modification of Polymers and Copolymers of Dienes The simplest diene monomer, butadiene, can polymerize through the two terminal carbon atoms to yield the cis-l,4-polymer or its trans-isomer: (I) --CH2

~c

(4) .CH2 .

.

.

(I) CH 2

.

c/

"qc

H

c/ (4)

cisi l ,4-polybutadiene

trans- l ,4-polybutadiene

Polymerization through C1 and C 2 results in the formation of the so-called vinyl (or 1,2)-polybutadiene having the structure

(I) (2) - - C H 2 --CH - - ~

L

(3) CH

II (4) CH2 1 , 2 - polybutadiene

Dienes with the hydrogen atom o n C 2 replaced by another group, such as methyl (isoprene) or chlorine (chloroprene), form isomers in the same way, with the additional complexity that polymerization through C3 and C4 is now structurally distinct from that through C1 and C2. CH3

t --~H2--C

(4) .

I(2)

(3) C H

II

(4) CH2 I, 2 - polyisoprene

.

.

.

.

.

.

(3)

CH2--CH--~

I

(2) C - - C H 3

II

(I) CH2 5, 4 - polyisoprene

The free-radical polymerization of diene monomers generally produces macro-

212

A. BRYDONAND G. G. CAMERON

molecules containing a mixture of all possible structures. The proportions present depend upon the nature of the monomer but only to a limited extent upon the polymerization conditions, such as temperature, solvent, etc. The advent of catalysts of the Ziegler-Natta and alkali metal types has provided the means of sterically controlling the polymerization of these monomers so that a high degree of structural and steric regularity has been attained. Thus, polybutadiene with more than 95 ~ of the double bonds in the cis-l,4 arrangement can be prepared, and 1,2-polybutadiene can be synthesized in the isotactic, syndiotactic, and atactic forms. The residual olefinic bond carried by each diene unit in the polymer chain is subject to the same ionic, radical, or other reactions as any other olefin. The facility of reaction, however, is often impaired in a polymer compared with a simple olefin. This is not surprising since a double bond in a macromolecule is less accessible than a structurally similar double bond in a small olefin. The steric barriers to reactions on polymers may become quite marked when attempts are made to carry a reaction to completion. Considerations of this kind are the subject of some recent articles. (a,4) In many modifications of polydienes, however, comparatively low degrees of reaction suffice. When high degrees of modification are required it is generally more satisfactory to carry out the reaction in solution since in the dissolved state steric and viscosity effects are minimized. It should be pointed out that chemical transformations of polymers are rarely quantitative, sometimes for steric reasons and sometimes because the modified polymer undergoes a marked decrease in solubility. The electron-rich region of the ethylenic bond results in these carbon atoms being subject to electrophilic attack in polar conditions. With polyisoprene Ca is the preferred site of attack by electrophiles because the resulting tertiary carbonium ion is more stable than the alternative secondary. For this reason polar addition reactions, like the ionic addition of HC1 to polyisoprene below, tend to conform to Markownikov's rule:

(~) "~'~"~CH2

c=

/CH2 - ' ' ' ' ~ H +

~

(4)

- - C.H2±

.CH2--"""

CH3

I

CI-

CL - - C - - C~---H C/HH3 H Other reactions, such as the acid catalysed addition of aldehydes and the addition of sulphenyl derivatives to polydienes, also occur by a polar mechanism. On the other hand, halogenation may take place by either an ionic or a free radical mechanism depending on the reaction conditions. The hydrogen atoms on Ca and C4 in 1,4-polydienes are comparatively easily removed by free radicals, and either of these atoms may become the site of substitutive attack. In polyisoprene the favoured site of attack is at (24,giving a resonance stabilized allylic radical, although a proportion of abstraction must also occur at C1 (s)"

213

C H E M I C A L M O D I F I C A T I O N OF U N S A T U R A T E D POLYMERS

CH3 I --CH2

--C

(0

CH3

R.

=CH

(2) (3)

-- CH z --

~

I

=~

i

N'-,~--CH 2 - - C - ~ - C H - - C H

(4)

---~'~

*RH

CH3

I ---CH z - - C - - C H-----CH - - , . , . ~

The corresponding reaction on 1,4-polybutadiene produces a similar radical which is somewhat more reactive than the polyisoprenyl radical. A similar abstraction reaction can occur at 1,2-units: (I) .....

(2)

CH2--CH--

i

~,~

A

R.

(3) CM II

CH

(4) CH?

CH2

II

--CH

+ RH

2 -- C . . . .

II cM

I "CH2

Addition of free radicals may also take place at double bonds, the pendant vinyl groups being more susceptible to this type of attack than the main chain double bonds. The double bonds in some polydienes such as polyisoprene undergo transposition in a concerted addition reaction with certain enophiles:

The hydrogen atoms on Ca and C4 in 1,4-polydienes are mildly acidic and may be replaced by alkali metals, especially lithium. This reaction opens up many avenues for further modification.

2.1. STRUCTURAL MODIFICATION OF THE POLYMER CHAIN

2.1.1. Hydrogenation Hydrogenation is one of the most familiar modifications of unsaturated polymers and has been studied continuously from the early work of Staudinger, t6) Harries, t~) and others38) These early studies employed nickel or noble metal catalysts in a heterogeneous system at high temperatures and pressures, with the polymer in dilute solution. Typical conditions might be nickel on kieselguhr at 230°C and 5 p.s.i, in decalin using 5-10% of catalyst. Under these conditions extensive degradation of the

214

A. BRYDON AND G . G . CAMERON

polymer occurs. Other disadvantages of these catalysts are the difficulty of preparation, ease of poisoning, and the problem of their separation from the viscous solution after hydrogenation. Although there is still some interest in using catalysts such as Raney nickel for hydrogenating polymers, ~9.1°~ these serious disadvantages have channelled investigations towards hydrogenation systems which operate under much milder conditions. Several catalysts meeting this requirement have been developed. Many of these are termed homogeneous, but this is not always an accurate discription since some at least exist as particles of subcolloidal dimensions rather than in true solution. Ziegler-type catalysts figure prominently in this group. These comprise an organic derivative of a transition metal reduced by a metal alkyl, usually aluminium triethyl. Nickel compounds such as the acetylacetonate, octanoate, naphthenate, and diisopropylsalicylate have been used most frequently, but the successful application of iron and cobalt compounds has also been reported. (1~-~5) Butyl lithium and lithium aluminium hydride (~6) as well as various compounds of aluminium and magnesium have been applied as second components of such catalysts. These catalysts permit hydrogenation of polydienes under mild conditions with no accompanying main chain scission. Thus it has been observed that the nickel diisopropylsalicylate/lithium aluminium hydride system can produce about 90 7oohydrogenation of polybutadiene in 4 h at 40°C and atmospheric pressure. ~6) A recent patent describes catalysts of this type activated by hexaalkylphosphorotriamides giving complete hydrogenation after 30 h at 70°C and 4 atm pressure. (17~ Other homogeneous catalysts which have been used to hydrogenate polymers are organoboron compounds, °s-2°) but boranes unfortunately often cause chain scission as a side reaction. Two very recent papers describe the application of diimide for hydrogenating unsaturated polymers, t2~,22~ Diimide has been known for some time as a useful hydrogenation reagent for non-polar olefinic double bonds in small molecules, and operates under comparatively mild conditions, t2a-26) Complete hydrogenation of various butadiene polymers and copolymers, polyisoprene, and polycyclohexadiene was achieved with diimide generated in situ from an excess of p-toluenesulphonylhydrazide in solution at 110-160°C. Alkyl branches at the double bond in the polymer reduced the efficiency of hydrogenation to less than 50 % and polychloroprene showed very low reactivity with less than 10 % conversion. Chain scission and a small amount of incorporation of p-toluenesulphonate groups on the polymer appear to be the main side reactions with this reagent. Hydrogenated polydienes show a steady change in physical properties with degree of saturation. For example, polybutadiene remains elastomeric up to 60 % saturation, but beyond this level of hydrogenation it shows properties of a thermoplastic. (~6~ As anticipated, the fully hydrogenated polymer has identical infrared and X-ray patterns to crystalline polyethylene, ~2°) but it must contain a limited concentration of ethyl groups arising from pendant vinyl groups.

2.1.2. Halogenation, Hydrohalogenation, and Related Reactions The chlorination of natural rubber has been known and used for a considerable time in the preparation of heat- and chemical-resistant paints. The traditional method

CHEMICAL MODIFICATION OF UNSATURATED POLYMERS

215

of passing chlorine into rubber dissolved in a chlorinated solvent has been supplemented by direct chlorination of rubber latex or of solvent-swollen rubber film. The mechanism of chlorination of polyisoprene is complex and, depending on the reaction conditions, involves substitution, addition, cyclization and crosslinking. ~27~ In polybutadiene, cyclization does not occur, but crosslinking is more pronounced. C28~ In a recent investigation of the direct bromination of polyisoprene it was reported that the reacted rubber contained the following structures :t29) CH 3 Br

I

CH2Br

I

I

--CH 2 -- C --CH--CH 2.

.

.

.

CH2 - - C - - C H - - C H 2 - -

I

I

Br

Br

"'~

--

I

Br

C H 2 - - C - - C H B r - - C H 2 --

CNz The reaction shows similarities to that of chlorination, t27) Direct halogenation of polymers using halogen-carrying reagents rather than free halogen has also been investigated. These reagents, like pyridinium perbromide, have the advantage that they are easily handled and can be added in precisely measured quantities to the reaction mixture. It is not entirely clear whether the reactions involve the same complexities as direct halogenation, but it seems likely that some side reactions occur. Complete bromination of cis- and trans-l,4-polybutadiene has been achieved with pyridinium perbromide in carbon disulphide-pyridine solution. ~a°) Subsequent dehydrobromination with potassium hydroxide and pyridine gave conjugated sequences of double bonds. Potassamide in liquid ammonia or sodium hydride in dimethyl formamide yielded a dark-brown insoluble substance which was the completely dehydrobrominated material. Phenyltrimethylammonium perhalogens have also been used for halogenation of polydienes.~3t) With these reagents mixed halogenated products, like the bromochloride, may be obtained. Various brominating agents in the presence of methanol were reported to yield methoxy-brominated products (a~) in accord with earlier observations, t27) while chloroethoxychlorination of 1,2-polybutadiene resulted from direct chlorination in the presence of 2-chloroethanolJ 32) The pseudointerhalogen iodine isocynate (ICNO) with polyisoprene gave a product in which 41% of the repeat units had reacted to form the structure :t33)

--CH 2 -- C --CH--CH 2--

NCO

This grouping is amenable to further modification to yield a variety of nitrogencontaining functional groups. Treatment with methanol yielded a proportion of carbamate units CH3

--CH2--

I

I I C--CH---CH2--~ I NHCOCH3

II o

216

A. BRYDON AND G. G. CAMERON

and with ammonia the substituted urea: CH3

I

--CH2 - - C

I

I

I

CH--CH 2--

NHCNH2 II 0

The modified polymers bore little resemblance to the parent polyisoprene and underwent complex cyclization reactions on heat treatment. The hydrohalogenation of polydienes has been extensively investigated, and films of the hydrogen chloride adduct of natural rubber have been used for a long time in food-wrapping because of their clarity, flexibility, and inertness. There is still interest in improving the hydrohalogenation process, and a product containing 95 % of the theoretical hydrogen chloride content has been obtained by treating cis-l,4-poly. butadiene with 35 % aqueous HC1 in butanol followed by the addition of tert-butoxychloride, ta4) The properties of isoprene rubber hydrochloride do not change smoothly with increasing degree of hydrochlorination. A sudden change in properties at 29-30 % addition is apparently caused by formation of a crystalline region in the polymer and by the simultaneous change of the amorphous region from the elastic to the glassy state. (aS) Hydrobrominated polybutadiene has been rendered water-soluble by treatment with trimethylamine. The resulting polymer contains quarternary ammonium groups on the backbone and can be used as a flocculating agent, c36~

2.1.3. Addition of Thiols and Related Sulphur Compounds The addition of thiols or mercaptans to low molecular-weight compounds has been known since the early part of the century. The reaction may occur by a radical mechanism, or be acid- or base-catalysed. In the realm of thiol addition to diene polymers and copolymers only the radical-induced reaction appears to have been investigated in detail. Much of the early work on thiol reactions with polymers involved alkane thiols which add to olefinic bonds as follows: Initiator. + R S H RS.

> InitiatorH+RS.

+ ~--CHz--CH----CH--CHz--~

=

~,~

--CH 2 --CH--CH

--CHz--CH--CH--CH2--~ I

2 --CH 2 --

I RS

+ RS- etc

The termination step in this chain process produces a vicinal bis (thioether) or a polymer-polymer crosslink. The addition of methanethiol has been fairly closely studied, and adducts of 97-98 % saturation have been produced, tzT-ag) The products of this reaction display improved solvent and oxygen resistance but poor mechanical properties. Attempts have been made {4°) to exploit this reaction with a wide range of alkane thiols in order to develop an elastomer possessing a near ideal balance of

CHEMICALMODIFICATIONOF UNSATURATEDPOLYMERS

217

useful properties. The results, though encouraging, were marred somewhat by the formation of odorous compounds during cure. This is one of the drawbacks to many processes involving mercaptans. Aromatic thiols have received rather less attention, but thiophenol will add to polyisoprene in the presence of radical initiators. The addition appears to be more efficient if the initiator is added in small increments rather than in one dose. (41) Useful functional groups such as carboxyl, nitrile, hydroxyl and thioester may be attached to diene polymers by means of the thiol addition reaction.(42-4s) More recently a number of investigators have employed thiol addition to attach organophosphorus groups to polydienes. Under free-radical conditions O,O-dialkyl dithiophosphates undergo the addition (.6-48) --CH 2 -- C =CH--CH2--

I

,,.,w-, ( R O ) z P S S H

,.~,.~ - - C H z --CH--CH-.-CH2 - - ~

I

CH3

I

CH3 S ~ ( O R l z S

It is claimed that the adduct of O,O-diisopropyl dithiophosphate with polyisoprene t48) is more resistant to thermal degradation than the parent polymer. Dialkyl phosphorous acids, (RO)2POH, have also been reported to add to unsaturated rubbers under similar conditions, t49) Nitrile rubber, polybutadiene, polyisoprene, and SBR have been reacted with diphenyl dithiophosphinic acid, (C6Hs)2PSSH, to produce usefully modified polymers having better oxidation stability, flame resistance, and adhesive properties than the unmodified materials. ~5°) The reaction occurs thermally or by Lewis acid catalysis but is complicated, at least with nitrile rubber, by a crosslinking reaction. There has been periodic interest in the modification of unsaturated rubbers using sulphenyl derivatives. These compounds have the general structure K S - X , where X is an electronegative function such as halogen (sulphenyl halides), --NR'E (sulphenamides), --OR' (sulphenyl esters) and --OCOR' (sulphenyl carboxylates). They may be regarded as derivatives of sulphenic acids, RSOH. A large number of these compounds are listed in recent reviews ~s~,52) which show that the sulphenyl chlorides have been the most intensively investigated. Many sulphenyl derivatives add readily, in some cases rapidly, to olefinic double bonds. Superficially the addition is similar to that of thiols but in this case the mechanism is a polar one involving the intermediate formation of the episulphonium ion: x =

/

N

~

=

/I RS

C\

l R

A number of patents describe the addition of aromatic sulphenyl halides to cispolybutadiene, ~53~ cis-polyisoprene, ~54) and butyl rubber. ~55) It is claimed in the last that the modification provides a rubbery material of high resistivity with excellent adhesion to metals and rubbery polymers. The application of related selenyl halides has also been covered, t55) Toluene-p-sulphenyl chloride adds rapidly at room temperaP.P,S.--H

218

A. BRYDON AND G. G. CAMERON

ture to cis-l,4-polybutadiene in solution, t56) Quantitative conversion of double bonds is easily reached, providing a new polymer with the repeat unit: Cl

I ,,~.~--CHz--CH--CH--CH2--,~,,,~

I S

CH3

As the degree of saturation is increased, the physical properties change from those of an elastomer to a leathery material and ultimately to a hard resin at complete saturation. There is no loss in solubility with increasing degree of reaction, but the glass transition temperature, as expected, increases356) The authors have also prepared adducts with 2-nitro- and 2,4-dinitrobenzenesulphenyl benzoate :t56~

OCOCeH 5

I Ar

--CH z --CH--CH--CH 2 --

I ArS

and

~

~

NO 2

N02

NO2

In these cases, saturation was incomplete due to the decrease in solubility of the modified rubber, particularly with the dinitro compound. The reaction of sulphenyl derivatives promises to be useful for attaching functional groups to unsaturated polymers. Holdschmidt et aL c57) successfully modified cis-l,4-polybutadiene with p-isocyanatobenzenesulphenyl chloride: CL

I N,,~--CHz--CH--CH--CH2--,-.,--,~

I S

219

CHEMICAL MODIFICATION OF UNSATURATED POLYMERS

The isocyanate group offers possibilities for further modification by reaction with hydroxy- and amino-compounds, including polymers terminated by these groups (see later). Organosilicon moieties have been attached to unsaturated polymers by the addition of 2-chloro-2-(trichlorosilyl)- and 2-chloro-2-(triacetoxysilyl)-ethylsulphenyl chlorides :(58,59) CL

I

- - C H 2 - - C H - - C H - - C H2 -

~

I S I CH 2 X = EL or O A c

I I

CHCL SiX3

These adducts are capable of.further modification since chloro- and acetoxy-silanes are highly reactive.

2.1.4. Cycloaddition Reactions During the past 10 years a number of papers describing cycloaddition reactions on unsaturated polymers have been published. Prominent among these is a group concerned with carbene addition. The reaction of carbenes with small olefins to form cyclopropane derivatives has been intensively studied. (6°) The first application of these compounds to modify unsaturated rubbers appeared in 1964.~61) Most subsequent work has been concerned with dihalocarbenes which are easily generated under mild conditions in solutions of rubber, t°2-66) Dichlorocarbene may be produced by reaction of a base with chloroform and added in situ to polyisoprene forming dichlorocyclopropane rings at the backbone CHCL 3

+

t-C4HgOK

CH3

1

. . . . ~,- - C H 2 - - C - ~ C H - - C H 2 - ~ v ,

=

"CCLz -- ~

:CCL2 + t - C 4 H g O H

+ KCL

CH 3

I

- - C H 2 - - C -.,/ --CH--CH2 .... C CLz

double bonds. A polyisoprene sample with 18% of its double bonds converted in this way gave a vulcanizate with physical properties signifcantly different from those of the vulcanized parent polymer,(63) e.g., a decrease in solubility in benzene, decreases in tensile strength and elongation at break, but an increase in Shore A hardness. Complete conversion of the double bonds to cyclopropane structures in polydienes was achieved by generating carbenes by thermolysis of phenyl trihalomethylmercury compounds} 64) a technique which was shown to be more efficient than that involving chloroform and a base. Using thermolysis of phenyl trihalomethylmercury compounds (C6HsHgCXX'Br with X and X ' = CI or Br), cis-l,4-poly-

220

A. BRYDON AND G. G. CAMERON

isoprene, cis-l,4-polybutadiene, the polymers

and trans-l,4-polychloroprene

were converted to

R

I

---CH 2 - - CN-~CH - - C H 2 - - , . , , ~

x/CNx , with R = CH3, H or C1; X and X'-----C1 or Br. Polyisoprene was the most reactive and polychloroprene the least reactive in these modifications. The product polymers were said to have high solvent, heat, and oxidation resistance, although elsewhere it was observed that the thermal stability of such modified polymers is limited. (67) In a later paper Pinazzi and Levesque describe the reduction of the dibromo-adduct to the dihydro-adduct R

I

--CH2 - - C - - C H - - C

\/

H2--"-"-~

c H2

using tributyltin hydride,(31).BuaSnH, and to the polycumulene R

I

- - C H z - - C -----C= C H - - C H 2 - -

by means of organolithium compounds. (31) The thermolysis of phenyltrihalomethylmercury compounds (and of sodium trihaloacetates, another carbene source) suffers the drawback that it also induces main chain scission. This does not appear to be a serious disadvantage with the base-induced decomposition of haloforms and trihaloesters. Other carbene additions to unsaturated polymers have produced the cyclic

structures(6 6,6 8) ~H3 " ' " ~CH2 -- CX----/CH--CH2~~

--CH 2 -- C~--~CI-I.--C H z--

x z

/c

/C\ C6H 5 CL

C6H5 S

CL

and CH3

I

--CH 2 --C --CH~CHz

/\ H

COzR

~

R = CH 3 or CzH 5

The last-mentioned substituent was obtained by heating cis- and trans-polyisoprenes with a diazoester in the presence of a copper salt

CHEMICAL MODIFICATION OF UNSATURATED POLYMERS heat

NECHCOOR

>

CU SO4

221

:CHCOOR + N2

to produce the carboxycarbene which reacted with the olefinic bonds in the rubber. ~68' Self-extinguishing high-impact styrene polymers have been formed from radical polymerization of styrene in the presence of polybutadiene which had been reacted previously with dibromocarbene to 5-20% saturation. (69) Although carbenes can also react with hydrocarbons by insertion mechanisms at a C-H bond, R--CHE--R'

:CX2

------~

R--CH--R' HCX 2

this reaction does not appear to compete significantly with addition to the olefinic bonds in polydienes.(64'67) 1,3-Dipolar cycloaddition reactions have been used both to modify(T°-72) and crosslink (7a-75) unsaturated polymers. Nitrile oxides and nitrones are interesting examples of polar compounds which can react with olefinic bonds in polymers to produce heterocyclic substituentsF 6) The nitrone which forms from the reaction of an aldehyde with a phenylhydroxylamine adds to polybutadiene to yield an isoxazolidine ring:

RCHO

+

C6HsNHOH

/ //","'~ --CH = C l i -

nCH ~CH--

/ \ R--CH /0

C6H5

The benzhydroxamic acid chloride-triethylamine system generates a nitrile oxide which adds to polybutadiene in situ forming an isoxazoline ring: CL

I

CsHs--C=NOH

N(C2Hs)3

=

[ C6Hs--C---~N-" O] --CH---~CH--~.M ~

--CH--CH--

/

C6Hs--C%N/O

222

A. B R Y D O N A N D G . G . C A M E R O N

The carbon-black-filled vulcanizates of low cis-polybutadiene containing these ring structures showed good tensile properties. This was attributed to the interaction between the active sites on the carbon black particles and the heterocyclic structures. ~76) Chlorosulphonyl isocyanate, SO2C1.NCO, reacts with polyisoprene in a 1,2cycloaddition forming a N-substituted fl-lactam{77) from which the chlorosulphonyl group may be removed by hydrolysis: CH3 I --CHz --C = C H - - C H 2

~H3 SOzCt.NCO --,-',,,,, - ~

--CH2-- C--CH--CHz--,--,',,,

I I I

N--CO

CH3

~ / ' ~ 2 0~

s02cl"

I ""' --CHz -- C --CH---CH2 --,,,,,,~

I I HN--CO

It has been suggested that the strained four-membered ring can undergo rearrangement to a six-membered ring structure:

I-

1

,-.-.~ t C Hz-- C--CH--CH z -I-

.

L

.

.

.

c.,,.

CHz-- C---~C~H .

.

.

.

.

C(CH3 )

i\\

/

C,H--C H2-- ,-'.,'.'

'co .

C,H3/ CH2--qHz I1/

}n .

.

.

.

.

)_

\

,,-',,~---CH2-- C Jr CH C{CH3 ) CH--CH 2 I j \ \ CO--NH / "n IC02H R CO NH

Addition of chlorosulphonyl isocyanate to polybutadiene appears to require a radical catalyst. When chloranil is heated with polybutadiene or nitrile rubber at 134°C it first abstracts hydrogen from the rubber forming a conjugated diene on the backbone. After this the rubber backbone can participate in a Diels-Alder reaction :(78)

/

t

o

l

o

t

o

l

o

Chlorine-containing polymers have been produced by reaction of the double bonds in polydienes with polyhalogenated cyclopentadiene derivatives. (79,8°) It is probable that the addition reaction is of the type(al)

CHEMICAL MODIFICATION OF UNSATURATED POLYMERS

~,,~--CH2--CH:CH--CH2--~N~

223

+ Y2

X=F, Cl orBr Y=F,C[ orBr

~,~,--CHz--CH--CH--CH2--~, -X.~_~-I~Y2

X

~

-X

x producing a halogenated norbornene group on the backbone.

2.1.5. Miscellaneous Modifications While the previous sections have considered most of the broad classes of chemical modifications of polydienes, many other reactions have been carried out on these polymers. Some of these, described here, have not been investigated in great depth but further illustrate the variety of chemical changes which can be imposed on unsaturated polymers. Trifluoroacetaldehyde (fluoral) reacts with the olefinic bonds in cis-1,4-polyisoprene in a variation of the well-known Prins reaction which is catalysed by Lewis acids, in this case boron trifluoride: ~a2) OH CHa

c cHo

I --CH 2 --C =CH-CH2--

~'~

I

H2 Hc 3

=- ,,~ - - C H 2 - - C - - C H - C H 2 --,--,v,~ BF3 e t h e r o f e

"-'-~' - - CH2-- C --C --CH z --'~,~

As shown above, fl-elimination of water on heating leads to conjugation. This investigation is a natural continuation of earlier work on the addition of carbonyl compounds to unsaturated polymers. (8a) The above reaction scheme is identical to that of the Lewis acid catalysed addition of chloral to polyisoprene. The reaction of aldehydes in this manner occurs easily only when the double bond is methylated, as in polyisoprene. Polybutadiene reacts significantly with aldehydes only under radical conditions, adding carbonyl residues such as .CC12CHO , as side groups. (8*-86) The Ritter reaction t87) converts a group capable of forming a relatively stable carbonium ion, into a substituted amide by reaction with a nitrile and a strong acid. Applied to polyisoprene ~aS) this reaction modified the double bond as follows:

224

A. BRYDON AND G . G . CAMERON

,,,v~ - - C H z

[~H3

CH3 [ --C=CH--CH2

CLzCH.COOH --,,,v~

, ~,,~- - C H z - - C --C Hz--C Hz--,,,~ +

CH3 [ RCN H20 .

.

.

.

I .

I

|

NHCOR

N=C --R +

With phenylacetonitrile 12.3 ,%0of the double bonds reacted in this way, but a further 5.7 % reacted by straight addition of dichloroacetic acid. Hydroboration of olefins is now an established reaction in organic syntheses. (89) Hydroboration followed by in situ oxidation with alkaline hydrogen peroxide is a convenient procedure for anti-Markownikov hydration of double bonds: c9°) BHa

3RCH

H202

----C H 2

>

[RCH2CH2]aB

>

NaOH

3RCH2CH2OH

Partially alkylated boranes can also be produced under controlled conditions. This reaction has been successfully applied to cis-polyisoprene and polybutadiene to produce polyhydroxylated polymers. ¢91,92~ Using chromic acid as oxidizing agent, the product from this modification contains carbonyl functions. ¢92~ Hydroboration may prove to be a more convenient route to hydroxylated polymers than the older method of epoxidation followed by ring opening. ¢93~ The product of the reaction between lower alkyl boranes and natural rubber has been suggested for use as a rocket fuel.(94) A recent paper describes the asymmetric hydroboration of diene polymers/95) Optically active triisopinocamphenyldiborane (TIDB) was added to cis- and trans1,4-polyisoprene. Subsequent oxidation with alkaline hydrogen peroxide yielded optically active polymers containing 85-100% of the theoretical hydroxyl content. CH3 I

--CH 2-- C~CH--CH2 -- "~"~

CH3 I

TIDB -

/

/

HJ

IPC

H

\/

H

B

/\ CH 3 I *

IPC IPC

--CH2--CH--CH--CH2--~,, I OH

IPC = (-) isopinocamphenylgroup Dextrorotary a-pinene yielded the laevorotary polymer and vice versa. The chromophore showing optical activity was the --OH group. The analogous hydroxylated products from cis-polybutadiene, butadiene-styrene, and random butadiene-acrylo-

225

CHEMICAL MODIFICATION OF UNSATURATED POLYMERS

nitrile copolymers showed no optical activity, but an alternating copolymer of butadiene and acrylonitrile did yield an optically active product. Hydrosilation bears a formal resemblance to hydroboration. The catalysed addition of Si-H to olefins is, however, specific to vinyl-type bonds: 1,t

R3SiH -I- ) C--CH2

catalyst

J > R3Si--CH2--CH

This reaction has recently been applied to high vinyl liquid polybutadiene converting a fraction of the pendant vinyl groups to trichlorosilane moieties which yielded triethoxy or trimethoxy groups on treatment with ethyl and methyl ortho-formates respectively. (96) Of course, other modifications of the trichlorosilane derivatives could be carried out, and these groups are particularly useful for crosslinking reactions (see later). Hydrosilation may also be promoted by free radicals which give rise to a much less discriminating addition reaction. Metallated polymers are highly versatile intermediates for further chemical modification since the metal-carbon bond is reactive towards a large number of other functional groups. The general field of synthesis and reactivity of metallated polymers has been reviewed fairly recently.(97) In the particular area of metallation of polydienes, lithiation has received most attention, and much of the effort has been directed towards preparing precursors for graft copolymerization as discussed later. Lithiation of polydienes occurs fairly readily by the direct reaction of butyl lithium, in the form of its complex with a tertiary diamine such as N,N,N',N'-tetramethylethylenediamine (TMEDA), with the polymer in ahydrocarbon solvent. The metallation is accompanied by main chain scission which increases with increasing reaction temperature and amount of butyl lithium used. (98) The main sites of lithiation are the allylic hydrogen atoms; BuLl/TMEDA --CH 2-CH~-~CH-CH

2-~,~

= ~

--CH--CH~CH--CH

I

2 --

Li

Lithiated polyisoprene and polybutadiene have been reacted with various reagents such as carbon dioxide, Michler's ketone, chlorosilanes, benzaldehyde, and ethylene oxide. (99'1°°) The products of these reactions are summarized in the scheme on page 226. The treatment of lithiated polybutadiene with 2-methyl-2-nitroso-propane yields, after subsequent hydrolysis and oxidation, a nitroxide-labelled polymer (see page 226). (1o1) Such polymers have been employed to study molecular motions,(1 o2)but in this case the rapid onset of insolubilization prevented a detailed investigation. Polydienes carrying alkali metal atoms at chain ends are formed during "living" anionic polymerization. These present possibilities for modifications similar to those above. Considerable effort has gone into the preparation of polydienes carrying functional groups at both chain ends since these end-groups are useful for further end-linking reactions to form block copolymers or network structures. (1°3-~os) As this subject is discussed in another contribution to this volume(1°6) it is not further elaborated here. Aluminium-containing polymers have been synthesized by Greber. c1°7) These are of interest mainly as macromolecular Ziegler catalysts which initiate graft copoly-

226

A. BRYDON A N D G. G. CAMERON

--CH--CH=CH--CHz - -

I

H÷lox~,lJ

t Bu/NO"

--CH--CH=CH--CH2 - - ,-~ ,

,~

I

/NO t Bu

CH--CH =CH--CH2--,~

I

CHZl CH20H

N O ~ ,w,~--CH--CH=CH--CHz--~ 1 //6'. /H "+ CO2H i + />~\ /CO2/H ~

cH2-CH

,w~--CH-CH:CH---CHz--,~ CsHsCHO ~-~--CH--CH:CH-CHz--~R3SiCL '~-CH--CH=CH-CH2 - ' ~

I

H COH I C6H5

I

H+

I

Li

SiR3 Michler's ketone/H +

--CH--CH:CH--CH

z --

I

N (CH3)z

N (CH3)2

.--CH--CH=CH--CH 2-

1

(CH3laN.~+ - ' < /

~

\N(CH3) z

[merization and are therefore discussed more tully in a later section. The aluminium-

containing pendant group, however, can be reacted with reagents other than monomers to yield structurally modified polymers: --CH z --CH--~,~

I

CH

H At (C2 H5)2

~

=

--CH 2 --CH--,~,~

I

CHz

~Hz

--CHz--CH--,,,.~

I

CH2

I

OH3

I --AL(C2 H5)2 CH2

~w, --CH z __CH__,.~,~

I

CH2

I

CH20H

~

--CH 2 - - C H - - ~

I

CH2

t

CH2 S02 H

CHEMICAL MODIFICATION OF UNSATURATED POLYMERS

227

2.2. CROSSLINKING OF POLYDIENES Linear unsaturated rubbers exhibit elastic behaviour, but if held for a long time in the stretched state they do not recover their original form but show irreversible deformation. This is caused by slippage of molecular chains one past the other. Goodyear's vulcanization process, (2) in which rubber was heated with sulphur, resolved the problems associated with the inferior properties of raw rubber on an empirical basis, but the chemistry of the process was not understood until many years later. Sulphur vulcanization is now known to result in the crosslinking of linear polymeric molecules through chains of sulphur atoms to form an infinite network. The local freedom of motion of segments of molecules with respect to segments of neighbouring chains is not affected, and so the rubber retains its elastomeric behaviour, but the possibility of bulk slippage of molecules, and hence irreversible deformation, is eliminated. At higher degrees of crosslinking the rubber becomes progressively harder until ebonite is obtained. Sulphur vulcanization is still the most widely used crosslinking method, although Goodyear's original process, which required large quantities of sulphur and long cure times at high temperatures, has been improved and refined to give faster and more satisfactory cures by the introduction of accelerators and subsidiary vulcanizing agents. The modification of unsaturated rubbers by sulphur vulcanization has been developed to the point where it is now one of the most sophisticated of the industrial arts with a science and technology of its own. An account of sulphur vulcanization is outside the scope of this article which is concerned in this section with crosslinking of polydienes by agents other than sulphur. For a recent account of sulphur vulcanization the reader is referred to refs. 108--I10. Non-sulphur vulcanization of polydienes has been achieved in many ways and most of the processes are described in the patent literature. The chemistry is often obscure. In spite of the optimistic claims which have been made, many of these methods show few, if any, advantages over sulphur vulcanization and remain of academic interest only. The major disadvantage of most non-sulphur vulcanization systems has been the inferior physical properties of the vulcanizates and the greater difficulty in processing. On the other hand, non-sulphur systems are often superior in the age- and heat-resisting qualities of the vulcanizate. This is especially true of peroxide-cured rubbers. As will be seen in this section, many crosslinking agents for polydienes are difunctional counterparts of the monofunctional reagents described in the previous section. Dithiols and disulphenyl derivatives are two such reagents.

2.2.1. Crosslinking with Sulphur-containing Compounds A difunctional counterpart of dialkyldithiophosphates (described in section 2.1.3) is 3,9-dimercapto-3,9-diphospha-2,4,8,10-tetraoxaspiro [5.5]-undecane-3,9-dithione:

S%p/O-~Oxp//S Hs/\0 y ~ g \SH

228

A. BRYDON AND G. G. CAMERON

This compound can vulcanize unsaturated rubbers like polybutadienec~1~ producing crosslinks of the structure --CH z --CH a --CH--CH2--,-~

SCs I

o~ ~0 I

--CH2 --CHa--CH--CH

2 --

It is claimed that this vulcanizing agent does not need accelerators or other additives yet works slowly enough for the rubber to be shaped before it cures. Bisthioladipic acid HSOC(CH2)4COSH crosslinks rubber in a similar manner, u~2) In this ease the reaction appears to be free radical in nature. Structurally simpler dithiols than the above have also been used as vulcanizing agents. Good vulcanizates of cis-l,4-polybutadiene and natural rubber have been reported using 1,12-dodecanedithiol and other simple dithiols in the presence of an accelerator such as N-cyclohexylbenzothiazole-sulphenamide.(l~a) Liquid polybutadiene has been cured with aliphatic dithiols, such as 1,6-hexanedithiol, in the presence of organic peroxides, u~4) The products were claimed to have comparable mechanical properties to those obtained from carboxyl-terminated liquid polybutadiene but with greatly improved stability against hydrolysis and air oxidation. Dithiomorpholine and other thiobisamines may function both as accelerators and sulphur-donors in vulcanization. These substances are considered as sulphurcuring agents by some authors (l°s) since, although no free sulphur is produced, the sulphur crosslinks formed are identical to those from some direct sulphur cures. The initial reaction ofthiobisamines is cleavage to active fragments, but the mechanism of cleavage is not clear. Free-radical, ~15) ionic, and hydrosulphide-promoted reactions (I~6) have been suggested, and in some circumstances all three may operate. In a recent paper on dithiomorpholine vulcanization~ 7 ) it was suggested that $4 crosslinks are formed in a free-radical reaction in which tetrathiomorpholine is the reactive sulphur-donating intermediate: crossllnked MS2 M

-"'- M ,

+

• SzM

=

MS4M

---

rubber

/---k M --O x M . _ j ~ Cyclic analogues of thiobisamines have also been used as vulcanizing agents/118) Many di- and multifunctional sulphenyl derivatives rapidly crosslink unsaturated rubber. Sulphur monochloride ($2C12) and dichloride (SC12) may be regarded as the simplest difunctional sulphenyl chlorides, and both have been known as crosslinking

CHEMICAL MODIFICATIONOF UNSATURATEDPOLYMERS

229

agents for rubber for some time. (119) Although there has been uncertainty in the past as to the mechanism of addition of these compounds, it now seems likely that a polar intermediate is involved. The reaction of the monochloride with olefins is not simple. Disulphide, trisulphur dichloride, and sulphenyl chlorides are formed as intermediates.(120) One of the earliest polyfunctional organic sulphenyl chlorides to be used for direct crosslinking of diene rubber was 1,3,5-triazine-2,4,6-trisulphenyl chloride :(~2i. ~22)

CLS~N SCL N~N SCL By first reacting this compound with isobutylene or some other simple olefin in controlled amounts, it was converted to a difunctional crosslinking agent. (122) Disulphenyl chlorides react with unsaturated polymers to produce crosslinks of the form: CL

I

--CH 2 --CH--CH--CH i S

2 -- ~

I

R S ~

--CH 2 --CH--CH--CH f

2 ....

CL

The sulphur atoms are contained in the crosslink as also happens with dithiol crosslinking. The related compounds, bis(alkylsulphenyl) trithiocarbonates, have been patented as vulcanizing agents. (12a) The crosslinking reactions discussed up to this point have been direct or one-stage in the sense that only one reaction occurs at each polymer chain involved in the crosslink. Crosslinking, however, may be a two-stage process, the first stage being the bonding of suitable functional groups to the polymer backbone and the second the further reaction of these groups to form crosslinks. The previous section outlined a number of reactions which could be used to place potential crosslinking functions on the polydiene backbone, e.g., the addition of p-isocyanatobenzenesulphenyl chloride to polybutadiene. Further reaction of the modified polybutadiene with a glycol or a diamine leads to crosslinking. (s7) A similar modification to introduce triacetoxysilyl and trichlorosilyl units on to the backbone has also been mentioned. These groups are readily hydrolysed, even by atmospheric moisture,

Cl I --CHz--CH--CH--CH2 I S I

? H2

CHCL I Si ( O A c ) a

Cl I -

'''~

H20 =

"~'~ - - CH 2 - - C H - - C H - - C H 2 - - ~ I S I

~H2

CHC1 I ( A c O ) = Si ( O H )

230

A. BRYDON AND G. G. CAMERON

and the resulting silanol can condense with a second silanol or other functional group to form a siloxane crosslink: tSs'sg~

I

I

I

--Si--OH -}- HO--Si--

I

I

> --Si--O--Si-- q- H20

r

I

I

This reaction provides the means for chain-extension and crosslinking in self-curing silicone rubbers t124) as well as in the example quoted here. The thiol addition reaction described earlier has been used to attach various functional groups to polydiene chains, c42-4s) The radical addition of thioglycolic and other mercaptoacids to rubbers is a well-known modification, and the resulting carboxylated elastomer, for example, --CH 2 --CH--CH z --CH 2 --

I s

I CH2C02 H

can be crosslinked by metal-salt formation using polyvalent metal oxides, hydroxides, salts of weak acids, etc. Metal-salt vulcanizates containing crosslinks of the general type

co I 0I co

have been comprehensively reviewed by Brown. t125) One of the main advantages claimed for these metal-salt vulcanizates is that the crosslink breaks down on heating above about 100°C, allowing the rubber to be processed and moulded easily, and reforms again on cooling. Carboxylated rubbers can also be crosslinked by covalent bond formation through reaction with polyfunctional compounds such as polyamines, polyepoxides, polyisocyanates, etc. ~12s) Several other types of reaction have been successfully employed to carboxylate elastomers. Some of these are discussed later in this section.

2.2.2. Crosslinking by Cycloaddition Reactions Since the previous subsection ended with a short discussion of carboxylated elastomers and their preparation via sulphur-containing compounds, it is convenient to open this subsection with a carboxylation reaction involving a 1,3-dipolar addition. The addition of a nitrile oxide generated in situ from benzhydroxamic acid chloride has already been discussed. The analogous addition involving the carboxylated benzhydroxamic acid chloride produces carboxyisoxazoline rings on the polymer backbone :tTo)

CHEMICAL MODIFICATION OF UNSATURATED POLYMERS

231

--CH 2 - - C H - - C H ' - C H 2 - -

/

\

HOzC ~

Carboxycarbenes also add to polydienes in a cycloaddition reaction (see section 2.1.4) to produce carboxylated elastomers which can be subsequently vulcanized. Difunctional analogues of the nitrile oxides and nitrones described in the previous section crosslink unsaturated rubbers by a 1,3-dipolar addition. The difunctional hydroxamic acid chloride NOH

II

C--CL

C--CL

II

NOH

is converted to a dinitrile oxide and can link two rubber molecules via two isoxazoline ringsF 5) Similarly, the dinitrone from terephthalaldehyde and phenyl hydroxylamine vulcanizes rubbers by isoxazolidine ring formation3TM Although the authors of refs. 74 and 75 describe their vulcanization systems as novel, the use of nitrile oxides and nitrones as crosslinking agents was covered in patent applications in 1966(126) and 1968.~12~) These patents describe the use of several other polyfunctional reagents such as 4,4-oxybis(phenylglyoxylohydroximoylchloride), which crosslink unsaturated polymers after intermediate formation of dinitrile oxide or dinitrone. A number of diazo- and azido-compounds have been developed as vulcanizing agents. (12s-131) Many of these compounds decompose easily, losing molecular nitrogen and forming radical intermediates which readily react with double bonds. Azido-compounds decompose to form nitrenes which attack double bonds in a cycloaddition reaction similar to that of carbenes. One such compound recently described is 1,3-bis-(p-azidobenzylidene)-5-methylcyclohexan-2-one which decomposes on irradiation with light to form molecular nitrogen and a dinitrene, tlaa) The latter adds to double bonds in unsaturated rubbers such as polyisoprene yielding crosslinks with the structure:

I

I H2

0

CH2

H... /-7-X II /--~ clH I i N - - ( ( ' ) ~ - C H = f " " ~ = C H - / ~ ~,--NI I cH~-c ~ / / ~ \C--Ca3 l

cH3

I l

232

A. BRYDON AND G. G. CAMERON

Disulphonylazides(132) and azidoformates°3a) have also been used as crosslinking agents. The latter were claimed to yield with polyisoprene and polybutadiene tough, odour-free vulcanizates with good resilience and solvent resistance.

2.2.3. Other Crosslinking Reactions Besides those considered in the two preceding subsections many more reactions have been utilized to crosslink unsaturated polymers. The mechanisms of these reactions are highly varied and they are included here under one heading only for convenience. Peroxide vulcanization of rubbers has been one of the more successful non-sulphur methods. The first record of a peroxide cure appeared in 1912 by Ostromislensky, who vulcanized natural rubber with benzoyl peroxide, tla4) Dicumyl and other peroxides are now more commonly used, and detailed investigations of the chemistry involved have been undertaken. The predominant reaction in dicumyl peroxide curing of natural rubber appears to be the removal of an allylic hydrogen atom CH3

CH 3

I

--CHz--C----CH--CH2

I

-~'~

+

RO'---"-~'~'~--CH2--C=CH--CH

-~'~

+ ROH R = Curny[

to form a resonance-stabilized allylic radical. (1as) Combination of two such polymeric radicals would lead to an intermolecular crosslink, but doubts have been expressed° 35) as to the feasibility of such a process in polyisoprene. Crosslinking may also occur by addition of the rubber radical to a double bond in another rubber molecule. Some degradation occurs during cure and branching as well as crosslinking reactions probably occur. A fuller account of the chemistry is given in ref. 135. The same amount of dicumyl peroxide causes a much higher degree of crosslinking in cis-polybutadiene. This has been attributed to the ability of the more reactive polybutadienyl radical to form a train of crosslinks by addition to double bonds. (s) It has also been suggested that in synthetic rubbers some addition of the primary radicals takes place at double bonds, o36)

RO. +

,~--CHz--CH=CH--CH

z - - ,',.','~--.---.~ ~

- - C H z - - C H - - ( ; H - - C H z -~-~

I

RO

Several maleic derivatives react with unsaturated compounds, including natural rubber, in which the reaction may be thermally induced at about 200°C or peroxide promoted at about 100°C. c137-139) Crosslinking occurs to a limited extent during the peroxide initiation reaction, but with maleic anhydride and the monomaleimide this well-studied reaction is not really an efficient vulcanization process. Nevertheless, this polydiene modification reaction still attracts some attention. (14°-142) Dimaleimides, on the other hand, crosslink unsaturated rubbers very efficiently. The peroxidepromoted reactions of anhydrides and imides have the same initiation step as straight

233

CHEMICAL MODIFICATION OF UNSATURATED POLYMERS

peroxide curing, viz. the abstraction o f an allylic hydrogen atom to form a polydienyl radical. Dimaleimides add to this radical CH~ I

CH--CO

~C--CH

CH3

I

--CH 2 --C ~CH--CH--~"

I

to yield a new radical which can abstract a hydrogen atom from another polymer chain forming a new rubber radical:

CH3 /

R'

" " ' ~ --'CH 2 - - " = CH--CHz - - " ' " ~

I

+

CH--CO

I

CH--CO

~:H 3

CH2 - - C R'=

1

-~--CH--~:H--,',',,-.,~

QC--CH

>NRN<~]

I[

OC--CH

R'

+

I

~FI--C~NRN O/C-OH CH2-CO

rubber molecule

~OOC--~H A

It is fairly obvious that the sequence of reactions above will not lead to crosstinking until the second maleimide group A also reacts the same way. Since this second reaction site is absent from maleic anhydride and monoimides, the reaction with these compounds is predominantly a simple modification; crosslinking occurs only as a termination step or a side reaction such as addition. The final structure of the crosslink with dimaleimide is CH3

I

--CH 2 - - C ~ C H - - C H

I

....

~ H - - C ~ NR N ~ C - - ~ H 2 CHz--CH

'

OC--CH

I

--CH 2 - - C - - - ~ C H - - C H - - ~

I

CH3

Although this reaction was elucidated some 15 years ago (143) and has been the subject o f several patent applications, (144'145) it still commands some interest as indicated

234

A. BRYDON AND G. G. CAMERON

by, several recent papers from the Soviet Union.t146-14a) It is probable that the diacetal

CH3 C H : C H

CH --~-CHCH3

which has been claimed as a useful vulcanizing agent for unsaturated rubbers, c149) functions in a similar manner to dimaleimide. Structurally related to the maleimides is 4-phenyl-l,2,4-triazoline-3,5-dione, a highly reactive enophile which reacts as follows:(~5°)

./%. H ,//.N\

"~\

CO CO-N \

Ph

H~ /N N

\CO

~CO--N~\ph

This reaction is formally identical to the non-radical addition reaction of maleic anhydride to olefins c15~) but proceeds with much greater facility. It offers further possibilities for chemically modifying polydienes. The corresponding difunctional, enophile, bis-(p-3,5-dioxo-l,2,4-triazolin-4-ylphenyl) methane, readily crosslinks rubber. (xs2) However, in common with many disulphenyl derivatives, it reacts far too rapidly to be of much promise as a commercial vulcanizing agent. Quinone dioxime and its derivatives have been known as vulcanizing agents for some time. The principal virtue of these vulcanizates is their good heat stability but, partly for economic reasons, commercial application of dioxime curing has been limited to butyl rubber. The reactive agent in quinone dioxime cures appears to be dinitrosobenzene formed via in situ oxidation by, for example, red lead, NOH

il

NOH

NO

NO

and this adds to two molecules of rubber. Dinitroso-compounds, including N-nitroso derivatives, have also been used directly/129'ls3-1ss) The mechanism of addition of nitroso-compounds to olefins is complex and not yet fully understood. Nitrone structures have been proposed as products of nitrosobenzene addition to natural rubber, ~156,157) which also undergoes main chain scission during reaction, ~58) but anils,<159) oxadiazole-N-oxides, (16°) and nitroxide radicals (a6~) are known to be formed in the reaction of C-nitroso compounds with olefins. Nevertheless, it has recently been claimed~162) that the original mechanism proposed for crosslinking by dinitrosobenzene is essentially correct c~s 3.163) and that in the presence of a sufficient amount of oxidizing agent uni-azoxycrosslinks (dinitrone) are formed:

CHEMICAL MODIFICATIONOF UNSATURATEDPOLYMERS

235

CH3

I ........... C H ~ C - - C - - C H

2 .....

II

NO

N0 II . . . . . . . . . CH ~ - C - - C - - C H z . . . .

I

CH3

Baker et al. (164) attempted to utilize the reaction of nitroso-compounds, such as p-nitrosophenol and p-nitrosoaniline, with rubber in order to attach - - O H or --NH2 groups to the rubber chain. The idea was to set up to a two-stage vulcanization system by subsequently linking these functional groups in a reaction with a di- or polyisocyanate. Although crosslinking was achieved, the vulcanizates were unsatisfactory, mainly because of scorching during milling. A more satisfactory one-stage system was derived by milling p-nitrosophenol and dicyclohexylmethane-4,4'diisocyanate simultaneously with the rubber. (164.x65) Vulcanization occurred rapidly at 180°C, but the expected reaction between the phenolic hydroxyl groups and the isocyanate did not occur. Instead, nitrosophenol reacted as its oxime tautomer to give a diurethane:

o

C)= Notch

oH2

0

0

By thermal dissociation this diurethane apparently generated both p-nitrosophenol and diisocyanate which then underwent the expected crosslinking reaction. It has been found (166,16v~ that p-nitrosophenols and -anilines react with olefins in the absence of oxidizing agent to form N-alkenyl-p-phenyl compounds:

""-N

HON

NH

L
X

"[

/

× X = NHa, NHR, NR2 and OH

x

236

A. BRYDON AND G. G. CAMERON

Consequently most of the crosslinks on natural rubber from the p-nitrosophenol/ diisocyanate system are expected to have the structure

I

i I

CH2 I

CHz

! CH2 ----C

x~/

II 0

x___/

x___/

II ~ 0

| C =CH 2

I

I

~;H2

CH2

I

The use of the diurethane may have other advantages because nitroso-compounds are toxic and by this means are not handled directly. Also formation of coloured byproducts, formed by reaction of p-nitrosophenol with itself or with intermediates in the reaction, is minimized. The reaction of diboranes with polydienes was mentioned in connection with hydroxylation of rubbers. Crosslinking by boranes has also been investigated. Reaction of diborane with simple olefins to give organo-boranes (see section 2.1.5) shows that if applied to unsaturated rubbers a hexafunctional crosslink should result. Boron hydrides have, indeed, been used as vulcanizing agents, (168) but their practical use is limited because of their extreme reactivity. For this reason Baker et al. (~64) used amine-boron complexes such as triethylamine-borane, (C2Hs)3N'BH~, to achieve crosslinking. Chlorinated complexes such as triethylamine-chloroborane (C2Hs)3N.BH2CI and triethylenediamine-bischloroborane /CHz--CHz Cl H2 B. N--CH2--CH 2 ~N. BH 2 CL ~CH 2 --CH2/"

gave less internal cyclization and accompanying hydrogen evolution than did the non-chlorinated borane complex. A study of the addition of triethylamine-chloroborane to 2-methylpent-2-ene (a one-unit model for natural rubber) showed an almost quantitative addition: 2

CH3\ CH3/C = CH - CH2-CH 3

+ Et 3 N, BH2Ct

L

CH3~cH-CH -CH2-- CH3 CH3/ l BCL I CH3"CH - CH - CH2- CH 3

+ Et3N

CH~

With cis.l,4-polyisoprene the reaction is more complex since, in addition to crosslinks of the type formed with the model, intramolecular reactions occur to form the groups

CHEMICAL MODIFICATION OF UNSATURATED POLYMERS

237

/CH2 /CH3 CH2

"CH

I

I

. . . . . . . . CH2--CH--C~. . / C H - - C H 2 - - ~ ' ~ '

e

CH3

I CL

and

.....

C~ L

CH2.. jCH3

cH L

CH2 ---CH--CH. .CH - B J ~CH 2

CH~

i

i

CH2~\C /CH2 LL CH i

The latter arises from elimination of HC1 as the amine hydrochloride. The major disadvantages of these chloroborane vulcanizates are their high degree of creep and stress relaxation. The nature of the stress-labile crosslinks which are responsible for these failings is not entirely clear. The hydrosilation reaction has also been used to crosslink unsaturated polymers. A thermosetting resin has been prepared by reacting a styrene-butadiene copolymer containing vinyl groups with a siloxane containing Si-H groups in the presence of a platinum catalyst.(t69) The application of the hydrosilation reaction to attach reactive functional groups to unsaturated polymers was discussed briefly in section 2.1.5. This reaction has been employed to prepare room temperature vulcanizing polymers and copolymers of butadiene by addition of diacetoxymethylsilane.(17°) The acetoxysilyl units are readily hydrolysed to silanols which condense to give a siloxane crosslink. This reaction has already been mentioned in section 2.2.1 in connection with sulphenyl chloride addition as a means of introducing acetoxysilyl units.

2.3. GRAFT COPOLYMERIZATION OF UNSATURATED POLYMERS It is common practice to consider graft and block copolymers together. This is quite logical since the syntheses of these substances have many features in common. Block copolymers, however, are linear species produced by some chain-extending reaction. In the present context we are concerned with backbone modification of polymers rather than chain-end reactions, and hence only graft copolymers, which fall within this definition, are considered. However, one may consider a block copolymer as a simple type of graft copolymer in which the graft chain is attached to the end backbone unit. The commercial advantage of graft copolymers rests on the fact that it is difficult to make a well-dispersed blend of polymers which have a low compatibility with one another. Furthermore, unless the components of a blend are mutually soluble (a rare situation in high polymers), they will tend to separate under stress, and the product

238

A. BRYDON AND G. G. CAMERON

will possess poor physical properties. This separation cannot occur in a graft copolymer because the components are connected by covalent bonds. High-impact polystyrene is a good example of a commercial exploitation of graft copolymers. This material may be prepared by polymerizing styrene in the presence of about 5 % ofpolybutadiene or butadiene-styrene rubber. The product contains a mixture of homopolymers together with some graft copolymer. The properties of this material reflect the contrasting contributions of the two components, polystyrene providing the hard, plastic properties, and the rubber the resilience. A similar combination of properties is shown by methyl methacrylate-natural rubber grafts. The incompatibility between the components of a graft copolymer is not confined to the solid state, and also gives rise to complex solution behaviour. The FloryHuggins theory of the thermodynamics of dilute homopolymer solutions has proved a useful tool for interpreting the properties of polymer solutions. With graft copolymers this theory is inadequate, and there appears to be no single theory or generalization which deals adequately at present with graft copolymers in solution. The solution behaviour of graft copolymers is not a simple function of the properties of the component homopolymers. Among the difficulties to be overcome in studying the properties of graft copolymers, either in solution or in the solid, is the preparation of well-characterized material. Much effort has been expended in obtaining "model macromolecules", i.e., graft copolymer samples which are "little polydisperse in mass, of satisfactory homogeneity in composition, and in which the molecular structure is defined unambiguously by the conditions of preparation".(171) In the field of grafting reactions to polydienes it is probably safe to say that no synthesis has met all these requirements without careful fractionation of the final product. Most grafting reactions lead to homopolymer formation and often leave unreacted polydiene. Even when these contaminants have been removed, the final product may still be a long way from meeting the requirements of a "model macromolecule" since the branches may not be of uniform length, may not be uniformly distributed along the polydiene backbone, the backbone may have undergone degradation or other side reactions, etc. These and other difficulties are apparent in the ensuing review of the main methods which have been used to prepare graft copolymers of polydienes.

2.3.1. Free-radical Methods

Free-radical polymerization of a vinyl monomer in the presence of a polydiene yields some graft copolymer. The proportion of the vinyl monomer incorporated as grafted chains depends mainly on the nature of the unsaturated rubber, the vinyl monomer, and the initiator. This grafting reaction is one of the simplest to perform but suffers the disadvantage that large amounts of homopolymer are formed. It has been applied to polydienes in solution, in latex s y s t e m s , 072'~7a) and, using y-irradiation as the source of initiating radicals, to polymer swollen with monomer.(174) Most of the early investigations used natural rubber as the polydiene substrate. A radical site on the polymer backbone may be formed by the abstractive or additive attack of an initiator radical (I.) on the polydiene,

239

CHEMICALMODIFICATIONOF UNSATURATEDPOLYMERS CH3 + ~

CH3

I

t

--CH2--C~CH--CH 2 . . . . . .

~'~

CH2 - C - C H - C H 2 ........ I I addition CH3 --CH 2 - c

I ~CH--CH--

~" +[H

abstraction

or by chain transfer from a growing polymer radical: CH3

I

M. + ,--,~--CH2--C~CH--CH2--~,,~

MH

+~

CH3 1 --CNz--C~CH--CH--

The addition of a rubber molecule by propagation o f the growing polymer radical through a double bond in the rubber backbone can be discounted as a significant

contribution to grafting on the grounds of the comparative unreactivity of 1,2disubstituted ethylenes in radical polymerization. There is fair agreement on the broad features of the reaction. Initiators, like benzoyl peroxide (BPO), which produce radical fragments with a strong tendency to abstract hydrogen atoms, give the highest grafting efficiencies.1- On the other hand, azobisisobutyronitrile (AZBN) gives the relatively weak hydrogen-abstracting 2-cyanoisopropyl radical (175) and yields mostly free homopolymer. Some authors (176-1s°) have claimed that AZBN is incapable of yielding any graft product from polyisoprene, but this has been disputed by others ~181-ls4) who maintain that AZBN yields a small proportion of grafted material with styrene and methyl methacrylate. This highly specific initiator effect rules out the macroradical transfer step as a significant source of graft formation. It follows that direct initiator radical attack on the rubber must be the major source of radicals which ultimately give graft copolymer. The hydrogen abstraction reaction yields a resonance stabilized allylic radical so that energetic considerations would seem to favour this process rather than addition which yields a much more reactive alkyl radical. The abstraction process is also favoured by the strong hydrogen atom affinity of BPO fragments already mentioned. It has also been argued ~s) that the addition reaction requires favourable orientation of the double bond under attack, and hence co-operative motions of the chains attached to the ends of the C = C bonds. Other authors have pointed out that, like peroxide vulcanization, the grafting reaction involves a negligible loss in unsaturation in the polymer3184) The weight of evidence is therefore in favour of a hydrogen abstracting reaction for initiation by BPO. Few reactions are absolutely exclusive, and some addition probably occurs as well, although this is unlikely to be as frequent as some of the early work in the field suggests. °s°) In the case of initiation by AZBN, the situation is not so clear, but for the few radicals that do attack the rubber, both addition and abstraction probably ~ Grafting efficiency=

Weight of monomer grafted Total weight of monomer polymerized

240

A. BRYDON AND G. G. CAMERON

occur, with the latter predominating3184) Styrene and methyl methacrylate are the two monomers which have been most frequently grafted to polyisoprene by radical means. With both monomers the rubber acts as a retarder. A few experiments have been carried out with other monomers, such as methyl acrylate and vinyl acetate, but these failed to produce significant amounts of graft copotymer, and the rubber severely retarded polymerization3176,178) This is almost certainly due to the low reactivity of the monomer towards the allylic rubber radical. Polybutadiene has been the subject of some recent radical-induced grafting experiments with styrene and methyl methacrylate,t185,186~ A pattern of behaviour very similar to that of natural rubber is evident with BPO t186~ and dicumyl peroxide t185~ giving grafting and AZBN ~186) or thermal polymerizationt185~ showing a very much lower efficiency. It is probable that the efficient initiators operate by first abstracting a methylenic hydrogen atom, as is the case with natural rubber, to produce an allylic radical which then initiates monomer addition. Radical addition and macroradical transfer are likely to be comparatively infrequent. Experiments with polybutadienes with different microstructures showed that the high-vinyl polymer was the most reactive. ~186~The present authors have also studied the grafting of styrene to cis-l,4polybutadiene and found that at 60°C in benzene solution BPO produced graft copolymer but AZBN formed only homopolystyrene.(56~ Under identical conditions, however, methyl methacrylate formed a small amount of grafted material with AZBN, indicating that the methyl methacrylate radical can transfer to the polybutadiene chain. (187~ These experiments were carried out in fairly dilute solution (e.g. 0.53 monomer-mole 1-1 of rubber, 2.5 × 10 .3 mole 1-1 of initiator, and 1-3 mole 1-1 of monomer) to prevent complications due to phase separation. The polymer retarded polymerization of methyl methacrylate, but the rate of styrene polymerization was unaffected by the polymer provided the concentration of the latter was kept below about 1.0 monomer-mole 1-1. A number of authors have attempted a kinetic analysis of radical grafting to polydienes. ~181,1as.~a8,~89~ The mathematical expressions for the rate of polymerization and graft efficiency are invariably complex, and experimental verification has been rendered near impossible by the unavoidable inclusion of unknown parameters. At best it has been possible to show that the expressions are not inconsistent with experimental observation,~181'185~ but this has necessitated rather drastic approximations in places. For example, Minoura et al. ~18~ were able to give theoretical justification to the observed dependence of the rate of polymerization on natural rubber and styrene concentrations: B[R]

R , / R o = 1/(1 + C+--~-]!

In this expression, Rp is the rate of polymerization of styrene, Ro being the value of Rp in the absence of rubber, [R] and [M] are rubber and monomer concentrations respectively, and B and C are constants. This derivation required the rather dubious assumption that the rate of attack of I. on rubber was very small compared with the initiation rate for styrene homopolymerization. The corresponding expression for the grafting efficiency is extremely unwieldy and incapable of full experimental verification, but it is consistent with the observations that the graft efficiency decreased with increasing initiator and monomer concentrations and decreasing rubber con-

CHEMICAL MODIFICATION OF UNSATURATED POLYMERS

241

centration. Most kinetic interpretations have been based on a set of elementary reactions of the type: i2

k~

2I.

Decomposition of initiator

I. + M

k,~

IM-

Radical attack on monomer

I ' + R

k,~

R.

Radical attack on rubber

~- R M .

Rubber radical attack on monomer

R-+M

k,~

RM. +M

k,

RM~ k ,

IM. + M

k,

1M½

> RM;, Propagation reactions

--

>

k,

>

IM~

RM; + RM,;, IM;, + IM m R-+RInactive products

Termination reactions

IM;, -I- RMm RM;, + R. IM;, + R. One of the complicating aspects of this scheme is the participation of the rubber radicals R. in termination. This is a form of degradative chain transfer which can retard the rate of polymerization. In the recent work on BPO-initiated grafting of styrene to polybutadiene in dilute benzene solution, (56) the reaction appeared to follow ideal kinetics with no retardation. This indicates a negligible participation in termination by R. radicals and greatly simplifies the kinetic analysis which yields the relationship 1 kiz[R] - - 1 + ~ (1-GE) ÷ k~l [M] where GE is the grafting efficiency. The experimental data were consistent with this relationship provided the polydiene concentration did not exceed about 1 monomer-mole 1-1. The ratio k~2/k~l was found to be 0.62 ~ 0.03 for both cis- and trans-l,4-polybutadiene, and was independent of the initiator concentration. Under the conditions of these experiments the graft efficiency was determined solely by the partition of initiator fragments between the monomer and polybutadiene. This fortunate situation is unlikely to be observed, however, in many other systems. The phenomenon of phase separation has been touched upon briefly. This arises when two or more of the components in the reacting system are incompatible, and there is insufficient solvent in the system to hold them in the same solution. For

242

A. BRYDON AND G. G. CAMERON

example, when styrene, containing a sufficiently high rubber concentration, is polymerized, two phases quickly form, one containing rubber in styrene and the other containing polystyrene in styrene,t~s s) This means that the concentration of monomer in the vicinity of the reacting rubber is lower than that initially present. Phase separation was noted in many of the investigations discussed in this subsection, but its possible effects on the detailed kinetics and mechanism of the grafting reaction have rarely been considered. It is most likely to occur in systems in which the polydiene concentration is high or where the monomer is polymerized to high conversions, particularly when the monomer also functions as solvent. Variable extents of phase separation may partly explain the conflicting observations of different authors reported above.

2.3.2. Ionic and Coupling Reactions As the previous subsection shows, the main problem with radical grafting is the difficulty in confining the propagation reaction to radicals produced on the polymer backbone. If some means could be found of generating radicals singly on the polydiene backbone, radical grafting would become more attractive since the only reactions leading to homopolymer would then be transfer reactions, e.g., to solvent or monomer. This does not appear to have been achieved with polydienes. The synthesis of macromolecular organometallic compounds would seem to offer a more promising alternative to radical methods since the polymer chain could function as a carbanion capable of adding to suitable monomers. Polybutadiene and polyisoprene can be lithiated at the ~-methylenic carbon as was mentioned in section 2.1. Minoura and Harada ~189) showed that the lithiated polybutadiene and polyisoprene readily polymerize styrene, methyl methacrylate, and acrylonitrile. The highest grafting efficiency they obtained, however, was 72 ~ (with styrene), indicating the occurrence of some side reactions. Halasa e t a/ . t9a,l°°) carried out similar experiments but employed much lower levels of lithiation. They too observed the formation of some homopolystyrene but suggested that this might be due to the formation of metallated TMEDA during the lithiation reaction or to a transmetallation reaction with unreacted styrene rather than to the presence of unreacted butyl lithium. By increasing the metallation reaction time the grafting efficiency was pushed up to 95-97~. However, increasing the metallation time or temperature (50-80°C in this instance), or increasing the butyl lithium concentration, results in main chain scission in the polydien6. The mechanism of this degradation reaction is not clear but it is certainly an undesirable side-effect. The grafting process described so far is obviously not as clean as might be imagined, and one has to compromise between maximizing grafting efficiency and minimizing degradation. The lithiation technique has been the subject of several patents.(191-~ 95) An alternative method of generating anions on the polymer is to have attached to the backbone functional groups which can be exchanged for lithium under mild conditions. Polybutadiene containing 3 - 5 ~ of copolymerized o-chlorostyrene was found to exchange readily the chlorine for lithium (from butyl lithium in TMEDA) at 25°C. (98) Lithium-halogen exchange proceeds with fewer side reactions in an aromatic-halogen compared with an alkyl-halogen system. The resulting copolymer containing aromatic lithium groups readily polymerized

CHEMICAL MODIFICATION OF UNSATURATED POLYMERS

243

styrene with a 100% grafting efficiency. It is not clear whether the metal-exchange reaction produced any significant degradation. A related but somewhat less successful route to grafting styrene to polybutadiene and other unsaturated polymers, involved carbanionic deactivation. This is a type of coupling reaction. The polymer was first brominated to a small extent then reacted with "living" polystyrene prepared by anionic polymerization. (196) The anticipated reaction is the elimination of alkali metal bromide, but this occurred to a limited extent because of significant transmetaUation, and much of the polystyrene remained ungrafted. Halogen-metal interchange occurs much less easily with silicon compounds, ~19v) and coupling of polystyryl lithium using 1,2-bis(methyl dichlorosilane) is virtually quantitative. (t98) This suggests that the carbanionic deactivation technique is more likely to succeed if the halogen involved in the reaction is attached to the polydiene through a silane structure such as"

--CH 2

--CH-I CH2 I

CH2

I

CH3--Si- - C H 3 I X This possibility does not appear to have been tested but, as shown in section 2.1.5, there should not be too much difficulty in bonding a substituted silane to a polydiene containing some vinyl unsaturation. Greber e t al. (199) have carried out a comprehensive series of investigations on the addition of organoaluminium hydrides to unsaturated polymers containing vinyl or vinylidene groups, e.g.: ....

--CN2--CH--CH 2

--CH..... +

HALR 2

CH H

CH2

. . . .

I

CH2 - - C H - - C H 2 - - C H . . . . . . . .

j..

i

CHzALR 2

By treating the products with transition metal halides such as TiCI4, macromolecular Ziegler-Natta catalysts of the type

244

A. B R Y D O N A N D G . G . C A M E R O N

--CH 2 - C H .

t-I2

CzHs~A 1 . . - / " "'... Cz 14/5 - . - . . - " "Ti

/Ct \CL

CL

were formed. These are capable of polymerizing ethylene and a-olefins to yield graft or block copolymers, depending on the position of the organoaluminium group on the chain. With propylene a graft copolymer having a stereospecific side chain was obtained. (x99'2°°) This technique leaves a proportion of the initial unsaturated polymer unreacted but no homopolymer of the olefin appears to form. u99,2°1) This is surprising since any organoaluminium compound unbound to the polymer would be expected to initiate homopolymerization on the addition of TIC14 and olefin. Fractional extraction showed that the graft copolymer formed in this reaction is highly heterogeneous with respect to composition. Cationic routes to graft formation on polydienes have not been explored in much depth. They are usually difficult to control, and with highly unsaturated polymers often lead to crosslinking. Transfer to monomer, which is common in many cationic polymerizations, leads to homopolymer formation. At least one successful cationic graft reaction has been claimed in the patent literature. (2°2) This involved grafting isobutylene to a polydiene containing about 1 Yoof bromine or other halogen. A Lewis acid like TIC14 functioned as catalyst at about --80°C, and the product yielded a graft copolymer containing about 20 ~ of polydiene. Another successful cationic grafting reaction was reported recently by Kennedy.(2°a) Again, the parent polymer contained halogen, e.g. chlorobutyl rubber, polyvinylchloride, etc. The principle of the synthesis is that the polymer chloride interacts with an alkylaluminium compound in an initiator-coinitiator system(2°'~) in which the carbonium ions are generated on the polymer backbone: +

--CH--~

I

CL

+

AIR 3 ~

~

--CH -~'~

[CLALR ]-

Polymerization of a cationically active monomer such as styrene or isobutylene leads to grafting and, provided the polymer does not contain too much unsaturation, side-reactions like crosslinking are avoided. It is claimed that under suitable conditions, pure graft copolymers are obtained. (2°3~ Apart from one instance, involving deactivation of living polymer ends, all the methods described in this section have dealt with grafting by the initiation of polymerization at an active site in the polydiene chain. A more general account of the preparation of graft and block copolymers by variations of anionic polymerization is given elsewhere in this volume.(1°'~ In an earlier part of this article (section 2.1.3) the addition ofp-isocyanatobenzenesulphenylchloride to polybutadiene was described. ('7) The pendant isocyanate groups may be coupled with other polymer molecules terminated by suitable functions such as hydroxyl or amino groups to yield graft

CHEMICAL MODIFICATION OF UNSATURATED POLYMERS

245

copolymers. Polyether chains have been grafted to polybutadiene in a similar type of coupling reaction. The low molecular-weight polybutadiene was first reacted at pendant vinyl groups with a trialkoxysilane (e.g. triisobutoxysilane) in the presence of a platinum catalyst: ~

--CH 2 --CH ....

I CH

+ (RO)3 SiFI

..... Pt

CFI 2 - - C H - I

Catalyst

CH2 I

II

CH2

Si(OR) 3

/ ~--CH

2--CH

H(OC~H~)~7{OC3 H6),30 -C4 H9

.....

I CHz I CH2 / (RO) 2

Si

--(OC 21-14)17(OC 5 H6)150 - C4 H 9

The resulting alkoxysilane adduct was then coupled with a polyoxyalkylated alcohol in the p r e s e n c e o f t r i f l u o r a c e t i c a c i d a n d finely d i v i d e d K O H to yield t h e g r a f t c o p o l y m e t as s h o w n a b o v e . (2°5) T h i s c o p o l y m e r c a n be u s e d as a s u r f a c t a n t in t h e m a n u facture of polyurethane foams.

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