IV. ANIONIC POLYMERIZATION AND ANIONIC POLYMERS

tertiary lithium compounds are required to rapidly initiate the polymerization. This is because primary organolithium compounds such as n-butyllithium are more associated than the secondary organolithium compounds

and thus are less reactive (31,32).

Functional organolithium reagents are used to make functional polymers (33). This technique is generally better than functionalizing a living

polymer by reaction with an electrophile, because there are fewer side

reactions with initiation. The reactivity of the lithium portion of the initiator

requires that the functional group be protected in most cases, but the available

functionality is surprisingly diverse. The key issues with functional initiators

are storage stability and solubility in solvents suitable for polymerization.

Lithiated acetals (34) and lithiated trialkylsilyl ethers (35) are used to form

hydroxyl-terminated polymers after deprotection. Amine-terminated polymers have proven to be more useful for the preparation of tire elastomers. The

synthetic routes diagrammed in Figure 10 can prepare these initiators.

The reaction of imine 1 with n-butyllithium produced initiator 2. SBR

was prepared with this initiator, but the number-average molecular weight

was much higher than predicted, which indicates that the alkyllithium

reaction with the imine produced less than 100% of 2 or that the initiator is

not completely efficient for initiation. The compounded SBR did exhibit

improved hysteresis compared to a butyllithium-initiated control (36,37). The

reaction of secondary amines with butyllithium seems like an easy way to

prepare n-lithium amides, but most of them are insoluble in nonpolar media.



Figure 10



Synthesis of lithium amide initiators. (From Refs. 36–38.)



Copyright © 2004 by Taylor & Francis



Cheng (38) prepared a series of simple secondary lithium amides, but in all

cases they were insoluble in hexane. The heterogeneous initiators were used to

polymerize dienes, but the polymerizations did not go to completion and the

resulting polymers most likely had a very broad molecular weight distribution. Lawson et al. (39a,39b) showed that preparation of lithium amides in the

presence of two equivalents of THF gave soluble initiators that could be used

to make a medium vinyl SBR at high conversion. The resulting polymer was

coupled with tin tetrachloride and showed a 40% reduction in hysteresis as

measured by tan y at 50jC compared to a butyllithium-initiated control

polymer. A partial list of the amide initiators studied and their solubilities is

given in Table 5.

Interestingly, although almost all of the amide initiators effectively

initiated polymerization, not all of the resulting polymers showed reduced

hysteresis on compounding.

N-Lithiohexamethyleneimine 3 and N-lithio-1,3,3-trimethyl-6-azabicyclo[3.2.1]octane 4 were studied further. They were both shown to be stable for

‘‘several days.’’ Initiator 4 produced polymers with a broader molecular

weight distribution than initiator 3 (40). One difficulty in working with these

initiators is that the amine group is lost during polymerization by the

mechanism shown in Figure 11. This reaction becomes more significant in

the presence of excess initiator and at temperatures above 80jC.

Initiators 5 and 6 (Fig. 12) can eliminate head group loss because the

additional carbon atom between the nitrogen and lithium prevents elimination (41).

The difficulty with the lack of solubility of simple lithium amides can be

overcome by in situ formation of the initiator. Immediately after charging a

reactor with solvent, monomer, randomizer (THF or potassium amylate),

and butyllithium, a secondary amine is added to the mixture. The amide is

made in situ, and high molecular weight polymers are formed that have lower

hysteresis than the corresponding polymers made with butyllithium. Approximately 85–90% of the chains have amine head groups when this procedure is

used (42).

Tin-containing initiators are also important compounds used to prepare

high-performance tire rubbers. Addition of lithium metal to tributyltin

chloride in an ether solvent produces a solution of the desired initiator that

is filtered to remove lithium chloride (43) (Fig. 13). The initiator is stable at

room temperature and can be stored for approximately 8 weeks before a loss

in activity is observed. Polymer with a lower vinyl content and narrower

molecular weight distribution is obtained if the initiator is made in dimethyl

ether rather than THF. This is illustrated in Table 6 for the polymerization of

butadiene. Carbon black compounds based on these polymers have lower

hysteresis than corresponding unfunctionalized controls.



Copyright © 2004 by Taylor & Francis



Table 5 Solubility and Effectiveness of Lithium Amide Initiators



Source: Ref. 39.



Copyright © 2004 by Taylor & Francis



Figure 11



B.



Head group loss in functional polymers.



Propagation



Propagation takes place at typical reaction temperatures (20–75jC) in inert

solvents such as hexane or benzene without chain transfer or termination. At

high temperature, however, the growing polymer chain can eliminate lithium

hydride, which stops the polymerization and broadens the molecular weight

distribution. The mechanism is shown in Figure 14.

Elimination of lithium hydride is a first-order process that yields a

polymer terminated with a diene. Addition of living polymer doubles the

molecular weight of the chain and provides an active site that can react with

additional butadiene to form a branched polymer (44).

The ratio of monomer to initiator has a major influence on the cis/trans

ratio in the homopolymerization of both butadiene and isoprene in unmodified polymerizations, as shown in Table 7 (45,46). The higher the ratio of



Figure 12



Functional initiators to avoid head group loss.



Copyright © 2004 by Taylor & Francis



Figure 13



Synthesis of tributyltin lithium.



monomer to initiator, the higher the cis/trans ratio produced with both

butadiene and isoprene.

Two kinetic factors affect the diene microstructure. The first involves

the relative rates of propagation versus isomerization of the initially formed

allyl anion. Monomer is inserted initially to form the allyl anion in the anti

form. If propagation is rapid, the microstructure of the penultimate unit will

be cis. If, however, the allyllithium has sufficient time to isomerize to the

thermodynamically more stable syn form, then the penultimate unit will be

trans. Thus, at a high monomer/initiator ratio that favors rapid propagation,

the microstructure is primarily cis. As the monomer is depleted and the

monomer/initiator ratio decreases, more trans microstructure will be formed

(Fig. 15) (47,48). The second factor is the relative rate of addition of monomer

to the syn or anti isomer. Butadiene will add approximately twice as fast to the

anti form as to the syn form. With isoprene the factor is eight times as fast (49).

In addition to increasing the rate of polymerization, polar solvents or

polymerization modifiers also affect the vinyl content and sequence distribution in polybutadiene, as shown in Table 8 (50,51).

Large amounts of weak complexing agents such as diethyl ether or

triethylamine must be used to significantly affect the microstructure, but

strongly chelating modifiers such as tetramethylethylenediamine (TMEDA)

or 1,2-dipiperidinoethane increase the vinyl content dramatically at low

levels. The effect of polymerization temperature and its interaction with

modifier is also illustrated by the data. Vinyl content is increased as the

temperature is reduced for all polymerizations, but the effect is more

pronounced at low modifier/lithium ratios.

In the copolymerization of styrene and butadiene, the sequence distribution is strongly affected by the addition of polar modifiers or salts. In



Table 6 Polymerization of Butadiene with Tributyltin Lithium

Solvent–initiator makeup

Tg (onset)

Vinyl content

Mn

Mw/Mn

Source: Ref. 43.



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THF



Dimethyl ether



À85jC

21%

223,000

1.25



À93jC

11%

206,000

1.11



Figure 14



Mechanism for branching in lithium polymerization. (From Ref. 44.)



hydrocarbon solvents without polar materials, most of the butadiene will

polymerize first, followed by the styrene. This process is used to prepare

‘‘tapered’’ block polymers where there is a butadiene block, a mixed butadiene–styrene block, and a styrene block (52).

Addition of polar compounds will randomize the styrene and increase

the rate of polymerization. Choice of modifier is critical to get the proper

degree of randomization and control the vinyl content. Modifiers such as

potassium tert-butyl alkoxide (t-BuOK) are used to randomize the styrene

without significantly increasing the vinyl content. At a ratio of t-BuOK/nBuLi of 0.1, there is only a small increase in vinyl content (Fig. 16), but this is

sufficient to randomize styrene in an SBR (53).

For higher vinyl SBR, a more powerful randomizer such as TMEDA is

used that produces high vinyl polymers at relatively low modifier/lithium

ratios (54). Very high vinyl SBR and polybutadiene can be prepared with a

modifier consisting of a mixture of TMEDA and an alkali metal salt of an

alcohol (55).



Table 7 Effect of Monomer/Initiator Ratio on Microstructure

Polymerization conditions

Monomer



Solvent



Initiator



Butadiene



Hexane



Li



Isoprene



Cyclohexane



Li



Source: Refs. 45, 46.



Copyright © 2004 by Taylor & Francis



Microstructure

Monomer/

initiator



Cis



Trans



1,2



3,4



5 Â 104

17

>5 Â 104

15



0.68

0.30

0.94

0.76



0.28

0.62

0.01

0.19



0.04

0.08

—

—



N/A

N/A

0.05

0.05



Figure 15 Microstructure formation during lithium polymerization. (Adapted

from Refs. 47 and 48.)

Table 8 Effect of Polar Modifiers on Polybutadiene Microstructure During

Lithium Polymerization

% 1,2-Addition at

Modifier

Triethylamine

Diethyl ether

Tetrahydrofuran

Tetramethylethylenediamine

1,2-Dipiperidinoethane

Source: Refs. 50, 51.



Copyright © 2004 by Taylor & Francis



Modifier/Li



30jC



50jC



70jC



270

12

96

5

85

1.14

1

10



37

22

36

44

73

76

99

99



33

16

26

25

49

61

68

95



25

14

23

20

46

46

31

84



Figure 16 Effect of potassium butoxide/lithium ratio on polybutadiene microstructure. (n) Percent trans; (E) percent vinyl. (From Ref. 53.)



C.



Termination



Termination is easily accomplished by reaction of the living polymer with an

electrophile. In early anionic polymerization studies, the electrophile was a

proton donor and termination resulted in a hydrocarbon polymer. Reaction

with other electrophiles such as carbon dioxide (carboxylic acid), sultones

(sulfonates), ethylene oxide (alcohol), or imines (amines) produce functional

polymers, but unless conditions are carefully controlled the functional polymer is contaminated with other materials (56). Virtually every electrophile

known has been tested as a terminating agent for lithium polymerizations. In

one patent alone, the following were claimed for terminating a living transpolybutadiene polymerization—isocyanates, isothiocyanates, isocyanuric acid derivatives, urea compounds, amide compounds, imides, N-alkyl-substituted oxazolydinones, pyridyl-substituted ketones, lactams, diesters,

xanthogens, dithio acids, phosphoryl chlorides, silanes, alkoxysilanes, and

carbonates (57), Amine- and tin-containing electrophiles provide the greatest

interaction with carbon black. Epoxy compounds and alkoxysilanes are most

beneficial for silica-filled compounds. The early work focused on termination

with amine-containing functional groups such as EAB [4,4V-bis-(diethylamino)benzophenone] (58–60). Black compounds made with these polymers

showed higher rebound, lower heat buildup, higher compound Mooney

Viscosity, and more bound rubber than the corresponding control rubber.

Another study by Kawanaka et al. (61) suggested that the mechanism of the



Copyright © 2004 by Taylor & Francis



rubber–filler interaction was through an iminium salt formed from the

reaction product of the amide and living polymer chain end (Fig. 17). The

authors inferred this because rubber functionalization with amides that could

not easily form iminium salts did not interact well with carbon black.

Termination with tin-containing compounds provides more flexibility

than with amine compounds. RxSnCly (where x + y = 4) can be chosen to

give different levels of branching and thus assist in macrostructure control.

Phillips pioneered the coupling of solution polymers with tin halides to make

radial polymers in the 1960s but the Japanese Synthetic Rubber Company

(JSR) was the first to use the nature of the carbon–tin bond for tire compounds. Tsutsumi et al. (62) outlined the synthesis of tin-coupled solution SBR,

the mechanism of how it improves hysteresis, structure–property relationships to maximize the effect of tin, and pitfalls to avoid in compounding (62).

They first demonstrated that coupling solution SBR with tin tetrachloride

provided a superior polymer compared to other coupling agents (Table 9).

The SBR polymerization was terminated with tin tetrachloride such

that 50% of the chain ends were coupled. The only major difference in

performance among the coupling agents was the low hysteresis exhibited by

the tin-coupled polymer. Tsutsumi et al. compared a series of tin-coupled

polymers with a polymer containing trialkyltin groups along the backbone.

Only tin located at the end of the polymer chain (or branch point) was

effective in reducing hysteresis (Fig. 18).



Figure 17

61.)



Termination of lithium polymerization with a cyclic amide. (From Ref.



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Table 9 Coupling of Solution SBRa,b



Coupling agent

None

Divinylbenzene

Diethyladipate

Silicon

Tetrachloride

Tin tetrachloride



ML-1+4

(100jC)



Compounded Tensile

strength Elongation Tan y Tan y

ML-1+4

(MPa)

at break at 50jC at 0jC

(100jC)



54

51

47

57



93

70

74

89



22.3

22.5

21.6

23.5



400

400

410

400



0.121

0.125

0.126

0.126



0.235

0.241

0.237

0.240



57



76



25.0



400



0.096



0.239



a



Formulation (phr): Polymer 100, HAF black 50, zinc oxide 3, stearic acid 2, antioxidant 1.8,

accelerator 1.8, sulfur 1.5.

b

SBR: 24% bound styrene, 40% vinyl.

Source: Ref. 62.



In another study the same group showed that putting the tin group on a

butadienyl chain end was more effective in reducing compound hysteresis

than putting it on a styryl chain end. Finally, they postulated that the

mechanism of interaction with carbon black is by formation of a bond

between the polymer chain and the quinone groups on the carbon black.

This was based on a model study of the reaction of tributyltin-capped low



Figure 18 Effect of tin content position on dynamic properties of tin-coupled SBR.

(x) Polymer modified on backbone. (n) Polymer modified at chain end. (From Ref.

62.)



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molecular weight polybutadiene with a series of compounds containing

functionality found on carbon black. Only the quinones reacted to any extent.

The ease of cleavage of the tin–carbon bond is the reason this chemistry can

take place, but it also puts some restrictions on how tin-containing polymers

can be isolated and compounded. Acid will cleave the bond and should be

avoided until late in the compound cycle. Mooney viscosity of coupled

polymers will drop if the tin–carbon bond is broken, but if the polymer is

capped with a trialkyltin halide there will be little change in Mooney viscosity.

The importance of complete functionalization is illustrated by a study

by Quiteria et al. (63). They examined the effect of the polymer end group and

the effect of unfunctionalized polymer (via incomplete coupling) on the

dynamic properties of tin-coupled polymer in a simple black formulation.

They synthesized a 25% styrene SBR with 32% vinyl via adiabatic polymerization and reacted the living polymer with a small amount of monomer

(butadiene, styrene, isoprene, or a-methylstyrene) to ensure a specific end

group. Tin tetrachloride was added to couple 40% of the polymer. The

residual polymer chains were terminated with tributyltin chloride. The loss

tangent as a function of temperature for these polymers is shown in Figure 19.

The most important feature of the graph is the effect of unfunctionalized

polymer on hysteresis (run 5). Compound made with polymer from run 5

(uncoupled polymer terminated with a proton) had 15–20% higher tangent



Figure 19 Loss tangent versus temperature for different tin–carbon bonds. Bd,

butadiene; St, styrene; Is, isoprene; MSt, a-methylstyrene; H, hydrogen; Sn, tin.

(From Ref. 63.)



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