Vd. 17 No. 4 CHINESE JOURNAL OF CHDllSTRY 1999

Ally1 chloride/A1C13/ether cationic initiating systems for polymerization of 1 3-pentadiene

~bshact ~arim ethers were used to mediate the pdymerimtiom of 1,3-pentadiene (a>) initiated by A I Q and by allyl chloride (AllyU)/AlQ. The introddm of the e h exert considerable effects on polymer yield and molecular weight due to its interaction with the pnpgathg cahwhon ' . The txhcatim reactivity is re- duced by this interaction which is dject to the ether' s nucleoptulicity determined by the steric hinmance of p p e adjacent to oxygen. ?he duction of ahcation reactivity gives rise to a decresse of polymer yield owing to inhibition of -on but results in an augmentation of mol& weight due to suppression of various side mtim such as terminatiolrs. By using suitably nucleophihc ethers such as diphenyl ether, the polymerization can be mediated to give an high molecular wei& polymer in high yield.

Keywomk cationic Polymerizat;on, initiating system, 1,3-pentadiene.

Introduction

Cationic polymerizations initiated by complex initiating systems containing various electron donors ( EDs) such as ethers have been widely studied in the recent years. l V 2 ?he interaction between ethers and electmphiles such as the growing carboaition can mediate the polymerization process. According to Sawamoto et d . ,3*4 the ether can insert the active center in the cationic polymerization and intemt with the growing carbocation to change the nature of active species:

e 0. -/ 8 - --, -C- - - -0- - - .G $ 8

-C---G + /O\ \

As shown above, the rearmngement of positive charge between carbon and oxygen can stabilize the active center and reduce the carbodon activity, which permits to inhibit or suppress diiTenmt side reactions such as chain transfer, cmdinking and termination.

We have described in the pmvious work' the cationic polymerizatiOn of 1,3-pentadiene (PD) initiat- ed by allyl chloride (AllyCl)/AlCl3/ether initiating systems in n-hexane. It was found that the intmduc-

Received Januruy 27, 1999; wised April 9, 1999. Roject (No. 29504032) supported by the National N d Science Foundation of China.

416 Chinese Journal of Chemistry Vol. 17 No. 4 1999

tion of the ethers could considerably suppress the formation of gel (crosslinked polymer) , and the reduc- ing effects of the ethers on cmlinking reaction is due to the reduction of carboation reactivity. The pre- sent work is aimed at the investigations on the effects of various ethers on the gel-free PD polymerization induced by AICl3 and AllyCl/AlC13 in order to further understand the interaction between the ether and the propagating carboation.

Experimental

PD from Fluka was distilled over C a H 2 , AUyCl, ethers and solvents were also distilled over C a H 2 , and AlC13 was purified by sublimation in vacuum.

Polymerization and measurements

The polymerization was pexfonned in a three-necked-flask in nitrogen atmosphere. After introduction of AlC13, the solvent, AUyCl and ether were added by syr inge, and PD was added finally at set tempera- ture. The polymerization was quenched by methanol after a given time. ?he solution was evaporated under reduced pressure and dried to constant after washing the resulting solution with water. The polymer intrin- sic viscosity was determined with an Ubbelohde Viscometer in toluene at 25°C.

Results and discussion

The results of the polymerizations of PD in toluene initiated by various AlCl3/ether initiating systems are shown in Table 1 . The addition of these ethers reduces polymer yield, indicating that the pmpagation is inhibited by reducing cabcation reactivity. The difference in reducing effects of the ethers on polymer yield is attributed to the difference in their nucleophilicity toward the carbocation which is determined mainly by the structure of the ether. For example, the yield is less reduced by addition of diphenyl ether because the steric hindrance of two phenyl p u p s makes this ether weakly nucleophilic. In fact, the poly- mer yield decreases in the order of diphenyl ether (PbO) , phenylmethyl ether (PhOMe) and ethyl ether (Et20) containing initiating systems, showing the hindrance of p u p adjacent to oxygen is the major factor decting the ether’s nucleophilicity. Table 1 also shows that the introduction of the ether gives rise

to a slight increase of the polymer intrinsic viscosity. This observation reveals that the pmper reduction in cahocationic reactivity can inhibit side reactions including -terminations, which would lead to an augmen- tation of polymer molecular weight. We note that cyclic ethers show strong nucleophilicity , e . g . , THF and dioxane lead to the lowest yield but the highest molecular weight.

We have reported that the combinations of AlCl3 with various active &yl halides turn to be highly

efficient initiating systems for cationic polymerization of PD . Table 2 gives the results of effects of ethers on the PD polymerization initiated by MyCI/AlCl3. It was observed again that addition of the ethers re- sults in a decline in polymer yield. However, the reduction scale is much s d e r than that of the poly-

PENG et al. Cationic polymerization 417

merization in the absence of AllyCl, indicating an enhancing effect of AllyCl on polymer yield. Particular- ly, the polymer yield is not decreased by weakly nucleophilic ethers such as diphenyl ether. Similarly, Table 2 also shows that the introduction of the ether gives an increase of the polymer molecular weight due to the reduction of cahxation reactivity and the inhibition of termination reactions.

TaMe 1 E$ect of ethers on polymerization of PD initiated by AlQ" ~~

Ether Yield ( % ) q ( U g )

100.0 0.046 Ethyl ether 55.0 0.048 Isopmpyl ether 80.0 0.058

-

THF D i O X a n e

22.4 15.0

0.071 0.070

Phenylmethyl ether 85.0 0.048 Diphenyl ether %.5 0.049

"[m] = 2 mVL, [etherl/[d(&] = O.S,[AKI~I =O.M moVL, Solvent: toluene, Temperatme: U)T, Reaction time: 4h.

Table 2 Effect of ethem on polymerization of PD initiated by AuycvAlQ'

Ether Yield (%) ?( U g )

100.0 0.036 -

Ethyl ether 68.0 0.051 Isopmpyl ether 99.0 0.042 THF 56.5 0.049 Dioxane 69.0 0.052 Phenylmethyl ether 100.0 0.040 Diphenyl ether 100.0 0.048

"[~fll/[AlCl~]/[ether] = 1/1/0.5, see Table 1 for other conditions.

The influence of the ether on the PD polymerization can be h the r illustrated by efiects of [ether]/ [ AlCl, ] ratio on the polymerizations induced by AUyCVAlCl3/ether systems containing &O, PhOMe and Et20, as shown in Fig. 1 and 2. Fig. 1 gives the dependence of polymer yield on [ether]/[ AlCb] ratio. The reducing effect order of the three ethers on polymer yield is: F'h20 < F'hOMe < Et20. "his order is f d y in accordance with their nucleophilicity order toward cadxmtions as well as the hindrance order of groupments adjacent to oxygen atom. The dependence of the polymer molecular weight on [ether]/[ AlC13] ratio is described in Fig. 2. ?he inhibiting &ect of the three ethem on terminations can

be mtionalized by the fact that the addition of the ethem leads to an higher molecular weight while the [ ether]/[ AlCl3 ] ratio falls in the range ammd 0 .5 . We also observed that the molecular weight of the polymer exhibits a drop with high ether concentrations, indicating that an over reduction of carbocation reactivity would considerably prohibit the propagation and depress the degree of polymerization. According to the results of Figs. 1 and 2, the cadmation reactivity can be properly reduced by introduction of suit- ably nucleophilic ethers such as diphenyl ether so that various side reactions such as terminations could be

418 Chinese Journal of Chemistry Vol. 17 No. 4 1999

prohibited but the propagation is not affected. As a result, both high polymer yield and high molecular weight could be achieved.

100

80

p 60 al > .-

40

20 0 1 2 3

[ Ether]flAlCk]

Fig. 1 Effect of [Ether]/[AlC13] ratio on the polymer

yield. See Table 2 for the -tion conditions.

Ether: 0 Et20, A PhOMe, b 0 .

.. 2

0.02 -

0.01 - 0 1 2 3

[Ether]l[AlCI3]

F&. 2 Dependem of the polymer intrinsic viscosity on

[ Ether]/[ AICl3 1 . See Table 2 for readtion

conditions. Ether: 0 4 0 , A PhOMe,

-0.

On the basis of the above results and discussion we can conclude that the PD polymerizations initiat- ed by AIC13 and AUyCVAlCl3 could be properly mediated by intrwluction of ethers. ?he interaction be- tween the ether and the propagating carbocation reduces the cabcation reactivity, and this interaction is subject to the ether' s nucleoptulicity determined by the steric hindrance of p p s adjacent to oxygen. The reduction of carbocation reactivity results in a decrease of polymer yield owing to reduction of propa- gation but gives rise to an augmentation of molecular weight due to suppression of various side reactions such as terminations. These observations make it possible to obtain better defined 1 ,3-pentadiene polymer through the control of the cationic polymerization. By using suitably nucleophilic ethers such 8s diphenyl ether, the polymerization could be mediated to produce an high molecular weight polymer in high yield.

References

2 . 3. 4 . 5 .

6 . 7 . 8.

Kennedy, J . P . ; Ivan, B . , L k s d Pd- by carbocatwnic Macromdecular Engineering: h r y and Rdice, Hanser, Munish, 1991, p.60. Gandini, A . ; Cheradame, H., A&. P o l p . Sc i . , 34/35, 167( 1980). Sawarmto, M . , Bog . P d p . Sci. , 16, lll(1991). Sawamoto, M . , M a k r o m d . Chan., Maawnd., Symp., 54/55, 4(1992). Peng, Y . X . ; Zhang, J . W . ; I iu , J . L . , Chin. Sci. B u l l . , 42, 1544( 19!37)[ C h . A b s t r . , 128, 75711~

Peng, Y . X . ; l i u , J . L . , J . Pdym. Sci . , Part A : Polym. G e m . , 33, 2087(1995). Peng, Y . X . ; Dong, Y . J . , J . Pdym. Sci., PartA: P d p . chsn., 34, 1815(19%). Peng, Y . X . ; Dong, Y . J . ; Liu, J.L., P d p . Bd., 36, 443(19%).

(1998)l.

(JIANG, X.H.; DONG, L . J . )