POLYMERIZATION OF PHENYLACETYLENE IN NEMATIC CHOLESTERIC AND ISOTROPIC SOLVENTS

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POLYMERIZATION OF PHENYLACETYLENE IN NEMATIC CHOLESTERIC AND ISOTROPIC

SOLVENTS

W. Bacon

To cite this version:

W. Bacon. POLYMERIZATION OF PHENYLACETYLENE IN NEMATIC CHOLESTERIC AND ISOTROPIC SOLVENTS. Journal de Physique Colloques, 1975, 36 (C1), pp.C1-409-C1-412.

�10.1051/jphyscol:1975167�. �jpa-00216246�

JOURNAL DE PHYSIQUE

Colloque C1, supplkment au no 3, Tome 36, Mars 1975, page C1-409

Ciassification Physics Abstracts

7.130 -

POLYMERIZATION OF PHENYLACETYLENE IN NEMATIC CHOLESTERIC AND ISOTROPIC SOLVENTS (*)

W. E. BACON

Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA

RL?sumB.

La vitesse de polymQisation est trks rapide dans les solvants nkmatiques et trBs lente dans les solvants isotropes. La formation d'oligom&res cycliques et la polymkrisation tEte

queue

sont facilitkes dans les solvants cristaux liquides. Les polym6res linkaires formks dans les solvants cristaux liquides presentent des poids moIkculaires plus Bleves.

Abstract.

The rate of polymerization is fastest in nematic and slowest with isotropic solvents.

Cyclic oligomer formation and head-to-tail polymerization .of phenylacetylene are promoted by liquid crystalline solvents. Linear polymers from the reaction dissolved in liquid crystalline solvents show higher molecular weights.

1. Introduction. - Solvents that order reactant trans-cyclohexane-1 ,Cdicarboxylate [2], used in this molecules exert an influence upon the kinetics of study. The cholesteric solvent was prepared by dissolv- chemical reactions. The liquid crystalline phase orients ing cholestanyl p-methylbenzoate in the nematic sol- reactant and solvent molecules parallel to one another vent and the isotropic solvents were p-xylene and the thereby influencing the kinetics of reaction. This isotropic phase of the nematic solvent.

investigation compares the kinetics and products of reaction for the thermal polymerization of phenylace- tylene dissolved in liquid crystalline and isotropic media. The purpose of this work is to compare the influence of the planar ordered nematic, the helical ordered cholesteric and the random order of the iso- tropic solvents upon the thermal polymerization of phenylacetylene. We have determined the rate of thermal polymerization of phenylacetylene, product ratio of cyclic oligomers and some physical characte- ristics of the linear polymers in the three different solvent systems.

In nematic liquids the preferred orientation is subject to continuous change and the angle by which the preferred direction changes is not defined as it is in cholesteric liquids. The cholesteric mesophase repre- sents a special case of the nematic structure in that it is also characterized by a parallel orientation of the b n g axes of the molecules. However, there is a uniform preferred direction within a given layer which is rotated in a uniform direction through a constant angle relative to that in the adjacent layer. The twisted structure and the sense of rotation are induced by the chiral form of the participating molecules. The choleste- ric solvent may exert a favorable influence upon the polymerization reaction if one considers the, head-to- tail polyphenylacetylene may have a helical structure containing three monomer units per turn [l].

We have earlier reported on the adiabatic calori- metry of the nematic solvent, di-(p-methoxypheny1)-

2. Experimental. - The preparation of di-(p-metho- xypheny1)-trans-cyclohexane-1,4-dicarboxylate [3] and cholestanyl p-methylbenzoate [4] was reported earlier.

The phenylacetylene was distilled, b. p. 73-40180 mm under nitrogen and stored over a nitrogen atmosphere.

The p-xylene was also distilled before use. The choleste- ric solvent was prepared by the addition of 4.4 mol percent of cholestanylp-methylbenzoate to the nematic solvent and the characteristic Grandjean texture was observed microscopically at 1 50° with and without the phenylacetylene solute.

The rate constants were determined by adding phe- nylacetylene to the liquid crystalline solvent contained in pyrex ampoules having a volume of about ten cubic centimeters. The air was removed from the ampoules with a stream of nitrogen and sealed. The ampoules were placed in an auxiliary bath heated at 1470

0.50 and shaken intermittently for exactly ten minutes to assure homogenous solutions. The samples were then transferred to a constant temperature bath controlled to + 0.050 and the time recorded. These samples were removed at selected times, cooled, opened and the contents of the ampoule extracted with anhydrous diethyl ether filtered and the ether removed. The residue was diluted to a standard volume with carbon tetrachloride and analyzed by infrared spectroscopy.

The change in intensity of the peak a t 3 280 cm-' due to the C-H stretching in acetylene [5] was followed with a Perkin-Elmer 337 equipped with an Ordinate Scale Expansion.

(*)

This work was supported

the National Science Founda- Larger quantities of linear polymer and oligomers tion under grant

GH

X. were obtained from the polymerization in ampoules

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975167

C1-410 W. E. BACON

having a volume of about twenty cubic centimeters.

Solutions of phenylacetylene were prepared as outlined previously and each reaction continued for 120 hours at 150 + 0.050. The contents of the ampoule were extracted in a Soxhlet apparatus with benzene. The volume of benzene was reduced by evaporation and the precipitated liquid crystalline solvent was sepa- rated by filtration. The residue was diluted to a standard volume with benzene and analyzed by gas chromatography [6]. The column was packed with 10 % vcc-w on 30/60 mesh grade chromosorb w, temperature 2600 and a nitrogen flow of 40-50 mls/min.

The internal standard was 1,3,5-triphenylbenzene.

After analysis for oligomers the benzene solutions were evaporated ; any liquid crystaIline solvent was separated by filtration and washed with n-pentane. The polyphenylacetylenes were obtained by evaporation of the n-pentane ,and the molecular weights were determined by a Hitachi Perkin-Elmer Model 115 Molecular Weight Apparatus.

3. Results and discussion. - The change in con- centration of phenylacetylene as a function of time in all solvents used in this work is described by a second-order rate equation. Previous work reports the thermal polymerization of phenylacetylene in benzene is second-order in monomer concentration C71. This is expected if the rate of thermal initiation, I, is also second order

I ⁼ ki[M-j2 .

Assuming the rate of initiation and termination are equal, then the concentration of polymer radicals is

The decrease in phenylacetylene, M, may then be written

and k, is a composite constant. The kM value and the activation energy for the thermal polymerization of phenylacetylene in p-xylene is reported in the following equation [8]

The activation energy of 26.5 kcal/mole is about 6 kcal/mole greater than the energy of activation for the thermal polymerization of styrene which accounts for the much slower rate of polymerization for phenyl- acetylene.

The rate constants for the thermal polymerization of phenylacetylene dissolved in the various solvents, as well as the relative rates in comparison with p-xylene are shown in Table I. The rate of polymerization is faster in liquid crystalline solvents than in isotropic solvents and fastest in nematic solvent. These results may be explained by considering the oriented phenyl- acetylene molecules in the nematic solvent have hin- dered rotation in contrast to the solution in isotropic solvent. The hindered rotation results in a smaller loss in entropy when two molecules combine to form an activated complex resulting in the subsequent increase in rate of reaction.

Rate constants for the thermal polymerization of phenylacetylene at 1500 Solvents k x 106 1.m-1 s-1 Relative rate

- -

p-xylene

& 0.3

Isotropic phase of

nematic (*)

5 0.4

Cholesteric

44.9 f 0.5

Nematic

f 0.5 36

(a)

Di(p-methoxyphenyl)-trans-cyclohexane-1,4-dic~boxylate.

(b) Prepared by the addition

4.4 mol % of cholestanyl p-methylbenzoate to the nematic.

An explanation for the difference in the rate of polymerization in nematic and cholesteric solvents is not apparent especially if one visualizes the nematic mesophase as a dynamic situation containing both helical and non-helical structures with the non-helical form possessing a lower free energy than the helical form 191. The addition of a chiral compound to the nematic mesophase alters the balance between helical and non-helical structures to where one of the helical structures become energetically more favorable thus forming the cholesteric mesophase [9]. The change in texture of the nematic solvent to cholesteric has decreased the rate of polymerization by one-half suggesting the helical structure of the, solvent hinders the polymerization reaction. However, it is reported the solubility of substituted aromatic moIecuIes in the cholesteric mesophase show positive heats and entro- pies of solution in contrast to the nematic meso- phase [lo, 111 indicating the solution process is more complex in cholesteric solvents. Additional work is under way to investigate the role of cholesteric solvents.

The rate constants for the thermal polymerization

reaction includes the formation of linear and cyclic

oligomers as wella as linear polymers shown in the

following equation :

POLYMERIZATION IN LIQUID CRYSTALLINE SOLVENTS C1-411

Polymerization of phenylacetylene at 1 50° for 120 hours

Conc. 1,3,5- 1,2,4- 1,2,4- Percent Percent MoI.

Solvents monomer T. P. B. (") T. P. B. 1,3,5 aromatic polymer weight

-

- - - - -

-

Nematic phase

0.77 M 43.7 mgs 53.9 mgs 1.2 11.2 62 917

Cholesteric phase (3 0.75 M 41.8 54.1 1.3 11.1 65 810

Isotropic phase

1.39 M 22.5 67.5 3.0 6.0 56 607

p-xylene 0.84 M 7.3 37.6 5.2 4.9 53 508

(") T. P. B. is triphenylbenzene.

( b )

Nematic phase of di-(p-methoxyphenyl)-trans-cyclohexane-l,4-dicarboxylate.

(") Prepared by adding 4.4 mol % of cholestanyl p-methylbenzoate to the nematic solvent.

( d )

Isotropic phase of the nematic solvent.

In addition to the products shown in the equation, we have identified trans-1,4-diphenylbutenyne [12] in less than 0.5 % in all thermal polymerizations of phenyl- acetylene in this work. This product arises by the addi- tion of phenylacetylene to the triple bond of a second molecule of phenylacetylene. This type of addition is in contrast to the polymerization of phenylacetylene catalyzed by arene - Group VI B tricarbonyls reported to proceed via ladder compounds to give polymers with a 12 000 number average molecular weight [13]. In thermal and most catalyzed polymeri- zations of alkynes only low molecular weight products are obtained probably since acetylenes show a strong tendency to trimerize, thus providing a ready termina- tion step.

A comparison of the ratio of 1,2,4-/1,3,5-triphenyl- benzene obtained from the polymerization reaction in nematic, cholesteric and isotropic solvents as well as the average molecular weight for the Iinear polymers is shown in Table 11.

Analysis of the results for the reaction in liquid crystalline'and isotropic solvents leads to the following conclusions :

(a) About twice the amount of triphenylbenzenes (TPB) is formed in liquid crystalline solvents as in isotropic solvents.

(b) The formation of 1,3,5-TPB is favored in liquid crystalline solvents.

(c) The ratio of 1,2,4/1,3,5-TPB in liquid crystalline solvent is much lower than in isotropic solvents.

(d) The average molecular weight of the linear polymer obtained from the liquid crystalline solvents is higher than from isotropic solvents.

The triphenylbenzenes are formed by the cyclization of linear trimers of phenylacetylene presumably through a diradical mechanism, and the liquid crystal-

line solvents not only favor the formation of cyclic oligomers but specifically the 1,3,5-TPB. The latter can only arise by the head-to-tail addition of three mole- cules of phenylacetylene followed by cyclization. There- fore, the greater amount of 1,3,5-TPB formed indicates the head-to-tail addition is favored in liquid crystalline solvents. Based upon Table I1 about six times more 1,3,5-TPB is formed in liquid crystalline solvents than in p-xylene and about twice as much compared to the isotropic phase of the nematic solvent.

The other cyclic isomer, 1,2,4-TPB forms by the cyclization of a different trimer than for 1,3,5-TPB and arises from the addition of a third molecule of phenyl- acetylene to the head-to-head or tail-to-tail dimer intermediate followed by cyclization. Alternately, it may also be formed from the head-to-tail dimer shown in the equation below. The ratio of 1,2,4- to 1,3,5-TPB shown in Table I1 indicates the 1,2,4-TPB is favored in thep-xylene and may be explained in the following way.

If one considers equal probability for the formation of the three dimer intermediates, i. e. H-H, T-T and H-T in p-xylene then the addition of a third molecule of phenylacetylene to the intermediates followed by cyclization gives a theoretical ratio of 1,2,4- to 1,3,5- TPB equal to five. The formation of three molecules of 1,2,4-TPB per molecule of 1,3,5-TPB in the iso- tropic phase of the nematic solvent suggests the size of the molecules composing the isotropic solvents may influence the ratio of intermediate adducts. The T-T dimer of phenylacetylene has two vicinal phenyl groups that cannot have a coplanar conformation, and the absence of this intermediate would give a statistical ratio of three molecules of 1,2,4-TPB to one of 1,3,5- TPB as shown in Table 11.

The ratio of 1,2,4- to 1,3,5-TPB equal to one obtained

in the liquid crystalline solvents may be rationized by

C1-412 W. E. BACON

assuming two molecules of phenylacetylene add head- mer units is modest. The net effect of the increased rate to-tail forming a dimer. Then a third molecule of of polymerization, higher molecular weight, and the phenylacetylene may add two ways to this intermediate mode of polymerization in liquid crystalline solvents as shown below to give a ratio of one. This mecha- indicates the ordered solvents exert an influence diffe- nism appears reasonable for the polymerization in rent than isotropic solvents.

liquid crystalline solvents.

he average molecular weight for the polymer in Acknowledgment.

It is a pleasure to acknowledge liquid crystalline solvents is higher than in isotropic the assistance of M ~ . ~ i ~ ~ - ~ i d ~ ~ k ~ . h ~ ~ d solvents although the difference of three to four mono-

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