lower 'growth' with protection and deprotection of react- ... the allyl groups of the initial dendrimer. ... group, capable of reacting with the terminal allyl groups.
Lreurn Crysrr.rs,1996,Vor.21, No. l, !-lZ,
Liquid crystallinecarbosilanedendrimers:First generation by S. A. PONOMARENKOT, E. A. REBROVI, A. Yu. BOBROVSKYf, N. I. BOIKOf, A.M.MUZAFAROV{ and V. P. SHIBAEV*f tChemistry Department,Moscow StateUniversity,Moscow 119899,Russia f Instituteof SyntheticPolymericMaterials,RussianAcademyof Sciences, 70 Profsoyuznayast.,Moscow 117393,Russia (Receiued 1995;accepted 22 December1995) 15 September An approachto the synthesisof a new classof liquid crystalline(LC) compounds,dendrimers of regular structure with terminal mesogenicgroups,was elaborated.LC dendrimersbased on the carbosilanedendritic matrix of first generation were synthesized.Cyanobiphenyl, methoxyphenyl benzoate and cholesteryl groups were used as mesogenic fragments. Individuality and structure of all compoundsobtained was proved by GPC together with 1H- and 2esiNMR methods.The mesomorphicbehaviourand structureof the LC dendrimers of the smectictype are synthesizedwere investigated.It is argued that different mesophases rcalizedin all cases.It is shown that the mesophasetype of these compoundsessentially dependson the chemicalnature of the mesogenicgroups.
1. Introduction A new field of chemistry of high molecular mass compoundsassociated with the synthesis of three-dimensional superbranchedpolymers and oligomers,called dendrimers,has recentlybeen activelydeveloped[ 1]. Obtainingthesecompoundsis very interesting,because eachelementaryact of a molecule'sgrowth is accompanied by an increasein the numberof branchingpointsin geometricalprogression.As a result, the shape and rigidity of the moleculesare changedwith increasein the molecular mass.That, as a rule, leads to strong propertiesof dendrivariationsin the physico-chemical mers, such as their characteristicviscosity,solubility etc. [2], density[3], phasetransitions, Present-daysyntheticapproachespermit the producmacromolecules tion of the so-calledregulardendrimers, of which have a strictly determinedmolecularmass.In addition,many propertiesof dendrimerssuch as glass transitiontemperature[4], solubilityand others,depend mainly on the chemical nature of the terminal groups which are located,as a rule, on the surfaceof suchballshapedmolecules. All the above-mentioned havestimulated great interest among researchersin the synthesis of dendriticmacromolecules [ 3, 5- 11] . In the literaturethereis someinformationconcerning the synthesisof dendriticblock copolymerscontaining hydrophobicphenyl 'surface'groupson one half of the 'molecular-ba11' and hydrophilic carboxylic groups on the other half [ 12], or with electron-withdrawing CN-groups on one half and electron-donatingbenzyl *Author for correspondence.
ether groups on the other half [13]. Moreover,many other publicationshave indicatedthe wide possibilities of moleculardesignof dendriticmacromolecules. Some time ago we suggestedan approach to the synthesisof a new type of dendrimer[14-17]-liquid crystalline(LC) dendrimers(figure 1). They differ from thosedescribedearlierin the literature[18,19] by the fact that the mesogenicgroups, responsiblefor realiza'surface' tion of the LC state,are disposedonly on a of regularstructure. layer of dendriticmacromolecules We considersuch LC dendrimersas very interesting materials for investigation for the following main reasons.First, the unusual, exotic character of the molecularstructureof suchcompoundsshouldbe noted; moleculecan be representedas a each superbranched sort of sphere,the internalpart of which cdnsistsof non-
Figure 1. Schematicrepresentationof the molecularstructure of the LC dendrimer.
0267-8292196 $1200 O 1996Taylor & FrancisLtd.
5rS.A. Ponomarenko et al.
mesogenicblocks while the outsidesurfaceof the sphere fragments.Sucha'microheis formedby the mesogenic terogeneous' structureof the moleculesshouldpredetermine a tendencyof the dendriticsystemsto microphase separation,just as it takes place in block and graft polymers copolymers,as well as in somecomb-shaped organizaIn this sense, the study of the structural [20]. tion of dendrimersconsistingof feterogeneousblocks, part of whichtendsto giveLC phaseformation,presents a subjectofessentialscientificinterestfrom the structural point of view, bearing in mind their molecular and supermolecular or ganizalion. The possibilityof the creationof an LC shell(Jacket') 'soft' dendritic around a centralnucleus,formed by the matrix, is also interesting from the practical point of for the applicationof such view.It opensup perspectives compoundsas active structural modifiers which can be introduced into usual polymers for the modification of rheologicaland tribologicalproperties, their mechanical, as well as for the creation of selectivemembranesand drug delivery systems. There are at least two fundamentally different approachesto the synthesisof dendrimers.The first is basedon uncontrolledreactionsof low molecularmass compoundsthat lead to the formation of branched
moleculesfrom, as a rule, a trifunctional monomer. In this caseall three functional groups participate in the reaction growth, forming a dendrimerwhosemolecular mass and dimensionsare determinedby the reactivity and a numberof kinetic factors. of the intermediates The second method, called usually the controlled synthesis,is based on the sequentialassemblyof individual molecular fragmentsusing a layer-by-layersynthetic method via reiterative sequencesof reactions of lower'growth'with protectionand deprotectionof reactive groups.The merits of this method have been well proved by the multistep synthesisof cholesteryl-containing polyorganosiloxanedendrimersof regular structure which have been describedfor the first time in the literatureU4,l5l. 'structure-properHaving in mind the elucidation of for LC dendrimers, till now practically ties'relationships the goalof the presentpublicanon-existent, we associate tion and future work in this field with the synthesisand systematic study of LC dendrimers of different but regular molecular structure. In accordancewith ll4,15) we have now extended our approach to the synthesisof new LC dendrimers based on the carbosilanedendritic matrix containins different mesogenicgroups,as shown below:
Sijr[(CHtroCOO-Mes]
:
@cru
i@coo@*n.
t
a-L-{\---< .+w
,\,\l
Liquid cry stalline dendrimers
This paper presentsa detailed description of the synthesisof the three different new dendrimersof the first generationt, containing only eight mesogenic groups,and our preliminarydata on their structureand phasebehaviour. 2. Resultsand discussion 2.1. Generalstrategyof the synthesis of LC dendrimers Elaborating the universal approach to the synthesis of dendrimersof regular structure with terminal mesogenic groups, we have concentratedour attention on two important points. First, we had to choosesuch a chemical reaction of the matrix with the mesogencontainingcompound,which would permit us to obtain LC dendrimerswithout occurrenceof side-chainreactionsand formationof by-products.Second,the reaction betweenthe matrix and mesogenic fragmentsshouldbe controlledin order to obtain the targetdendrimerswith strictly definedmolecularmass.Finally, the mesogenic groupsshouldbe completely'indifferent'withrespectto the chemicalreactionsusedfor their attachmentto the dendritic matrix. Taking into accountall the abovementionedpoints, hydrosilylation,satisfyingall the enumeratedconditions [21], was chosenas the basereaction.Using this reaction, some carbosilanedendrimersU0,221 have been successfully synthesizedrecently,and that is why we also decidedto choosethis reaction for synthesisof LC dendrimers. The structure of the initial carbosilane dendriticmatrix Si|(AllVl)I is shownbelow. \ ,
r\
;(
r / rSi.
Sir
\i-
Si
./\
\
*r\r 4
The detailedmethod of synthesisof this dendrimerhas beendescribedpreviously 1231.In figure 2 the synthetic route to the synthesisof the dendriticmatrix is briefly summarized. In our work we have started with the carbosilane dendrimer of the smallestgeneration(G:1) owing mostly to its availability,and the simplicityof isolation and identificationof final products.However it does not limit the universalityof the elaboratedsynthetic approach;using the samesyntheticroute, subsequent generationsof LC dendrimerscan be synthesized. are tAccordingto Tomaliaand Durst [1], generations definedasstepwise reiterative reactionsequences in controlled dendrimersynthesis. {In the fomula Siv,(Z):x is the number of silicon atoms in the dendritic molecule;y is the number of terminal groupsZ, shownin the parentheses.
Me
Hiicr. ( Y' ) si(cH2-cH-cH sillcHr)jsiclr4zh ffi sil(Auyl) si!(cl) CH2:CH-CH2MgCI Yt-CU,-Cs:cu) .-._' ^.[^-_ s1(uH2)3s1
