Silicon-Containing Dendritic Polymers

During the last two decades, dendritic polymers, particularly dendrimers and hyper-branched polymers, have become one of the fastest growing areas of interest in polymer science [1]. This can be easily seen from the impressive growth in the number of publications on these unique polymers, which soared from less than a dozen in the 1970s, to over 10,000 (in scientific journals and patent literature) by the end of 2007 [2]. At present, new publications continue to appear regularly, and only the future will tell how much further this trend will continue.

Of many different reasons that may have caused such a great interest in dendritic polymers, the following seem especially important. First, the natural beauty and symmetry of dendritic, particularly dendrimer, structures is hard to resist and it has certainly inspired many scientists to design novel chemical compositions, architectural arrangements and artistic presentations of these unique molecules. Regardless of whether they are shown as simple schematics, or as elaborate computer-generated 3D images, dendritic structures have great aesthetic appeal, and are very inspirational for creative thinking, further modifications or potential applications (see Fig. 1.1).

Three different original protocols for the synthesis of siloxane dendrimers have been described. However, after appearing in the literature at much the same time they did not result in any follow-up, unlike many other strategies proposed simultaneously or even later. To appreciate the suddenness of the introduction of siloxane dendrimers and the reasons for their subsequent neglect we need to make an excursion into orga-nosiloxane chemistry. Silicones are based on a relatively small number of starting functional monomers. Moreover, their combination into new structures is restricted by the reversibility of most reactions and as a consequence by the statistical character of the main methods for the synthesis of siloxane polymers [1–3]. Although the existence of some selectivity results in the formation of useful polycyclics, the possibilities of structural control of linear and especially of branched oligomers and polymers are seriously limited. The nonequilibrium polymerization of strained cyclosiloxanes [4] is a rare exception to this rule which emphasizes the hopelessness of attempts to overcome the “statistical ill fate” that haunts the siloxane structures. All efforts to apply heterofunctional processes to achieve structural diversity are limited by this specificity of the siloxane bond and by a scanty number of appropriate reagents.

Because of this, it is not surprising that after the discovery of sodiumoxyorganoalkoxy-silanes, compounds unique in their synthetic versatility and often called Rebrov's salts [5], the branched functional oligomers and acyclic methylsilsesquioxanes came into the focus of our research attention. Furthermore, most of the oligomers obtained using this approach became the first generations of the corresponding dendrimers, but only after the finding of an appropriate method for quantitative transformation of alkoxy groups into the chlorosilyl ones, and a way to reiteratively use them in pairs did it become evident that possibilities of the new approach for dendrimer synthesis were enormous (see Chapter 1). By that time, dendrimers had become well known in the field of organic polymers, largely due to the efforts of D. Tomalia [6, 7].

The concept of highly symmetrical, perfectly branched macromolecules prepared in a generational fashion was introduced in 1978 [1]. The synthesis of polylysine dendrimers [2] and the seminal research by Tomalia and Newkome in the mid-1980s established that such molecules could indeed be prepared [3, 4]. Tomalia et al. used trifunctional nitrogen branch points and Newkome chose tetrafunctional carbon branch points. These dendrimers contained ether, ester, amine and amide polar bonds

Polysilanes, –(Si)_n–, are polymers that contain catenated silicon atoms, and their chemistry has attracted considerable interest during the last 30 years because of their electronic, optical, structural, and chemical properties [1]. In particular, the σ-conjugation of the –Si–Si– backbone has attracted much attention compared with analogous carbon polymer systems. Although, in contrast to the numerous reports on polysilanes with linear main chains, little attention has been devoted to their branched counterparts; hyperbranched polysilanes [2], ladder polysilanes [3], and organosilicon nanoclusters [4] have nevertheless been described. However, with the exception of ladder polysilanes, the precise structures of these branched polymers have not been sufficiently elucidated. This chapter deals with polysilane dendrimers from the initial [6] to the most recent report [20]. These dendrimers, which contain silicon atoms attached to three or four other silicon atoms, exhibit some interesting properties compared with their linear homologues [5]. As in other chapters of this book, the ‘1G(4,⁰3) end-group’ nomenclature system (see Fig. 4.1) is used. In this system, 1G denotes generation 1, the first number in parentheses denotes the branching functionality of the core (in this example a tetradendron dendrimer with first digit 4); the following superscript represents the number of spacer silicon atoms between the core and the next branching point; the following numbers represent functionalities of the silicon atoms in each subsequent generational layer; and finally the end-groups are specified by their chemical formulas

Linear polysilazanes and polycarbosilazanes are well-known members of the organosilicon polymer family and can be prepared by a variety of methods [1, 2]. These polymers are characterized by having either a –Si–N– backbone (polysilazanes) or a –R–Si–N– backbone (polycarbosilazanes). On the other hand, dendritic analogs are relatively rare. Polysilazane dendrimers are essentially nonexistent, and the syntheses of polycarbosilazane dendrimers have been reported by only two groups. A primary reason for the paucity of examples is the relative reactivity of the Si–N bond. Since many reagents involved in conventional organosilicon dendrimer synthesis would react with Si–N bonds, syntheses of these systems are difficult to design. This problem has been partially circumvented in polycarbosilazane dendrimers in which the nitrogen atoms are bonded to three silicon atoms, a bonding situation that is considerably less reactive than nitrogen bonded to only one or two silicon atoms

Despite the synthetic difficulties, organosilicon dendrimers with nitrogen in the structure are of interest from a fundamental perspective. First, the nitrogen atoms represent potential binding sites, as in the poly(amidoamine), PAMAM, poly(amidoamine-organosilicon), PAMAMOS (see Chapter 11), or poly(ethylene imine), PEI, dendrimer systems. Secondly, the lability of Si–N bonds raises the possibility of controlled degradation of the dendrimers. Furthermore, the presence of planar trisilyl-substituted amine groups [3–5] throughout the structure would impose some rigidity and interesting configurational constraints on the dendrimer. As a consequence, when combining these factors with the underlying synthetic challenges involved, one can see that this field should hold much interest for the synthetic dendrimer chemist

About three decades ago a remarkable cascade-type molecule was reported by Vögtle and his co-workers [1, 2]. This development set the stage for new types of polymers with a high degree of isomolecularity that are now widely known as dendrimers (see Chapter 1). In the years that followed, a number of different compositions of dendrimers, including amidoamine-, ether-, amine-, and ester-type dendrimers, etc. as well as their hetero-atom homologues has been prepared by many organic and inorganic chemists [3–6]. Among these, the introduction of silicone and organosilicon moieties into dendrimer structures has resulted in very unique silicon-containing dendrimers with considerable structural versatility [7–9]

One of the most versatile groups of organosilicon dendrimers is the carbosilanes (see Chapter 3), which often have four allylic branches emanating from a single central silicon atom (core) (0G(4-n); where n = 1–2). Physical properties of such dendrimers are gradually altered with increasing number of silicon-based moieties, branch units, and by addition of other functional groups to the silicon atoms in the peripheral region. The siloxane dendrimers with Si–O bonds in their main skeleton were reported earlier than the carbosilane dendrimers with Si–C bonds by Muzafarov et al. (see Chapter 2). The synthesis of these dendrimers, where silicon atoms form branch junctures with three Si–O bonds, was performed starting from trichloromethylsilane as a core, 0G(3)-Cl, using repetitive substitution of the chlorosilyl bond with ethoxy groups and their subsequent conversion back to chlorosilanes by thionylchloride (see Chapter 2)

Dendrimers have been prepared with a wide variety of core molecules since the first patents and publications in the early 1980s (see Chapter 1) [1–3]. The most common core molecules (e.g. ammonia, ethylenediamine, pentaerythritol) permit 2–4 branches although some molecules may give greater branch multiplicity. Polyhedral oligo-meric silsesquioxanes (POSS) allow eight branches to radiate from a silicon-oxygen core. Dendrimers based on POSS were first reported in 1993 and have resulted in many publications to date [4].

Siloxanes are molecules with the general formula [RSiO_x/2] where R is an organic group or silicon species. Siloxanes may be discrete molecules, two-dimensional ladders or networks, or three-dimensional cages or polymers. The siloxane linkage, Si–O–Si is formed when different units join to form larger molecules. Siloxy groups [R₃SiO_1/2] are good terminal groups because they halt formation of larger siloxane networks, siloxane [R₂SiO_2/2] groups are ideal candidates for forming long chain-like molecules, while silsesquioxanes [RSiO_3/2] and silicates [SiO_4/2] are most commonly found in three-dimensional structures, both random polymers and oligomers, due to the number of siloxane linkages that can be created. In all cases, a large number of structures with a large number of functionalities have been reported [5]

Dendrimers constitute a unique class of macromolecular architectures that differs from all other synthetic macromolecules in its perfectly branched topology, which is constructed from a multifunctional central core and expands to the periphery that becomes denser with increasing generation number (see Chapter 1) [1–5]. Since the pioneering works published in the late 1970s and the mid-1980s [6–8], the design and synthesis of these tree-like, well-defined molecules, which exhibit a unique combination of chemical and physical properties, is a field which has sustained dramatic growth and has generated enthusiastic studies at the frontiers of organic, inorganic, supramolecular and polymer chemistry, and more recently in the fields of nanoscience, biotechnology and medicine [1–5, 9, 10]. Whereas the initial interest in dendrimers was focused on the synthetic and structural characterization challenges that pose their fractal geometries, nanometer sizes and monodisperse nature, in the last decade the emphasis has been placed mainly on modification of the properties of dendritic molecules by their functionalization

Nowadays, one of the most active and promising research areas in dendrimer chemistry is in the integration of transition metals into dendritic structures to create metallodendrimers. Thus, the dendritic scaffold may be used for the spatial arrangement of a large number of transition metal-containing functionalities, either at the periphery or inside the dendritic skeleton (at the core or within the branches) and for the tailoring of properties through the interplay of metallic subunits. Since the first transition-metal containing dendrimers were reported in the early 1990s [11, 12], advances in the synthesis and chemistry of these molecules have not ceased to blossom. Besides the pleasant aesthetics and fundamental synthetic challenges of metallodendrimers, these molecules are also attractive because of their potential applications as functional materials in such diverse fields as catalysis, sensors, molecular electronic devices, light-harvesting antennas, nanoparticles and medical diagnostics [13–21]

The attachment of catalytic species to support materials is a widely applied method to combine the advantages of homogeneous and heterogeneous (supported) catalysis. The commonly used organic supports are insoluble polymeric materials, which have been developed with great success for solid phase organic synthesis and have a long history and importance. Obvious difficulties with these materials are their restricted loading capacity, the wettability issues, the often restricted accessibility of active (supported) sites, their reactivity or incompatibility towards reactive reagents, such as organometallics, and last but not least their high polydispersity. The use of soluble support materials can solve some of these problems, and for this reason soluble dendrimers have been explored as supports for homogeneous catalysts. Some of the advantages of dendrimers over many other types of macromolecules are their well defined structures and low polydispersity, good solubility in common organic solvents, and the presence of well-defined end-groups for the anchoring of catalytic species (see Chapter 1), all of which facilitate analysis of the (loaded) dendrimers often with atomic precision.

During the last decade, several reviews appeared describing the use of dendrimers as soluble supports for catalysts [1–11]. Among these, the silicon-based carbosilane dendrimers (see Chapter 3) assume a special position because of their structural robustness and stability towards highly reactive reagents. These are important prerequisites for any derivatization of the dendritic structure as well as for the introduction of catalytic metal sites, vide infra. Carbosilane dendrimers derive their kinetic and thermodynamic stability from the relatively high dissociation energy (306 kJ/mol) and low polarity of the Si—C bond [1]. The first demonstration of the potential of these unique properties was the successful synthesis of a carbosilane dendrimer 1 functionalized at its periphery with catalytically active NCN-pincer nickel catalysts (NCN = [C₆H₃(CH₂NMe₂)₂−2,6]⁻) [12]. Due to their molecular size of about 2 nm, such catalytic species can be separated from the reaction solutions by nanofiltration, which in principle opens the way for recycling of the catalyst as well as for continuous use of such catalysts in membrane reactors.

It is well-known that one of the main features of low-molar-mass liquid crystals and liquid-crystalline (LC) polymers is the presence of anisometric molecular fragments (mesogenic groups) responsible for LC phase (mesophase) formation. The majority of mesogenic groups consist of rigid rod-, board- (or lath-) and disk-shaped molecular moieties, which play a role of specific “building blocks”, a spontaneous ordering of which leads to the formation of different LC phases. A compound that under suitable conditions of temperature, pressure, and concentration can exist as a mesophase is usually called a mesogen or mesogenic compound.

Figure 10.1 shows various types of the best known and wide-spread mesophases. Depending on the orientational and positional organization of molecules, these mesophases may be roughly divided into nematic, smectic, and columnar LC phases. All these types of mesophases are usually formed by melting of crystalline organic solids (or cooling of an isotropic melt) and are, therefore, called thermo-tropic liquid crystals. The temperature at which the transition between the mes-ophase and the isotropic phase occurs is called the clearing (T_cl) or isotropization temperature (T_iso). Detailed information relating to low-molar-mass and polymer liquid crystals and their nomenclature may be found in a comprehensive three-volume handbook [1] and in the IUPAC Recommendations of basic terms associated with liquid crystals [2].

Silicon can play a variety of different roles when incorporated in or combined with otherwise “purely” organic dendritic branch cells which in this chapter are meant to include those that contain carbon and some combination of hydrogen, nitrogen, oxygen and sulfur. As a result, a diversity of compositional and architectural hybrid dendrimers can be formed, containing one or more of the following types of building blocks (see Fig. 11.1):

Because of these structural differences, the properties of the resulting dendrimers may range from being quite similar to those of their purely organic counterparts to being dramatically different from them. Hence, the introduction of silicon into organic dendritic structures opens up vast new areas of molecular design and engineering aimed at unique new materials for specific targeted applications.

Nucleophilic substitution reactions involving organomagnesium (Grignard) [1] and organolithium reagents have been used extensively for many years to form Si—C bonds (see Reaction Scheme 12.1). However, their use for the construction of hyperbranched polymers whose backbone contains, as a major structural component, silicon—carbon bonds, i.e., polycarbosilanes [2] is relatively more recent. This chapter focuses on the application of such nucleophilic substitution reactions toward the synthesis of hyperbranched polycarbosilanes, with particular emphasis on those preparations that have resulted in relatively well characterized products. These syntheses are organized by the type of AB_n monomer unit used (see Section 1.2), where A and B refer to the (C)X and (Si)X_n, respectively, functional ends of the monomer unit and where the nature of the coupling reaction leads to entirely or primarily Si—C bond formation. In most cases, these are “one-pot” reactions that employ monomers that bear halogen or alkoxy groups on the C and Si ends of the unit. Indeed, hyperbranched polycarbosilanes have been described, in general, as “obtained in one synthetic step via a random, one-pot polymerization of multifunctional monomers of AB _n type” [2]. Treatment of the AB_n monomer with either elemental Mg or an organolithium reagent, ideally (but not always) forms a complexed carbanion (the nucleophile) by reaction with the C–X end of the monomer unit, resulting in an intermediate of the type, (X_xM)CSiX_n, where M = Mg or Li, X = halogen or alkoxy, and x = 1 (Mg) or 0 (Li). Self-coupling of this reagent via reactions of the type shown in Reaction Scheme 12.1 leads to oligomeric and polymeric products that are connected primarily through Si—C bonds and yield an inorganic MX_x by-product.

As pointed out in Chapter 1, silicon chemistry offers a variety of quantitative, high yielding reactions, i.e. hydrosilylation, Grignard reactions and controlled condensation of silanols that are suitable for the synthesis of organic-inorganic hybrid materials. Thus, silicon-based chemistry played a prominent role in the evolution of dendrimer chemistry [1–4], and it did not take long until the first examples of silicon-containing hyperbranched polymers were reported. Hyperbranched polymers are generally prepared by one-pot polymerization of AB_x (x ≥ 2) (see also Section 1.2) monomers and are characterized by polydispersity as well as a randomly branched structure due to the multifunctional polycondensation or polyaddition process. The statistical treatment of such polyfunctional polycondensations was achieved in the early 1950s by Flory, who calculated both molecular weights and polydispersity in such systems, as is discussed in Section 13.3 of this chapter [5, 6]. The properties of hyperbranched polymers are significantly different from their linear analogs and are characterized by good solubility, low viscosity and a large number of end-groups that can be used for further functionalization. Despite imperfections in branching and structure of hyperbranched polymers compared to monodisperse dendrimers, these properties render them easily accessible competitors for dendrimers, particularly in applications where structural perfection is not a mandatory prerequisite.

Recent developments in the control of macromolecular architecture have led to important progress in dendritic structures, such as dendrimers and hyper-branched polymers, which exhibit fundamentally different properties from their linear counterparts, including impeded crystallization, minimized chain entanglements, unusual viscosity profiles and solubility behavior (see also Chapter 1). Furthermore, in contrast to linear polymers where the number of functional end-groups quickly diminishes as the molecular weight is increased, in these highly branched macromolecules the end-group functionality increases directly proportional to the molecular weight. This combination of high molecular weight, large number of terminal functional groups, overall globular shape of their molecules, a lower than expected viscosity and low degree of entanglements provides potential advantages that can be utilized in various applications.

Most common synthetic approaches to hyperbranched polymers have been based on a divergent growth method, where the monomer contains two types of functional groups that can react with each other but cannot react with themselves, and the overall functionality is greater than two (i.e., reacting monomer molecules are of the type AB_x, where x ≥ 2) (see also Section 1.2). The simplest suitable monomers of this type contain a single functional group A and two functional groups B (i.e., AB₂ type), and polymerize as shown schematically in Reaction Scheme 14.1:

Theoretical descriptions of AB_n (n ≥ 2) hyperbranched polymerization systems have been known for some time [1], but in them, cyclization is a factor that is generally and largely ignored. However, it is now understood that cyclization is prevalent in polymerizations of this type, and that it can often affect to a significant extent both polydispersity and molecular weights of the polymer products. Since research in hyperbranched polymers has increased dramatically in recent years [2–4], a number of experimental and theoretical studies have focused on the presence and effects of cyclization in these systems (see, for example [5–10] and references cited therein). In essence, intramolecular cyclization of an oligomer in an AB_n polymerization results in the consumption of the focal A group (see Section 15.3), which converts the oligomer into a B_x core. Although the newly formed core can continue to grow through the reaction of other A groups with the B groups, this growth is limited, especially if other A groups in the polymerization system are also consumed through similar intramolecular cyclization reactions. Thus, for the control and optimization of the resulting polymer molecular weight, it is necessary to understand these issues and the methods that can be used to avoid excessive amounts of cyclization

This chapter describes cyclization in organosilicon hyperbranched polymer synthesis, and techniques that have been used to minimize its occurrence. The focus is primarily on AB_n (n ≥ 2) systems, although many of the principles apply to the similar A₂ + B₃ bimolecular systems that are now gaining more research attention (see Chapter 16) [11]

Bimolecular non-linear polymerization, BMNLP (see Reaction Scheme 16.1), represents ‘the other method’ for preparation of hyperbranched polymers by the step-growth reaction mechanism. In contrast to the monomolecular polymerizations of AB_x monomers, discussed for the two most prominent groups of silicon-containing hyperbranched polymers in Chapters 12 and 13, this polymerization type involves, as its name implies, two reactive monomers A_x and B_y, where A and B denote two types of mutually reactive functional groups while x and y are integers which must both be equal to or larger than 2, while one of them (either x or y) must be equal to or larger than 3 (see Section 16.3). Thus, the most common BMNLP systems include A₂ + B₃, A₂ + B₄, and A₃ + B₄ monomer combinations. General representation of the simplest of these, the A₂ + B₃ system in which the minor component has completely reacted, is shown in Reaction Scheme 16.1