Polypropylene preparation, dendrimer

Weener et al. prepared photo-responsive monolayers from azobenzene modified polypropylene imine) dendrimers which also hold promise in the area of optical data storage [74], A fifth generation polypropylene imine) dendrimer was functionalized with equal amounts of palmitoyl and azobenzene containing alkyl chains, resulting in the formation of an amphiphilic copolymer with a random shell structure (Figure 16.5). 

A number of groups have reported the preparation and in situ application of several types of dendrimers with chiral auxiliaries at their periphery in asymmetric catalysis. These chiral dendrimer ligands can be subdivided into three different classes based on the specific position of the chiral auxiliary in the dendrimer structure. The chiral positions may be located at, (1) the periphery, (2) the dendritic core (in the case of a dendron), or (3) throughout the structure. An example of the first class was reported by Meijer et al. [22] who prepared different generations of polypropylene imine) dendrimers which were substituted at the periphery of the dendrimer with chiral aminoalcohols. These surface functionalities act as chiral ligand sites from which chiral alkylzinc aminoalcoholate catalysts can be generated in situ at the dendrimer periphery. These dendrimer systems were tested as catalyst precursors in the catalytic 1,2-addition of diethylzinc to benzaldehyde (see e.g. 13, Scheme 14). 

The convergent growth approach to dendrimers [1] first introduced [2] in 1989 at the IUPAC meeting on macromolecules in Seoul, Korea, has provided a useful alternative to the divergent methods exemplified by the work of Tomalia et al. on PAMAM dendrimers [3] and Meijer et al. on polypropylene imine) dendrimers [4], Today several hundred papers have exploited the convergent approach to dendrimers to prepare a variety of synthetic functional macromolecules of unparalleled structural precision. 

Polypropylene imine) dendrimers (see Scheme 1 for structure) have been constructed step by step onto an amine functionalized polystyrene [38, 130, 131]. The challenge in this synthesis is finding conditions for the polypropylene imine) synthesis under which the low MW polystyrene (MW=3200) is soluble [38]. Similar poly(imine) dendrimers with carboxylic acid end groups have also been prepared [130]. The polypropylene imine) dendrimer has also been synthesized on an amino-terminated poly(2-methyl-2-oxazoline) [132]. 

Nonetheless, it was a fairly short step from octopus compounds to dendrimers, and the step was taken by Vogtle in the late 1970s when he attempted to use a cascade reaction to prepare a molecule of the dendrimer type that would now be considered a dendron rather than a fully developed dendrimer. It began with the addition of acrylonitrile to an amine, followed by reduction of the nitrile to amine. This was followed by a further reaction with acrylonitrile, and the process was repeated several times to yield highly branched macromolecules. There were initially problems with the reduction step but these were overcome, and the preparation of these polypropylene imine) dendrimers was later commercialized. 

Meijer and co-workers also used a divergent dendrimer synthesis to prepare AB diblock structures (Figure 7.13B) in which the polystyrene linear block is used to initiate growth of the polypropylene imine) dendritic block [45], An amino end group had to be introduced in the polystyrene as a core for subsequent growth of the dendritic fragment via an iterative protocol of sequential... 

Since the first synthesis of a polypropylene inline) (PPI) dendrimer [1], several families of dendrimers have been prepared including the poly(amidoamines) (PAMAM), polyethers, polyphenylenes), polyphenylacetylenes), polysilanes, silicon, phosphorus and nitrogen-based dendrimers... 

Polypropylene amines, poly(amidoamines) (PAMAM) and silicon dendrimers set records for the number of generations, whereas the polyacetylene dendrimer prepared by Moore et al. [6] with the formula C1398H1278 and a mass of 18079.53 holds the heavyweight championship title of pure hydrocarbons. Mullen et al. have recently published a series of aromatic hydrocarbon dendrimers consisting of phenylene-bridged benzene rings. [7] Interestingly, these species form two-dimensional planes rather than three-dimensionally extended structures. 

Co(III) and NaBH4. Iterative repetition of these two steps allows one to obtain cascade polymers , as Vogtle initially described them, but in poor yields. Optimization of Vdgtle s original inefficient synthesis by Meijer and Mulhaupt in 1993 resulted in the preparation of poly(propylenimine) dendrimers (Scheme 13.9). The high yield and degree of purity obtained in this synthesis has made these dendrimers one of the most popular architectures in dendrimer chemistry. They have also been produced on an industrial scale and can be found under different names, in particular PPI (for polypropylenimine), DAB (for diaminobutane) and POPAM (for polypropylene amine), depending on the nature of the core. 

Ihe literature " describes a number of dendrimers and the closely related star-like 120-123 polysiloxanes. Ihe hyperbranched polysiloxanes are the primary example of more random structures. Although the emphasis has been on synthesis and characterization,i i modeling on hyperbranched polymers has also been carried out. Some of the most interesting species involve polysiloxane chains. Star polymers, some with nanosized silica cores, have also been synthesized.i °-i i Hyperbranched polysiloxanes have been prepared with controllable molecular weights and polydispersities,i -1 - with epoxy terminal groups some are UV-curable ° and some serve as a source of molecular silica. Hyperbranched polysiloxanes have also been used in the sol-gel preparation of polypropylene/silica nanocomposites. ... 

The morphology of ferrocene-containing polymers has been examined in some detail. In the block copolymer of poly(ferrocenyldimethylsilane-b-dimethyl-siloxane) (PFDMS-b-PDMS) with a 1 6 block ratio long rodlike micelles are observed rather than spherical structures in a variety of PDMS-selective n-alkane solvents when the solutions are prepared at or near ambient temperature. The reasons for these observations are subsequently discussed in detail. A further series of mixed ferrocene cobaltocenium dendrimers have been obtained these have been prepared in the reaction of the acyl chlorides of ferrocene and the cobaltocenium PF " with polypropylene dendrimers. ... 

---