Catalytic polymerization routes

The poly(siloxane) polymers are usually prepared by the acid or base hydrolysis of appropriately substituted dichlorosilanes or dialkoxysilanes, or by the catalytic polymerization of small ring cyclic siloxanes [71-75]. The silanol-terminated polymers are suitable for use after fractionation or are thermally treated to increase molecular weight and in some cases endcapped by trimethylsilyl, alkoxy or acetyl groups [76,77]. Poly(siloxanes) synthesized in this way are limited to polymers that contain substituent groups that are able to survive the relatively harsh hydrolysis conditions, such as alkyl, phenyl, 3,3,3-trifluoropropyl groups. Hydrosilylation provides an alternative route to the synthesis of poly(siloxanes) with labile or complicated substituents (e.g. cyclodextrin, oligoethylene oxide, liquid crystal, amino acid ester, and alcohol) [78-81]. In this case... 

In addition, two synthesis strategies were studied (Figure 6.11). In route 1, the clay was first reacted with an MMAO heptane solution and then, after saturation with the monomer, the polymerization initiated by the addition of Fe precatalyst. Instead, in route 2, the precatalyst was activated by using the clay-immobilized cocatalyst and the polymerization initiated by monomer addition. The catalytic activities with the different montmorillonites and precatalysts used, following polymerization routes 1 and 2, are summarized in Figure 6.12. 

There are a variety of routes toward the synthesis of polyacetylene. These syntheses can be classified into four categories catalytic polymerization of acetylene, noncatalytic pofymerization of acetylene, catalytic polymerization of monomers other than acetylene, and, finally, precursor methods. 

A variety of routes have been proposed for the synthesis of polyacetylene. These can be classified into four categories as summarized in Table 7.1. The first is via the catalytic polymerization of acetylene. The second is noncatalytic polymerization. So far, spontaneous polymerization of acetylene has been reported under high pressure. The third type of route is the catalytic polymerization of monomers other than acetylene. The fourth is a so-called precursor route in which linear conjugated polyene chains are formed either by decomposition or by isomerization of soluble precursor polymers. 

Chiang et al. [372] describe an interesting variant of the catalytic in situ polymerization route described above for fabrication of P(Ac)/PE composite fibers. PE fibers are first spun firom a PE/mineral oil solution. The fibers are then soaked in a standard P(Ac) Ziegler-Natta catalyst (Ti(OBut)4/AlEt3). Polymerization of P(Ac) from acetylene gas is then carried out in the standard maimer. The excess catalyst and mineral oil in the fibers are extracted with toluene and HCl/methanoI. The P(Ac)/PE composite fibers so obtained have conductivities of 1,200 S/cm as prepared, or 6,000 S/cm when drawn to a 2.2 draw ratio, and can have P(Ac) contents up to 82 w/w%. The Young s Modulus, Tensile Strength and Elongation at Break of as prepared composite fibers are respectively 0.5 GPa, 0.1 GPa and 170% respectively. Unfortunately, the fibers also suffer from the usual environmental instability of P(Ac). 

Hexafluoiopiopylene and tetiafluoioethylene aie copolymerized, with trichloiacetyl peroxide as the catalyst, at low temperature (43). Newer catalytic methods, including irradiation, achieve copolymerization at different temperatures (44,45). Aqueous and nonaqueous dispersion polymerizations appear to be the most convenient routes to commercial production (1,46—50). The polymerization conditions are similar to those of TFE homopolymer dispersion polymerization. The copolymer of HFP—TFE is a random copolymer that is, HFP units add to the growing chains at random intervals. The optimal composition of the copolymer requires that the mechanical properties are retained in the usable range and that the melt viscosity is low enough for easy melt processing. 

Oxalamidinate anions represent the most simple type of bis(amidinate) ligands in which two amidinate units are directly connected via a central C-C bond. Oxalamidinate complexes of d-transition metals have recently received increasing attention for their efficient catalytic activity in olefin polymerization reactions. Almost all the oxalamidinate ligands have been synthesized by deprotonation of the corresponding oxalic amidines [pathway (a) in Scheme 190]. More recently, it was found that carbodiimides, RN = C=NR, can be reductively coupled with metallic lithium into the oxalamidinate dianions [(RN)2C-C(NR)2] [route (c)J which are clearly useful for the preparation of dinuclear oxalamidinate complexes. The lithium complex obtained this way from N,N -di(p-tolyl)carbodiimide was crystallized from pyridine/pentane and... 

Eisch s work promoted investigation into the preparation of cationic metallocene complexes of Group 4 metals. Several preparative routes to cationic group 4 metallocene complexes are illustrated in Scheme II. Catalytic activities of some selected cationic metallocene complexes for the polymerization of a-olefins are summarized in Tables 5 and 6. The catalyst systems based on these cationic complexes are just as active as M AO-activated metallocene catalysts for the polymerization of a-olefins. 

The use of porphyrinic ligands in polymeric systems allows their unique physio-chemical features to be integrated into two (2D)- or three-dimensional (3D) structures. As such, porphyrin or pc macrocycles have been extensively used to prepare polymers, usually via a radical polymerization reaction (85,86) and more recently via iterative Diels-Alder reactions (87-89). The resulting polymers have interesting materials and biological applications. For example, certain pc-based polymers have higher intrinsic conductivities and better catalytic activity than their parent monomers (90-92). The first example of a /jz-based polymer was reported in 1999 by Montalban et al. (36). These polymers were prepared by a ROMP of a norbor-nadiene substituted pz (Scheme 7, 34). This pz was the first example of polymerization of a porphyrinic macrocycle by a ROMP reaction, and it represents a new general route for the synthesis of polymeric porphyrinic-type macrocycles. 

Biopolymers are certainly a product of the future. Their high eco-efficiency in some applications drives the development of these plastics [133]. In some niche markets, their higher production costs resulting from time-consuming purification and less than ideal raw materials can be neglected, but for large-scale applications of several million tons per year and real competitiveness with commodity polymers, new catalytic routes that lead directly to the polymeric material without generation of side-products are necessary. 

---