Polymerization mechanisms for

The polymerization mechanism for the dual-side catalysts is totally different from the C2-symmetric complexes. Due to their geometry, the dual-side complexes show different stereoselectivities for monomer coordination and insertion. It was shown that the introduction of the stereoerror formation by the 5-substituted asymmetric catalysts originates predominately from the kinetic competition between chain back-skip and monomer coordination at the aspecific side of the catalyst [9],... 

The Brookhart laboratory has contributed much of the knowledge of the polymerization mechanism for the late transition metal a-diimine catalysts. The review by Ittel provides a concise summary of the mechanistic understanding as of the year 2000 [26]. Some of the early findings will be reviewed here and additional insights reported afterward will be presented. In addition to the experimental work, many theoretical and computational studies worthy of discussion have also been carried out. These efforts have been most important in providing insight into the mechanistic details of the highly reactive nickel system, which is often difficult to study experimentally. 

Polymerization inhibitors miscellaneous, 23 383 in styrene manufacture, 23 338 Polymerization initiators alkyllithiums as, 74 251 cerium application, 5 687 peroxydicarbonates as, 74 290 Polymerization kinetics, in PVC polymerization, 25 666-667 Polymerization mechanism, for low density polyethylene, 20 218 Polymerization methods, choice of,... 

Figure 5. Anionic polymerization mechanism for alkylcyanoacrylate monomers (R = n-butyl).

The living cationic ring opening polymerization (CROP) of 2-oxazolines was first reported in the 1960s [61, 62]. The polymerization can be initiated by an electrophile such as benzyl halides, acetyl halides, and tosylate or triflate derivatives. The typical polymerization mechanism for 2-alkyl-2-oxazoline initiated by methyl tosylate is shown in Scheme 6. 

Fig. 32 (a) Topochemical polymerization mechanism for a typical diacetylene monomer, (b) Crystal structure of a typical poly diacetylene... 

The generally accepted polymerization mechanism for heterocyclic polymers such as poly(pyrrole) is shown in Figure 7.3 [23]. 

The authors propose that the dimers are formed by a ring-expansion mechanism comparable to the polymerization mechanism for cyclic acetals proposed by Plesch... 

It seems that formation of dimers and trimers from soybean oil is the slow step of polymerization, an evidence for step polymerization mechanism. Once formed, these dimers and trimers polymerize very quickly to a high molecular weight and exhibits chain reaction kinetics. This phenomenon is caused by the soybean oil molecule itself, a large molecule with low activity, due to mid chain double bond location. However, the formed dimers and trimers have more unsaturated carbon-carbon double bonds per molecule, and they are easily polymerized to high molecular weight polymers. We propose further investigation on the possible combination of two polymerization mechanisms for the polymerization of soybean oil in SCCO2. Kinetic study of soybean oil polymerization currently is carried out in our laboratory. 

The polymerization mechanisms for vinyl chloride and acrylonitrile (and also styrene) with coordination catalysts are also uncertain [222] and the copolymerization of butadiene/acrylonitrile (q.v.) also shows some features suggesting the formation of free radicals (or possibly radical-ions from charge transfer complexes). As these polar monomers can react, or form strong complexes, with the organo-metal compound it is likely that the kinetic schemes will be complex. As with styrene there is a good deal of scatter in the experimental kinetic data with these monomers which detracts from the certainty of the deductions, and much work will be required to put their polymerization by coordination catalysts on a sound mechanistic and kinetic basis. 

Scheme 2. Proposed a-olefm polymerization mechanism for divalent samarocene complexes.

Polymerization mechanisms for polyaniline have been proposed in the literature [45, 46], Figure 2 illustrates some of the basic steps occurring during polymerization of aniline. The oxidation states of PANI, and of polyanilines in general, are indicated by an index for the degree of oxidation (Y). It is in its completely reduced form (leucoemeraldine) when Y = 1, and its completely oxidised form (pemigraniline) is dominant when Y = 0. At Y = 0.5, the 50% intrinsically oxidized polymer (emeraldine) is ambient [49, 50], The molecular structures of the different forms (oxidation states) of PANI are illustrated in Figure 1. 

Three kinds of structural isomerism are observed in polymers regioisomerism, stereoisomerism, and geometrical isomerism, depending on the polymerization mechanism. For example, with modern catalysts, structures contained in polyolefins can be controlled to... 

Scheme 11 Polymerization mechanism for methyl methacrylate with 1,1,2,2-tetraphenyl-l,2-bis(trimethylsiloxy)ethane...

Figure 14.8 Putative mechanism of CALB-catalyzed polymerization of [3-lactam adopted from the assumed polymerization mechanism for (3-lactone [7h].

Strelko and co-workers (71-73) investigated the properties of the siloxane bond and the polymerization mechanism for silicic acid on the basis of the difference in the electronic structure of the Si-O bonds in silanol and the siloxane groups of silica. A kinetic equation was proposed to describe the polymerization of silicic acid throughout the entire pH range. The authors believe that the molecular mechanisms for the formation of globular skeletons in silica gels are based on the polymerization and depolymerization of silicic acids. 

A recent paper presented a Schulz-Flory polymerization mechanism for the synthesis (22). This scheme is a simplified form of SCG obtained by setting the branching parameter f equal to zero. 

The mechanisms of ionic and coordination polymerizations are more complex and are not as clearly understood as those of free radical polymerization. Here, we will briefly highlight the essential features of these mechanisms, and more details will be given in Chapter 7. Initiation of ionic polymerization usually involves the transfer of an ion or an electron to or from the monomer. Many monomers can polymerize by more than one mechanism, but the most appropriate polymerization mechanism for each monomer is related to the polarity of the monomers and the Lewis acid-base strength of the ion formed. 

Figure 3.8 Step-growth polymerization mechanism for silicones.

When catalysts containing more than one type of active sites are used, the conventional approach is simply to repeat the model equations derived for single-site-type catalysts, using distinct kinetic rate constants for each site type. This simply means that the multiple-site catalyst is considered to be a collection of single-site type catalysts. Sometimes, site transformation steps may also be included in the model to permit the conversion of one site type to another, but the polymerization mechanism for each site type remains the one previously described. 

The arrangement of the two types of monomeric units a and b of a bipoiymer is determined by the polymerization mechanism, for example,... 

FIGURE 1 The electrochemical polymerization mechanism for the formation of polypyrrole begins when a pyrrole radical cation forms from the oxidation of the monomer. Then two radical cations react to create a dimer and split off two protons. The dimer undergoes further oxidation to generate a radical, which reacts with another radical to propagate chain growth and eventually produce polypyrrole. 

A variety of Group 4 metal complexes, in combination with common olefin polymerization activators, have been evaluated as potential catalysts for syn-diospecific polymerization of styrene (for reviews, see Refs. 114, 115, 123, and 426). Monocyclopentadienyl and monoindenyl titanocenes generally exhibit the highest activities (eq. 5) (112-127). Curiously, half-sandwich titanium-trifluoride-based catalysts are more active than their trichloride analogues (124,427,428). The polymerization mechanism for sPS formation is under debate. Kinetic studies and spectroscopic investigations of the catalytic systems suggest a cationic Ti(III) complex as the active species (123). 

SCHEME 21.2 Polymerization mechanisms for acetylenes where M represents the active catalyst site. 

Presents the possible polymer microstructures, best catalyst systems, and polymerization mechanisms for each monomer discussed... 

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