Double bonds, direct polymerization

When monomer units add directly to one another, the result is an addition polymer. Table 23.1 lists some of the more familiar synthetic addition polymers. You will notice that each of these is derived from a monomer containing a carbon-carbon double bond. Upon polymerization, the double bond is converted to a single bond ... 

It is possible to introduce the double bond directly into the polymer chain. Griener and coworkers polymerized ethylene with a variety of aromatic dibromides via a Heck reaction to produce both PPV and soluble deriva-... 

The validity of this statement is confirmed by the rates of IC1 additions (see Table 12). Because for these additions the formation of a cationic intermediate by direct attack of the electrophile on the double bond is rate determining, their order of rates is comparable to those of polymerizations. It is therefore understandable that the polymerization rates correlate much better with the reactivities of the monomers during an electrophilic addition of cationogenic agents (such as IC1) than with the relatively unspecific EDA complex formation. 

Due to the retractive forces in stretched mbber, the aldehyde and zwitterion fragments are separated at the molecular-relaxation rate. Therefore, the ozonides and peroxides form at sites remote from the initial cleavage, and underlying mbber chains are exposed to ozone. These unstable ozonides and polymeric peroxides cleave to a variety of oxygenated products, such as acids, esters, ketones, and aldehydes, and also expose new mbber chains to the effects of ozone. The net result is that when mbber chains are cleaved, they retract in the direction of the stress and expose underlying unsaturation. Continuation of this process results in the formation of the characteristic ozone cracks. It should be noted that in the case of butadiene mbbers a small amount of cross-linking occurs during ozonation. This is considered to be due to the reaction between the biradical of the carbonyl oxide and the double bonds of the butadiene mbber [47]. 

Ziegler-Natta catalyst for polymerization of alkenes. Considerable attention has been directed to double-bonded Fischer carbenes of Cr and W, the Schrock carbenes of Ta and Ti, and cyclic polyene ligands of Fe, Co, Cr, and U. Carbonyls of transition metals from groups 6 to 10 of the periodic table include both the monomeric compounds such as Cr(CO)g, Fe(CO)5, Ni(CO)4 and those with two metal groups such as Mn2(CO)io and Co2(CO)s, which is used industrially for hydroformylation. Although their source has not been identified, it has been shown that volatile compounds from landfills contain carbonyls of Mo and W (Feldmann and Cullen 1997). 

In the polymerization of butadiene, Teyssie (52-54) has shown that certain electron donors, such as alcohols or phosphines, can convert tt-allylnickel chloride from a catalyst which forms c/j-polybutadiene to one which produces frans-polybutadiene. These ligands presumably block a site on the nickel atom, forcing the butadiene to coordinate by only one double bond. While alcohols cannot be added directly to the hexadiene catalyst (as they deactivate the alkylaluminum cocatalysts), incorporation of the oxygen atom on the cocatalyst places it in an ideal position to coordinate with the nickel. The observed rate reduction (52) when the cri-polybutadiene catalyst is converted into a fra/w-polybutadiene catalyst is also consistent with the observed results in the 1,4-hexadiene synthesis. 

Until now the discussion has centered on the addition polymers obtained fiom unsaturated monomers by reaction of the C=C or C=0 double bond. However, polymers obtained by other methods (ring-opening polymerization, polycondensation, etc.) offer interesting stereochemical phenomena also. As a rule, in these classes of macromolecular compounds the monomer units are clearly defined, the direction of the chain is often distinguishable and the stereo-isomeric elements present in the chain already preexist in the monomer. There are, however, numerous exceptions and further clarification is called for. 

Monomers with cumulated double bonds, such as substituted allenes and ketenes, produce a great variety of structures. Stereoisomerism is found both at the saturated (iso- or syndiotacticity) and at the unsaturated carbons where the substituents in the plane of the chain can be oriented in either direction (forward or backward). With regard to 1,3-disubstituted allenes, four stereoregular strac-tures, 43-46 (Scheme 10), are predicted. Porri, Rossi, and Ingrosso succeeded in polymerizing 2,3-pentadiene (1,3-dimethylallene) samples of different optical purity (87). In their experiments they recognized the existence of sequences 43. 

The first route relies on the ROP of cyclic ketene acetals [1-3]. The electron-rich double bond is prone to react with radicals and electrophiles. Therefore, this class of monomers undergoes cationic and radical polymerization. For example, radical initiators react with the double bond to provide a new tertiary radical (Fig. 2). Two distinct mechanisms of polymerization can then take place direct vinyl polymerization or indirect ring opening of the cycle accompanied by the formation of a new radical, which is the propagating species (Fig. 2). The ester function is formed... 

An important quantity that can be deduced from the reaction profile is the rate of the cross-linking polymerization (Rp), i.e., the number of double bonds polymerized or of cross-links formed per second. Rp values were determined from the maximum slope of the kinetic curves (usually reached for conversion degrees between 20 and 40%). Table I summarizes the Rp values for the two photoresists tested under various conditions, namely conventional UV and continuous or pulsed laser irradiation at different light intensities. According to these kinetic data, Rp increases almost as fast as the light-intensity the ratio Io/Rp which is directly related to the product of the light-intensity and the required exposure time was found to vary only in the range 10-8 to... 

It is not possible at our present stage of knowledge to place all of the catalysts in exact position relative to their ionic nature. The "mid point may be displaced some to either direction. Most catalysts contain several different components with different degrees of ionicity. Which component acts as the active catalyst for a particular double bond is unknown in most cases. Only crude presentations are possible until techniques have been developed to determine the actual ionic nature of the propagating species in isotactic ionic polymerization s such as ESR is capable of in free radical polymerizations. 

This type of halogenation procedure involving active centers should be carefully examined since like a,a -dibromoxylene it is applicable to unsaturated polymeric anions such as poly(butadienyl)lithium and poly(isoprenyl)lithium whose double bonds would react directly with halogens. 

The polymerization of sucrose derivatives (esters, ethers, acetals) bearing a carbon-carbon double bond has been studied (Scheme 45). Polymers can be obtained by polymerization or copolymerization.146,404 414 The monomers are prepared either by multistep synthesis, leading to defined compounds and subsequently rather well-controlled polymerization processes,302,415,416 or by direct functionalization of unprotected sucrose, leading to mixtures of isomers and... 

Polymeric (SN)X has some unusual properties. For example, it has a bronze color and metallic luster, and its electrical conductivity is about that of mercury metal. Values of the conductivity of (SN) depend on the purity and crystallinity of the polymer and on the direction of measurement, being much greater along the fibers than across them. A conjugated single-bond/double-bond system can be formulated, in which every S-N unit has one antibonding tt electron. The half-filled overlapping tt orbitals combine to form a half-filled conduction band, in much the same way as the half-filled ns orbitals of alkali metal atoms... 

Functionalised a-olefins capable of undergoing insertion polymerisation with Ziegler-Natta catalysts are, in principle, monomers in which the heteroatom (X) does not electronically interact with the double bond to be polymerised in such monomers, the heteroatom is separated from the double bond CH2=CH-(CH2)x X [326,384,518,522-528], Monomers with the heteroatom directly bound to the double bond, i.e. those of the CH2=CH-X type, may also undergo polymerization, but when the heteroatom is silicon or tin (X= Si, Sn) [522-526], Representative examples of the insertion polymerisation of functionalised a-olefins and their copolymerisation with ethylene and a-olefins in the presence of heterogeneous Ziegler-Natta catalysts are shown in Table 3.7 [2,241,326,384,518,522-528],... 

For the preparation of poly(isoprene), the monomer 2-methyl-1,3-buta-diene (= isoprene = IP) is required as feedstock. This monomer can be obtained by various condensation methods that utilize four principles to create the C5 skeleton. In the more modern process IP is obtained from the C5 cracking fraction which contains various double-bond containing hydrocarbons with 5 C-atoms (e.g. among other C5-compounds the fraction contains cyclopentadiene, various pentadienes and pentenes) [478]. The preparation of pure IP by either of these two routes is cost intensive. By the direct and selective polymerization of IP in the crude C5 cracking fraction the cost intensive isolation of pure IP is avoided. Thereby production costs for IR are considerably reduced [264,265]. The selective polymerization of IP in the crude C5 cracking fraction is achieved by the application of a NdP-based catalyst system. The latest patent of Michehn claims a process in which dehydrogenation of the C5 cut is applied prior to polymerization. In this way an IP-enriched C5-fraction is obtained which does not contain a high quantity of disubstituted alkynes, terminal alkynes and cyclopentadiene. The unpurified C5-fraction is used as the feedstock for polymerization [591,592]. 

The simplest chemical compounds used directly in synthesis reactions and which are incorporated into the macromolecular chain as a structure sequence are called monomers. Monomers are either unsaturated, that is they have one or more double bonds or are bifunctional compounds. The corresponding plastic (polymer) is produced by a technical polymerization reaction of either a free radical chain reaction (unsaturated monomers) or an intermolecular condensation reaction (bifunctional). 

Cationic Polymerization. Cationic polymerization is initiated by the transfer of a cation from the catalyst to the monomer. It allows a wider choice of monomers with double bonds, including carbonyls, cyclic ethers, and lactones. The ion may be within a carbonium or an oxonium ion. Friedel-Crafts halides, like AlCls or A CoHsJCL, are strong Lewis acids and initiate the polymerization directly. Weak Lewis acids need a... 

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