Polymerization, initiators anionic type

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

Carbonyl monomers can be polymerized by acidic initiators, although their reactivity is lower than in anionic polymerization. Protonic acids such as hydrochloric and acetic acids and Lewis acids of the metal halide type are effective in initiating the cationic polymerization of carbonyl monomers. The initiation and propagation steps in polymerizations initiated with protonic acids can be pictured as... 

The initiation of this type of polymerization can be achieved with a polymeria initiator, e.g. a polymer containing peroxide or azo end groups. As the different types of synthesis which have been described for the preparation of such polymeric initiators, have generally the disadvantage to need several reaction steps (3- 5), it seemed interesting to prepare these initiators in a one Step reaction, by keeping the different advantages of the anionic polymerization. 

Unlike cationic polymerization initiated by a conventional catalyst, the propagating species in the present system would bear different type of counter-ion or would be much more free. The counter-anion obtained in this entirely organic system would be large and unstable. The problem of the counterion in charge transfer ionic polymerization certainly requires further study. 

Another investigation along this line is the pulse radiolysis study of the electron transfer reactions from aromatic radical anions to styrene this type of reaction is commonly used to initiate anionic polymerization of styrene [35], The electron transfer rates from the unassociated biphenyl radical-anions to styrene derivatives in 2-propanol were found to increase along the... 

The polymerization of anions is a special type of irreversible anodic processes. Of these the oxidation of sulphate to persulphate ions has been studied in the deepest detail. In the production of pcrsulphuric acid the yield is increased to a certain limit by a higher concentration of the initial sulphuric acid and an increased current density at the anode of smooth platinum. In too concentrated sulphuric acid the pcrsulphuric acid is already hydrolysed to a considerable extent to monopersulphuric acid (Caro s acid), which then acts as a depolarizer and lowers the required high potential at the anode. Electrolysis of sulphate solutions also gives persulphates and in this reaction the current efficiency will depend on the nature of the cation the efficiency increasing in the order of Na+, K+ and NHj. 

A Rohm and Haas group in 1958 (13, 21), reported that methyl methacrylate can be polymerized stereospecifically by an anionic type initiator under homogeneous conditions. Table 6 summarizes their results. 

When lithium alkyl catalysts are used in non-solvating media such as aliphatic hydrocarbons, the polymer-lithium bond is not sufficiently ionic to initiate anionic polymerization so that the monomer must first complex with vacant orbitals in the lithium. A partial positive charge is induced on the monomer in the complex, and this facilitates migration of the polymer anion to the most electrophilic carbon of the complexed monomer. This type of polymerization is more appropriately termed coordinated anionic and will be discussed in the next section. There does not appear to be any evidence that alkyl derivatives of metals which are less electropositive than lithium and magnesium can initiate simple anionic polymerization. 

We have carried the theme of a primary electrophilic attack on monomer through the discussions on coordinated anionic polymerizations with simple alkyl metals, and on coordinated anionic and radical polymerizations with Ziegler type catalysts. When a Ziegler type catalyst is comprised of one or more strongly electrophilic components, the Lewis acidity can become great enough to initiate polymerization by a cationic mechanism. Naturally, this will occur most readily for those monomers which are prone to cationic initiation because of their ability to stabilize the carbonium ion 278). 

In cationic polymerizations, initiation occurs by attachment of a proton or some other Lewis-acidic cation X" to the H2C=CR2 double bond of a vinyl monomer to form a new carbon-centred cation of the type XH2C-CR2, which then grows into a polymer chain by subsequent H2C=CR2 additions (Figure 2, bottom). This type of polymerization works well - and is used in practice - only for olefins such as isobutene, where 1,1-disubstitution stabilizes the formation of a cationic centre. Since side reactions, such as release of a proton from the cationic chain end, occur rather easily, cationic polymerization usually gives shorter chains than anionic polymerization. 

Dormant Species and Pseudocationic Propagation The majority of propagating chain ends in most cationic polymerizations initiated by protonic acids and/or cocatalyzed by Lewis acids do not exist as carbenium ions, but are instead dormant species. The two major types of dormant species are onium ions and covalent esters or halides. The covalent species are formed by reversible reaction of carbenium ions with nucleophilic anions onium ions are generated by reaction of carbenium ions with noncharged nucleophiles such as ethers, sulfides, and amines. Because the majority of propagating chain ends exist as dormant species, they are often the only species that can be detected spectroscopically ... 

Several approaches used to prepare hybrid polymers in which polyacetylene is an electroactive component are presented. Specifically, these involve the preparation of (1) composites by in-situ polymerization, (2) graft copolymers utilizing carbanions in n-type (CH)X as polymerization initiators, and (3) A-B diblock copolymers exploiting anionic-to-Ziegler-Natta transformation reactions. 

Ring-opening metathesis polymerization was also used recently for the preparation of amphiphilic star-block copolymers [25]. Mo (CH-f-Bu) (NAr) (0-f-Bu)2 was used as the initiator for sequential polymerization of norbornene-type, unfunctionalized and functionalized, monomers. The living diblocks were reacted with endo-ris-endo-hexacyclo- [ 10.2.1.1.3/115>8.02>l 1. O 1-9] heptadeca-6,13-diene, a difunctional monomer in a scheme analogous to the use of DVB in anionic polymerization, to form the central core of the star (Scheme 10). 

The possibilities inherent in the anionic copolymerization of butadiene and styrene by means of organolithium initiators, as might have been expected, have led to many new developments. The first of these would naturally be the synthesis of a butadiene-styrene copolymer to match (or improve upon) emulsion-prepared SBR, in view of the superior molecular weight control possible in anionic polymerization. The copolymerization behavior of butadiene (or isoprene) and styrene is shown in Table 2.15 (Ohlinger and Bandermann, 1980 Morton and Huang, 1979 Ells, 1963 Hill et al., 1983 Spirin et al., 1962). As indicated earlier, unlike the free radical type of polymerization, these anionic systems show a marked sensitivity of the reactivity ratios to solvent type (a similar effect is noted for different alkali metal counterions). Thus, in nonpolar solvents, butadiene (or isoprene) is preferentially polymerized initially, to the virtual exclusion of the styrene, while the reverse is true in polar solvents. This has been ascribed (Morton, 1983) to the profound effect of solvation on the structure of the carbon-lithium bond, which becomes much more ionic in such media, as discussed previously. The resulting polymer formed by copolymerization in hydrocarbon media is described as a tapered block copolymer it consists of a block of polybutadiene with little incorporated styrene comonomer followed by a segment with both butadiene and styrene and then a block of polystyrene. The structure is schematically represented below ... 

The majority of miniemulsions reported in the literature have been stabilized with anionic surfactants such as sodium dodecyl sulfate (SDS), probably because of the widespread use of anionic surfactants in conventional emulsion polymerization, and due to their compatibility with neutral or anionic monomers and anionic initiators. Other types of surfactant have also been used, such as cationic, nonionic, and mixed nonionic/anionic, reactive as well as nonreactive surfactants. The surfactants useful for miniemulsion polymerization should meet the same requirements as in the conventional emulsion polymerization. 

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