Polymerization terminator

In ionic polymerizations termination by combination does not occur, since all of the polymer ions have the same charge. In addition, there are solvents such as dioxane and tetrahydrofuran in which chain transfer reactions are unimportant for anionic polymers. Therefore it is possible for these reactions to continue without transfer or termination until all monomer has reacted. Evidence for this comes from the fact that the polymerization can be reactivated if a second batch of monomer is added after the initial reaction has gone to completion. In this case the molecular weight of the polymer increases, since no new growth centers are initiated. Because of this absence of termination, such polymers are called living polymers. 

In anionic polymerization, as in carbonium ion polymerization, termination does not involve bimolecular reaction between two growing chains. Neither can recombination of ions lead to termination, since a carbon-metal bond is highly polar, in the case of alkali metals frequently completely ionized, and in every case very reactive. The termination step leading to the formation of a terminal C=C double bond is not too probable. This reaction involves the formation of a metal hydride, and this does not contribute greatly to the driving force. Consequently, such a termination is observed at higher temperatures only and it is probably more common in coordination polymerization where the metals involved are less electropositive. 

It has been shown recently (10) that such block structures could be tailored precisely by the general method summarized hereabove. It is indeed possible to convert the hydroxyl end-group of a vinyl polymer PA (f.i. polystyrene, or polybutadiene obtained by anionic polymerization terminated with ethylene oxide),into an aluminum alcoholate structure since it is well known that CL polymerizes in a perfectly "living" manner by ring-opening insertion into the Al-0 bond (11), the following reaction sequence provides a direct access to the desired copolymers, with an accurate control of the molecular parameters of the two blocks ... 

Ionic polymerizations are characterized by a wide variety of modes of initiation and termination. Unlike radical polymerization, termination in ionic polymerization never involves the bimolecular reaction between two propagating polymer chains of like charge. Termination of a propagating chain occurs by its reaction with the counterion, solvent, or other species present in the reaction system. 

The theoretical molecular weight distributions for cationic chain polymerizations are the same as those described in Sec. 3-11 for radical chain polymerizations terminating by reactions in which each propagating chain is converted to one dead polymer molecule, that is, not including the formation of a dead polymer molecule by bimolecular coupling of two propagating chains. Equations 2-86 through 2-89, 2-27, 2-96, and 2-97 withp defined by Eq. 3-185... 

Fontanille, M., Carbanionic Polymerization General Aspects and Initiation, pp. 365-386 and Carbanionic Polymerization Termination and Functionalization, pp. 425 432 in Comprehensive Polymer Science, Vol. 3, G. C. Eastmond, A. Ledwith, S. Russo, and P. Sigwalt, eds., Pergamon Press, London, 1989. 

The results of the polymerization terminated lOmin after the initiation are shown in Table IV. As compared with the results of the polymerization for 5hr, the yield and molecular weight of the polymer and oligomer were lower, but the number of butyl carbonyl group in one polymer molecule was almost constant regardless of the polymerization time. This may indicate that the butyl lsopropenyl ketone unit in the polymer molecule locates at or near to the forefront of the chain. 

The first polymerizations reported by Kops and Schuerch147 were those of l,4-anhydro-2,3,6-tri-0-methyl-/3-D-galactopyranose and 1,4-anhydro-2,3-di-0-methyl-a -L-arabinopyranose. The latter compound was slightly contaminated with l,4-anhydro-2,3-di-0-methyl-a-D-xy-lopyranose, but the course of the polymerization could nevertheless be monitored reasonably accurately. For the most part, the polymerizations were conducted at 10% concentration (g/mL) in dichloro-methane, or aromatic hydrocarbons, with 1-5 mol% of phosphorus pentafluoride, or boron trifluoride etherate. At low temperature (—78 to —97°), the d.p. of both polymers produced was —90 at increasing temperatures of polymerization, termination processes became more severe, and the d.p. lower. Usually, the reaction times were long (perhaps unnecessarily so), and the conversions were 50 to 90%. The specific rotations of the D-galactans prepared at —28 and —90° differ by only —10° ( — 85 to — 95°), but those of the L-arabinans varied from + 6... 

Which mode of termination occurs can be determined by measuring the number of initiator fragments per polymer molecule. If there are two initiator fragments in each molecule, termination must have occurred by combination. One initiator fragment per molecule indicates disproportionation. Apparently, ethenylbenzene polymerizations terminate by combination, but with methyl 2-methylpropenoate, both reactions take place, disproportionation being favored. 

If the diazonium groups result from the diazotation of poly-/>-amino-styrene, the macroradicals will initiate grafting. Contrarily, if >-(N-acetyl) phenylenediamine is diazotized and used as initiator of a first monomer, a polymer is obtained with an acetamino. phenyl end group (-CGH4-NH-Ac). After hydrolysis of this last and diazotation of the free amine group, the polymeric terminal diazonium salt can be used with ferrous ions for the synthesis of block copolymers. 

In a conventional polymerization, termination is the irreversible step which prevents the attainment of an equilibrium between polymer and its monomer. Hence, if a sufficiently large amount of initiator is available, all the monomer will be converted eventually into polymer. This is in principle impossible in a polymerization involving living" polymers. 

Figure 1. Polydispersity index of the polymer produced in Interval II of an emulsion polymerization terminated solely by combination as a function of the average number of free radicals per particle...

The cationic polymerization terminates by loss of a proton to afford the alkene. 

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

Childers (5) found that a cobalt-catalyzed polymerization terminated with C14 labelled alcohol resulted in activity in the polymer, whereas H3 hydroxyl labelled alcohol gave inactive polymer. He deduced, therefore, that the mechanism was cationic. The organometal compound used in this catalyst was ethylaluminum sesquichloride, and his observations have been confirmed (26). We carried out virtually the same experiment as Childers except that AlEt2Cl was used instead of AFEtsCh with the opposite conclusion—namely, the growth was anionic (8). 

The triphenyl methyl or trityl radical behaves as a radical trap and favors the polymerization-termination which is thermoreversible and thus allows the insertion of a new polymeric sequence. In 1982, Otsu et al. [49,213,214] proposed an interesting example involving phenylazotriphenylmethane as Initer (initiator-terminator) able to initiate a free radical polymerization from the phenyl radical. Alternatively, the trityl end-capped polymer can be utilized as an original macroiniter for the polymerization of a second monomer and yields block copolymers as follows ... 

The THT and SMe2 adducts have structures of the type (18-B-V). Their chemistry has been extensively studied and it is summarized in Fig. 18-B-7. The diverse, and in some cases unique, reactivity of these compounds includes substitution with preservation of the geometry or with conversion to (MX4)2(/t-X)2 species, oxidative-addition,53 cluster formation, splitting of C—N bonds,54 and above all coupling of the molecules with triply bonded carbon atom.55 They catalytically trimerize and polymerize terminal acetylenes, and dimerize nitriles and isonitriles with incorporation of the new ligand into the complex. Another remarkable reaction of M2C16L3 is the metathesis of M=M and N=N bonds into two M=N bonds upon reaction with azobenzene. 

A relatively high-molecular-weight, crystalline polyphenylacetylene of molecular mass of the order of 104 is formed by ionic or coordination polymerization (with WCI6 or (acac),Fe) [77]. In these polymerizations, termination is also a function of chain length [78]. 

Polymerizations terminating by combination produce polymers with a distribution narrower than the most probable one. As a again very nearly approaches one, v 3/2. 

In many carbocationic polymerizations, termination is negligible (cf., Section VI). In this case, the total number of chains equals the total number of chains generated by transfer to monomer [A/lrM], by spontaneous transfer [/Vlr], and the number of macromolecules [A/] still growing. The latter should be equal to the initial initiator concentration [I]o if initiation is completed, or to [I]0 - [1]/ at time t. The total number of macromolecules generated by transfer ( [A/tr]) equals the sum of Eqs. (117) and (118), assuming that initiation is rapid ([ZV]o [I]o). The number of macromolecules formed by transfer which is first order in monomer is proportional to conversion, whereas that formed by transfer which is zero order in [M] is proportional to time. 

It is useful to re-examine Chapter 3, Section VI describing conventional carbocationic polymerizations. Termination reactions are not very common and involve either the formation of too stable carbocations, unreactive alkyl halides, usually alkyl fluorides, or unreactive onium ions via reaction with very strong nucleophiles (impurities or intentionally added compounds) ... 

As you, by now, have doubtiess anticipated, cationic polymerizations involve an active site where there is a positive charge because, in effect, there is a deficit of one electron at the active site (Figure 3-21). Cationic polymerizations can be initiated by protonic acids (Figure 3-35) or Lewis adds (the latter sometimes combined with certain halogens). Uulike anionic polymerization, termination can occur, by anion-cat-... 

Termination. As in anionic polymerization, termination by coupling or disproportionation cannot occur, leaving chain transfers as the most likely mechanisms. 

As the functionality of the (acrylic ester-diene) copolymers l8,19> is higher than two, the disproportionation mechanism is unimportant and may be neglected. That means that in the diazo-initiated polymerization, termination mostly takes place by recombination. 

The reactions that limit chain growth and initiate polymerization have not been defined. Is polymerization terminated by impurities Does solvent participate in transfer Does polymerization start by the reaction with a monomer or with impurities in the system These questions must be answered. For example, we noticed that successful formation of the monomodal high-molecular-weight polysilane requires the addition of a few drops of monomer to sonicated sodium dispersion prior to the addition of the main part of disubstituted dichlorosilane. 

For anionic polymerization, termination can occur by neutralizing the live polymer Rj to Py... 

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