Multifunctional initiation

Polymeric azo-compounds and multifunctional initiators with azo-linkages are discussed elsewhere (see 3.3.3 and 7.6.1) as are azo compounds, which find use as iniferters (see 9.3.4). 

Multifunctional initiators contain two or more radical generating functions within the one molecule. They can be considered in two distinct classes according... 

The use of initiators such as 68 has been promoted for achieving higher molecular weights or higher conversions in conventional polymerization and for the production of block and graft copolymers. The use and applications of multifunctional initiators in the synthesis of block and graft copolymers is briefly described in Section 7.6.1. 

Knowledge of kui/kii is also important in designing polymer syntheses. For example, in the preparation of block copolymers using polymeric or multifunctional initiators (Section 7.6.1), ABA or AB blocks may be formed depending on whether termination involves combination or disproportionation respectively. The relative importance of combination and disproportionation is also important in the analysts of polymerization kinetics and, in particular, in the derivation of rate parameters. 

Multifunctional initiators contain two or more radical generating functions within the one molecule. The chemistry of these initiators has been the subject of several reviews.252 25 As long as the radical generating functions are sufficiently remote their decompositions are independent events. If decomposition occurs... 

The multifunctional initiators may be di- and tri-, azo- or peroxy-compounds of defined structure (c.g. 20256) or they may be polymeric azo- or peroxy-compounds where the radical generating functions may be present as side chains 57 or as part of the polymer backbone."58"261 Thus, amphiphilic block copolymers were synthesized using the polymeric initiator 21 formed from the reaction between an a,to-diol and AIBN (Scheme 7.22).26 Some further examples of multifunctional initiators were mentioned in Section 3.3.3.2. It is also possible to produce less well-defined multifunctional initiators containing peroxide functionality from a polymer substrate by autoxidalion or by ozonolysis.-0... 

The success of the multifunctional initiators in the preparation of block and graft copolymers depends critically on the kinetics and mechanism of radical production. In particular, the initiator efficiency, the susceptibility to and mechanism of transfer to initiator, and the relative stability of the various radical generating functions. Each of these factors has a substantial influence on the nature and homogeneity of the polymer formed. Features of the kinetics of polymerizations initiated by multifunctional initiators have been modeled by O Driscoll and Bevington 64 and Choi and Lei.265... 

A final class of multifunctional initiators is based on the use a (muUi)functional polymer and a low molecular weight redox agent. Radicals on the polymer chain arc generated from the polymer bound functionality by a redox reaction. Ideally, no free initiating species are formed. The best known of this class are the polyol-redox and related systems. Polymers containing hydroxy or glycol and related functionality are subject to one electron oxidation by species such as ceric ions or periodate (Scheme 7.23).266,267 Substrates such as cellulose,... 

The use of mono-, di- and multifunctional initiators provides scope for designing polymer architectures. The use of 14, 18 and 19 in the production of block or star polymers has been demonstrated.41 4445 Homopolymers of 20 or copolymers of 20 with S or MMA have been successfully used in photoinitiated... 

Puskas, J.E., Pattern, W.E., Wetmore, P.M., and Krukonis, A. Multiarm-star polyisobutylene-polystyrene thermoplastic elastomers from a novel multifunctional initiator, Polym. Mater. Set Eng., 82,42 3, 1999. Brister, L.B., Puskas, J.E., and Tzaras, E. Star-branched PIB/poly(p-t-bu-Styrene) block copolymers from a novel epoxide initiator, Polym. Prepr., 40, 141-142, 1999. 

There have been a few approaches to calculate the effect of cyclization on the MWD [75-77]. Since each cyclization event generates a new multifunctional initiator, it becomes clear that cyclization is expected to narrow the MWD. 

For SCVCP, the PDI is decreased in proportion to the comonomer ratio, y=[M]o/[I]o M, /Mn=l-I- /(/+ ) for y l [73]. The addition of a multifunctional initiator again affects the polydispersity index [72]. In the batch process it decreases with initiator functionality as M /Mn Pn/(y+l)/, similar to homo-SCVP. The effect is even more pronounced for the semi-batch process where the concentration of the inimer and the comonomer is kept infinitesimally low and M, /Mn=l-i-l//. This result is identical to the value obtained in homo-SCVP,that is, addition of comonomer does not decrease polydispersity any further. 

Hexaepoxy squalene, HES (Scheme 70) was used as a multifunctional initiator in the presence of TiCU as a coinitiator, di-f-butylpyridine as a proton trap, and N,N-dimethylacetamide as an electron pair donor in methylcy-clohexane/methyl chloride solvent mixtures at - 80 °C for the synthesis of (PIB-fc-PS)n star-block copolymers [145]. IB was polymerized first followed by the addition of styrene. The efficiency and the functionality of the initiator were greatly influenced by both the HES/IB ratio and the concentration ofTiCL, thus indicating that all epoxy initiation sites were not equivalent for polymerization. Depending on the reaction conditions stars with 3 to 10 arms were synthesized. The molecular weight distribution of the initial PIB stars was fairly narrow (Mw/Mn < 1.2), but it was sufficiently increased after the polymerization of styrene (1.32 < Mw/Mn < 1.88). 

Benzenetricarbonyl trichloride and l,2,4,5-tetrakis(bromomethyl) benzene were employed as multifunctional initiators for the synthesis of 3-and 4-arm PTHF stars, respectively [147]. The living ends were reacted with sodium 2-bromoisobutyrate followed by reduction with Sml2. The samarium enolates, thus formed were efficient initiators for the polymerization of MMA to give the (PTHF-fo-PMMA) , n = 3 or 4 star-block copolymers, according to Scheme 71. 

A combination of anionic and ATRP was employed for the synthesis of (PEO-b-PS) , n = 3, 4 star-block copolymers [148]. 2-Hydroxymethyl-l,3-propanediol was used as the initiator for the synthesis of the 3-arm PEO star. The hydroxyl functions were activated by diphenylmethyl potassium, DPMK in DMSO as the solvent. Only 20% of the stoichiometric quantity of DPMK was used to prevent a very fast polymerization of EO. Employing pentaerythritol as the multifunctional initiator a 4-arm PEO star was obtained. Well-defined products were provided in both cases. The hydroxyl end groups of the star polymers were activated with D PM K and reacted with an excess of 2-bromopropionylbro-mide at room temperature. Using these 2-bromopropionate-ended PEO stars in the presence of CuBr/bpy the ATRP of styrene was conducted in bulk at 100 °C, leading to the synthesis of the star-block copolymers with relatively narrow molecular weight distributions (Scheme 72). 

Multifunctional fibers, 13 391 Multifunctional initiators, 24 706 Multifunctional processes, in life cycle assessment, 14 813-814 Multifunctional teams, in the development process, 24 348... 

The core first method starts from multifunctional initiators and simultaneously grows all the polymer arms from the central core. The method is not useful in the preparation of model star polymers by anionic polymerization. This is due to the difficulties in preparing pure multifunctional organometallic compounds and because of their limited solubility. Nevertheless, considerable effort has been expended in the preparation of controlled divinyl- and diisopropenylbenzene living cores for anionic initiation. The core first method has recently been used successfully in both cationic and living radical polymerization reactions. Also, multiple initiation sites can be easily created along linear and branched polymers, where site isolation avoids many problems. 

Multifunctional initiators are found to be more effective in carbocationic than in carbanionic polymerization, because of the enhanced solubility of the less polar dormant initiating complexes. For example, the formation of a six-arm star polystyrene starts from... 

The metal-centered complexes can also be used as multifunctional initiators. For example, Fe2+(4,4 dichloromethyl-2,2 -bipyridine)3 or the Ru2+ complex have been used as initiators for the living cationic polymerization of 2-ethyl-2-oxazoline [120],... 

Interestingly, the Fe2+ ion in the core can be easily removed by base, the complex dissociates and the individual polymer dimers can be analyzed. Block copolymers of 2-ethyl-2-oxazoline with other substituted oxazolines have also been made [121]. Ru2+(4,4 dichloromethyl-2,2 bipyridine)3 has also been used as the multifunctional initiator for the ATRP of styrene at 110°C [122], It is interesting to note that the Cu+ ions necessary for the polymerization reaction are solubilized via complexation with other bipyridine species. 

Linear-dendritic star copolymers [5], most frequently obtained via processes in which dendrimers function as multifunctional initiator cores for the poly-... 

Fig. 27. Molar mass dependence of [rj] for a fractionated comb macromolecule. The fractionation was made with a SEC/LALLS/VISC set-up. The comb macromolecule consists oi a polyimidazole backbone prepared by free radical polymerization. The imidazol side groups acted in a melt with phenylglycidylether and phthalic anhydride as multifunctional initiator for the anionic growth of polyester chains. The straight lines correspond to the behavior of unattached polyester chains and the comb polymers at low and high molar masses respectively [136]...

The core-first approach is based on the initiation of polymerization by a multifunctional initiator. The number of arms is then defined by the number of functional units present on the core. In order to have a good control of the molecular structure of star-shaped polyesters, the initiation must be quantitative and fast. It is also mandatory to avoid possible side-reactions between the initiating species on the core. 

Multifunctional initiators made up of metal alkoxides rather than alcohols have been less used for the synthesis of star-shaped polyesters than have the tin (II) bis-(2-ethylhexanoate)/alcohol system. Nevertheless, Kricheldorf initiated the polymerization of sCL using a spiro-cyclic tin(IV) aUcoxide to obtain a tin-containing height-shaped polyester whose final hydrolysis resulted in the formation of a star-shaped polyester (Fig. 35) [25, 159-161]. 

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