Copolymers synthesis reactions

In a third type of block copolymer formation. Scheme (3), the initiator s azo group is decomposed in the presence of monomer A in a first step. The polymer formed contains active sites different from azo functions. These sites may, after a necessary activation step, start the polymerization of the second monomer B. Actually, route (3) of block copolymer formation is a vice versa version of type (1). It has been shown in a number of examples that one starting bifunctional azo compound can be used for block copolymer synthesis following either path. Reactions of type (3) are tackled in detail in Section III of this chapter. 

The reactions of polymeric anions with appropriate azo-compounds or peroxides to form polymeric initiators provide other examples of anion-radical transformation (e.g. Scheme 7. 6). ""7i However, the polymeric azo and peroxy compounds have limited utility in block copolymer synthesis because of the poor efficiency of radical generation from the polymeric initiators (7.5.1). 

Another consequence of the absence of sponataneous transfer and termination reactions is that the polymer chains formed remain living 3), i.e. they carry at the chain end a metal-organic site able to give further reactions. Block copolymer synthesis is probably the major application 12 14), but the preparation of co-functional polymers, some chain extension processes, and the grafting onto reactions arise also directly from the long life time of the active sites. 

It should be emphasized that for Markovian copolymers a knowledge of the values of structural parameters of such a kind will suffice to find the probability of any sequence Uk, i.e. for an exhaustive description of the microstructure of the chains of these copolymers with a given average composition. As for the composition distribution of Markovian copolymers, this obeys for any fraction of Z-mers the Gaussian formula whose covariance matrix elements are Dap/l where Dap depend solely on the values of structural parameters [2]. The calculation of their dependence on time, and the stoichiometric and kinetic parameters of the reaction system permits a complete statistical description of the chemical structure of Markovian copolymers to be accomplished. The above reasoning reveals to which extent the mathematical modeling of the processes of the copolymer synthesis is easier to perform provided the alternation of units in macromolecules is known to obey Markovian statistics. 

Ruthenium-catalyzed ATRP was employed in the synthesis of PMMA-fr-PnBuMA block copolymers. Subsequent reaction with the divinyl compound 1 (Scheme 82) resulted in the synthesis of the star-block structures in almost quantitative yield [157]. The divinyl compound 2 was also employed for the Unking of PnBuMA-fr-PMMA through the PMMA blocks. Narrow molecular weight distribution products were obtained in all cases. 

Various block copolymers have been synthesized by cationic living polymerization [Kennedy and Ivan, 1992 Kennedy, 1999 Kennedy and Marechal, 1982 Puskas et al., 2001 Sawamoto, 1991, 1996]. AB and ABA block copolymers, where A and B are different vinyl ethers, have been synthesized using HI with either I2 or Znl2. Sequencing is not a problem unless one of the vinyl ethers has a substituent that makes its reactivity quite different. Styrene-methyl vinyl ether block copolymer synthesis requires a specific sequencing and manipulation of the reaction conditions because styrene is less reactive than methyl vinyl ether (MVE) [Ohmura et al., 1994]. Both monomers are polymerized by HCl/SnCLj in the presence of (n-CrikjtiNCI in methylene chloride, but different temperatures are needed. The... 

Initial reports on chemoenzymatic block copolymer synthesis focus on the enzymatic macroinitiation from chemically obtained hydroxy-functional polymers (route A in Fig. 4). The first report on enzymatic macroinitiation was published by Kumar et ah, who used anionically synthesized hydroxy-functional polybutadiene of various molecular weights ranging from 2600 to 19,000Da (Fig. 5) [16]. In a systematic study, the authors investigated the efficiency of the macroinitiation of CL and PDF by Novozym 435 as a function of the polybutadiene macroinitiator. The reaction profile showed that polybutadiene consumption steadily increased with the reaction... 

Fig. 6 Left Strategy for consecutive chemoenzymatic and simultaneous one-pot block copolymer synthesis combining enzymatic ROP and ATRP. Right Influence of ATRP-catalyst system on the conversion of CL in the enzymatic ROP of MMA at 60 °C using ATRP-3 as initiator filled squares reaction in absence of ATRP-catalyst open circles CuBr/PMDETA (1 1 1 ratio with respect to initiator) jiWed triangles CuBr/dNbpy (1 2.1 1 ratio with respect to initiator) open inverted triangles CuBr (1 1 ratio with respect to initiator) yiHed diamonds CuBr2 (1 1 ratio to initiator). CL conversion was determined with H-NMR [26]...

In addition to block copolymer synthesis by subsequent polymer growth along one polymer chain, Eq. (21), and the reaction sequence of Eqs. (19, 22—24), preformed polymer blocks can be linked via reactive end groups. Polynorbomene with one titanacyclobutane end group was reacted in a Wittig-type reaction with... 

Amine functions readily react with oxonium sites. This reaction was used for the synthesis of graft copolymers involving reaction of living poly-THF with either poly(p-vinylpyridine) or poly(p-dimethylaminostyrene) S6K Attempts to synthesize macromonomers by a similar route were made57 but their characterization is difficult because they contain quaternary ammonium sites ... 

Another consequence of living polymerization systems is that they can be used to synthesize block copolymers. Under these conditions, once the initial quantity of monomer in a given reaction is consumed, the resultant polymer chains are terminated with metal carbene end groups that are still active for aUcene metathesis. As long as these carbenes do not react rapidly with the acyclic aUcenes in the polymer chain, addition of a second monomer will result in the synthesis of a block copolymer. This reaction is illustrated in equation (13) for the synthesis of a polymer that consists of block of x repeat units of monomer A followed by a block of y repeat units of monomer B. 

In addition to a better understanding of the effect of reaction variables on products, rates and molecular weights, this work has also led to new mechanistic concepts such as termination by hydridation and to the optimization of cationic graft-copolymer synthesis. 

Propyl trimethoxysilane 4a exhibits a much lower reaction rate than that of trimethoxy vinyl silane 4b. This is the result of a reduced electron density at the silicon atom in 4a compared to 4b bearing the vinyl groiq). In contrast, the high reactivity in die case of 5, the saturated counterpart of 3a, is not only retained, but enhanced. This can be explained by a more effective electron transfer from the ester moiety to silicon in the case of 5 compared to 3a due to the missing electron delocalization. The high hydrolysis speed found for compound 5 indicates that the high reactivity of 3a can be transferred into a polymer in the case of a copolymer synthesis. 

Radical Vinyl Polymerization. Conventional radical polymerization and telomerization can also be beneficial for block copolymer synthesis, because in some cases they polymerize monomers inactive for metal catalysts, although side reactions often render the block copolymers in low yield and with ill control of molecular weights. However, a combination with conventional radical polymerizations affords novel block copolymers in higher yields than before (Figure 21). 

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