Copolymers by sequential addition of monomers

Some of the drawbacks of the metallocene catalysts are their limited temperature stability and the production of lower-molecular-weight materials under commercial application conditions. It follows that they have a limited possibility for comonomer incorporation due to termination and chain-transfer reactions prohibiting the synthesis of block copolymers by sequential addition of monomers. This led to the development of half-sandwich or constrained geometry complexes, such as ansa-monocyclopentadienylamido Group IV complexes (67) 575,576... 

B. Block Copolymers by Sequential Addition of Monomers to Living Systems... 

In the formation of block copolymers by sequential addition of monomers it generally does not matter which monomer is polymerized first, and diblock or multiblock copolymers of narrow MWD and of any desired sequence length are readily prepared. Termination is usually effected by reaction of the living ends with aldehydes ketones can be used for terminating titanacyclobutane ends, while unsaturated ethers are used for terminating ruthenium carbene complexes. 

The living polymer technique is particularly suitable for preparing block copolymers by sequential addition of monomers to a living anionic polymerization system. Depending on whether monofunctional or difunctional initiators are used, one or both chain ends remain active after monomer A has completely reacted. Monomer B is then added, and its polymerization is initiated by the living polymeric carbanion of polymer A. This method of sequential monomer addition can be used to produce block copolymers of several different types, as classified above. 

Block copolymers by sequential addition of monomers to living systems... 

Block Copolymer Synthesis by Three-Step Sequential Monomer Addition The preparation of block copolymers by sequential addition of monomers using living anionic polymerization and a monofunctional initiator is the most direct method for preparing well-defined block copolymers. Detailed laboratory procedures for anionic synthesis of block copolymers are available [37, 230], Several important aspects of these syntheses can be illustrated by considering the preparation of an important class of block copolymers (Scheme 7.22), the polystyrene-fe-polydiene-( -polystyrene triblock copolymers. 

Block copolymers are closer to blends of homopolymers in properties, but without the latter s tendency to undergo phase separation. As a matter of fact, diblock copolymers can be used as surfactants to bind immiscible homopolymer blends together and thus improve their mechanical properties. Block copolymers are generally prepared by sequential addition of monomers to living polymers, rather than by depending on the improbable rjr2 > 1 criterion in monomers. 

ABA triblock copolymers (as well as multiblock copolymers) cannot be made by sequential addition of monomer A, monomer B, and again monomer A, except in the few cases in which the electron affinity of both monomers is almost the same 27,28, 118>. There are however two ways to circumvent this difficulty ... 

The living character of the ROMP promoted by the initiator Ru(CHPh)(Cl)2 (PCy3)2 (Cy = cyclohexane) was tested with the synthesis of diblock, triblock, and tetrablock copolymers of norbornene derivatives carrying acetyl-protected glucose, [2,3,4,6-tetra-O-acetyl-glucos-l-O-yl 5-norbornene-2-carboxylate], A or maltose groups, [2,3,6,2/,3/,4/,6/-hepta-0-acetyl-maltos-1-O-yl 5-norbornene-2-carboxylate], B, shown in Scheme 41 [102]. The AB, ABA, and ABAB structures were prepared by sequential addition of monomers with narrow molecular weight distributions to quantitative conversions. 

Anionic polymerization and suitable Unking chemistry were employed for the synthesis of 3-arm PCHD-fc-PS star-block copolymers with PCHD either as the inner or the outer block (Scheme 77) [153]. The block copolymers were prepared by sequential addition of monomers. It was shown that the crossover reaction of either PSIi or PCHDLi was efficient and led to well-defined block copolymers. However, in the case of the PCHD-fc-PSLi copolymers, longer polymerization times were needed for long PCHD... 

By also using the LASIP procedure, grafted PS-b-PI and PBd-b-PS block copolymers have been prepared (Fig. 7) [72]. Using silane and thiol-DPE initiators, polymerization was carried out on the SiOx and Au surface by sequential addition of monomers. Typically, after allowing this first reaction to reach completion, the second monomer was added to the living anion, and polymerization of the second block was allowed to proceed. The polymerization was also investigated by SPS [80], AFM, ellipsometry, FT-IR, and XPS. The schematic diagram for the reaction on Au surfaces and the formation of the block copolymers is shown in Fig. 6. The results are summarized in Table 2. 

Figure 11. DSC diagrams of totally hydrogenated derivatives of copolymers diblock copolymer, 9-1CW-29, by sequential addition of monomers with 49/51 wt % of PB/P1 (I) random copolymer, 9-1CW-88, by direct addition of monomers with 48.8/51.2 wt % of PB/P1 (2).

When the propagating species are long-lived it is possible to make block copolymers of controlled chain length by sequential addition of monomers, for example with ClC=CC4H9/ClC=CCi4H29686 693 and with acetylene/norbornene396,628. 

Block copolymers can be prepared by sequential addition of monomers. This is a special case of (a) above. 

Table 14.1 Block copolymers made from norbomene (Mi) by sequential addition of monomers... 

As mentioned earlier, the living nature of the growing chain in anionic polymerization makes this mechanism especially suitable for the synthesis of block copolymers, by sequential addition of different monomers. Since such copolymers have markedly different properties than simple copolymers, they will be discussed separately (in Section 2.10). 

For the synthesis of end-functionalized block copolymers with one dimethyl-amino end group 3-(Dimethylamino)propyl lithium (DMAPli) was used as initiator, in a nonpolar solvent (i.e., benzene). The diblocks were prepared by sequential addition of monomers starting with the block to which the end... 

It is, of course, the anionic mechanism which is most suitable for the synthesis of block copolymers, since many of these systems are of the living polymer type, as described previously [144,150,165,194,262,263]. Thus it is possible to use organoalkah initiators to prepare block copolymers in homogeneous solution by sequential addition of monomers, where each block has a prescribed molecular weight, based on monomer-initiator stoichiometry, as well as a very narrow molecular weight distribution (Poisson) [185]. As would be expected, such block copolymers are very pure, due to the absence of any side reactions during the polymerization (e.g., termination, monomer transfer, branching). 

Formation of living polymer chains such as synthesis of true block copolymers by sequential addition of different monomers. 

The synthesis of the poly[styrene-6-(hydroxystyrene-g-ethylene oxide)-6-styrene] (25), P[S-6-(HS-g-EO)-6-PS], has been reported. The backbone was a triblock copolymer, poly(styrene-6-t-butoxystyrene-6-styrene) (8), prepared by anionic polymerization by sequential addition of monomers. The protected t-butyl group was removed by treatment with HBr leading to the formation of P(S-6-HS-b-S) triblocks (9) (eq. 11). The metallation of the hydroxyl groups was performed in THF using either cumyl potassium or diphenylethylene potassium (eq. 12). The addition of EO generated the block graft copolymers (eq. 13). 

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