Block polymers, formation

One of the earliest examples of this methodology involves the reaction of a polymeric anion (formed by living anionic polymerization) with molecular oxygen to form a polymeric hydroperoxide which can be decomposed either thermally or, preferably, in a redox reaction to initiate block polymer formation with a second monomer (Scheme 7.25). However, the usual complications associated with initiation by hydroperoxides apply (Section 3.3.2.5). 

In situ block polymer formation is obtained by using a mixture of water-soluble and oil-soluble monomers. An example involves dissolution of acrylic acid or methacrylic acid in water followed by... 

The obvious, but experimentally very difficult route to block polymer formation by charge neutralization between living anionic and cationic polymers was successful for polystyrene and poly(THF). ... 

There is some evidence that the molecular iodine initiated polymerization of vinyl ethers in chlorinated solvents at low temperature contain dormant chain ends which are capable of block polymer formation by sequential monomer addition e.g. with />-methoxystyrene. 

Another type of adventitious block polymer formation occurs in the partial inter-randomization in the melt of mixtures of homopolymers such as polyesters and polyamides which can undergo ester-ester, amide-amide or ester-amide interchange reactions. 

In addition to graft copolymer attached to the mbber particle surface, the formation of styrene—acrylonitrile copolymer occluded within the mbber particle may occur. The mechanism and extent of occluded polymer formation depends on the manufacturing process. The factors affecting occlusion formation in bulk (77) and emulsion processes (78) have been described. The use of block copolymers of styrene and butadiene in bulk systems can control particle size and give rise to unusual particle morphologies (eg, coil, rod, capsule, cellular) (77). 

Anionic polymerization, if carried out properly, can be truly a living polymerization (160). Addition of a second monomer to polystyryl anion results in the formation of a block polymer with no detectable free PS. This technique is of considerable importance in the commercial preparation of styrene—butadiene block copolymers, which are used either alone or blended with PS as thermoplastics. 

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. 

Formation of block polymers is not limited to hydrocarbon monomers only. For example, living polystyrene initiates polymerization of methyl methacrylate and a block polymer of polystyrene and of polymethyl methacrylate results.34 However, methyl methacrylate represents a class of monomers which may be named a suicide monomer. Its polymerization can be initiated by carbanions or by an electron transfer process, the propagation reaction is rapid but eventually termination takes place. Presumably, the reactive carbanion interacts with the methyl group of the ester according to the following reaction... 

Formation of living polymers is not restricted to norbornene. For example, Grubbs successfully polymerized cyclooctatetraene to polyacetylene, and demonstrated the living nature of this polymer by forming block polymers with cyclooctadiene 19). 

The formation of IBVE-aMeSt block polymers was further supported by results of film-casting experiments (12). These data also show that the starting quasiliving poly(IBVE) dication is sufficiently reactive to initiate effectively subsequent aMeSt polymerization. 

Similarly, blocking MVE from quasiliving poly(IBVE) dications was accomplished. The products were fractionated with a heterogeneous mixture of n-heptane and water (5/2, v/v) the former is a good solvent for poly(IBVE) only, the latter a good solvent for poly(MVE) only. The 1H NMR spectra of the n-heptane-soluble fractions exhibited a sharp resonance at 6 3.3 ppm (-OCH3), characteristic of MVE units, and a doublet at 6 0.9 ppm (—i (CH3)2), characteristic of IBVE units. The presence of poly(MVE) segments in these fractions indicates the formation of IBVE-MVE block polymers. 

Block copolymers at high styrene contents behave similarly, with no break around the micellar region. Two of the block copolymers are shown separately in Figure 7c. The low M.W. BC 90 moves from an apparently adequate stabilization in CCI4 to a new level of modest protection at higher CyH g volume fractions. The block polymer of 42% styrene gives a hint of a discontinuity at the non-solvent content for micelle formation, but thereafter stabilizes the silica until the conditions approach those for phase separation. 

Thanks to its versatility, the general approach described hereabove gives a good grip of the synthesis of many different families of block polymers. However, in several cases where the formation mechanisms of the two types of chains are not amenable to this quantitative active end-group conversion technique, a still more general answer is needed. 

Ewen was the first to report the synthesis of stereoregular propene polymers with soluble Group 4 metal complexes and alumoxane as the co-catalyst [13], He found that Cp2TiPh2 with alumoxane and propene gives isotactic polypropene. This catalyst does not contain an asymmetric site that would be able to control the stereoregularity. A stereo-block-polymer is obtained, see Figure 10.6. Formation of this sequence of regular blocks is taken as a proof for the chain-end control mechanism. 

Hunter RL, Bennett B (1984) The adjuvant activity of nonionic block polymer surfactants. II. Antibody formation and inflammation related to the structure of the triblock and octablock copolymer. J Immunol 133 3167-3175... 

Copolymers with only short acrolein blocks can by synthesized. In these conditions, the formation of homopolyacrolelns can be avoided (differences between the PAI3 and the chromatograms Figures 1 and 7). In fact, it would better to define this approach as functionalization rather than block polymer synthesis. 

VVThen two chemically different polymers are mixed, the usual result is a two-phase polyblend. This is true also when the compositional moities are part of the same polymer chain such as, for instance, in a block polymer. The criterion for the formation of a single phase is a negative free energy of mixing, but this condition is rarely realized because the small entropy of mixing is usually insufficient to overcome the positive enthalpy of mixing. The incompatibility of polymers in blends has important effects on their physical properties, which may be desirable or not, depending on the contemplated application. 

Transformation of epoxy resins, which are viscous liquids or thermoplastic solids, into network polymers is a result of interaction with alkali or acid substances by means of to polyaddition and ionic polymerization mechanisms.10 A resin solidified by to the polyaddition mechanism, is a block copolymer consisting of alternating blocks of resin and a hardener or curing agent. A resin solidified by the ionic mechanism is a homopolymer. Molecules of both resin and hardener contain more than one active group. That is why block copolymer formation is a result of multiple reactions between an epoxy resin and a curing agent.11... 

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