Sequential polymerization

Sequential Polymerization. The sigmoidal reaction curves, which indicate a tendency for the molecular weight to increase during the course of the reaction, and other considerations led to the suggestion that the polymerizing chains had long lifetimes (9), similar to the chains in a living polymerization. If this is the case, sequential addition of different monomers would lead to block copolymer formation. To check this hypothesis, PMDS and HMDS were polymerized sequentially with suflScient time between additions for the first monomer to be consumed. In one experiment, PMDS was the first monomer, and in another experiment, HMDS was the first. In... 

Yu et al.180 have synthesized cyclic diblock copolymers of ethylene oxide and propylene oxide. They polymerized sequentially propylene oxide and ethylene oxide by using the difunctional initiator (I) shown in Scheme 89. The cyclization reaction of the a,a>-hydroxyl-ended diblock macromolecules was carried out under Williamson conditions. A solution of the triblock precursor in a mixture of dichloromethane and hexane 65 35 (v/v) was added to a stirred suspension of powdered KOH (85% w/v) in the same dichloromethane/hexane mixture (Scheme 89). After separation and evaporation of the organic phase, the cyclic diblocks were isolated by fractional precipitation. The dilute solution properties of the cyclics and the corresponding linear triblock and diblock copolymers with the same composition and total molecular weight were compared. By examining the micellar behavior in water they found that the aggregation numbers were on the order Nj < Nc < No, where N Nq, and No, are the aggregation numbers of the triblocks, cyclic, and diblocks, respectively. 

Block Type The preparation of molecular brushes with block copolymer backbones has been reported [39, 58, 90, 110-112]. These examples are mostly brush-coil block copolymers, in which one block is a cylindrical brush while the other is composed of a Hnear polymer chain. As an example, comonomers of octadecyl methacrylate (ODMA) and TMS-HEMA were polymerized sequentially via ATRP to afford a PODMA-b-P(TMS-HEMA) (PODMA = poly(octadecyl methacrylate)) diblock copolymer main chain. The poly(HEMA-TMS) block was converted into PBIEM polyinitiator, which was used for the polymerization of nBA this formed a PnBA block brush with a PODMA coil at the end of the main chain [28]. Owing to the crystalline nature of the PODMA segments, the self-assembly of the brush-coil block molecular bmshes was observed using AFM. This type of material gives rise to a new class of supersoft thermoplastic elastomers [95,113]. 

A novel route to prepare bifimctional initiating systems susceptible to anionic polymerization was recently devel-oped. For the synthesis of PMMA-PBd-PSt-PBd-PMMA, a bis(aryl halide) was metalated by a lithium/halide exchange reaction. The initiator that resulted was used to polymerize sequentially St, Bd, and MMA (Scheme 11). To ensure solubilization of the initiator in nonpolar organic media (benzene). 

The synthesis was carried out by anionic polymerization, sequential addition of monomers, and the use of 2-(chloro-methylphenyl)ethyldimethyl chlorosilane as a specific heterofunctional linking agent. The PIP, PDMS, and P2VP domains form triple coaxial cylinders with a hexagonal shape packed in a hexagonal array in the PS microphase, to form the honeycomb-shaped matrix. The potential applications of such systems include multifunctional sensors and multiselective catalysts for sequential or simultaneous chemical reactions of various kinds. 

Finally, NMRP has been used for the preparation of ABA triblocks through sequential monomer addition. Benzoyl peroxide (BPO) as the initiator, 2,2,6,6-tetramethylpiperidinoxy (TEMPO) as the nitroxide stabilizer, and camphorsulfonic acid as the accelerator were used to polymerize sequentially AcOSt, St, and again AcOSt. The obtained PAcOSt-PSt-PAcOSt can be... 

Sequence The difference in reactivity between comonomers affects the composition and also alters the placement of the monomer units along the chain. In the case of living polymerization, sequential monomer addition leads to the formation of block copolymers. However, when a random copolymer is targeted, reactivity differences can lead to nonrandom distribution of monomer units. If the incorporation of a comonomer B is intended to disrupt crystallinity of poly( A), uninteimpted sequences of A can lead to domains of crystallinity. For example, block copolymers of ethylene-propylene are highly aystaUine, while random copolymers are completely amorphous. 

Commercial interpenetrating polymer networks (not including thermoplastic compositions) include artificial teeth (Dentsply) made Ifom crosslinked PMMA mixed with MMA monomer and polymerized (sequential IPN), sound and vibration damping compositions (e.g., vinyl-phenolic Hitachi) and sheet molding compositions (acryhc/urethane/polystyrene Ferro Chemical) [164]. Water-borne acrylic methane semi-IPNs are commercial Ifom several sources where acrylate monomers are polymerized in the presence of a polymethane water dispersion. A silicone/polytetrafluoroethylene composition described as an IPN is offered by Biomed Sciences under the Silon tradename. The fluoropolymer provides the mechanical strength and the silicone rubber offers the softness and oxygen and moisture permeability for applications in the wound care area. 

A brief review has appeared covering the use of metal-free initiators in living anionic polymerizations of acrylates and a comparison with Du Font s group-transfer polymerization method (149). Tetrabutylammonium thiolates mn room temperature polymerizations to quantitative conversions yielding polymers of narrow molecular weight distributions in dipolar aprotic solvents. Block copolymers are accessible through sequential monomer additions (149—151) and interfacial polymerizations (152,153). 

The conversion of aromatic monomers relative to C-5—C-6 linear diolefins and olefins in cationic polymerizations may not be proportional to the feedblend composition, resulting in higher resin aromaticity as determined by nmr and ir measurements (43). This can be attributed to the differing reactivity ratios of aromatic and aHphatic monomers under specific Lewis acid catalysis. Intentional blocking of hydrocarbon resins into aromatic and aHphatic regions may be accomplished by sequential cationic polymerization employing multiple reactors and standard polymerization conditions (45). 

The main industrial use of alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. Decomposition of unsymmetrical diperoxyesters, in which the two peroxyester functions decompose at different rates, results in the formation of polymers of enhanced molecular weights, presumably due to chain extension by sequential initiation (204). 

In acidic solutions, equiUbtium is achieved more slowly. Polymerization of smaller species appears to occur sequentially a given polymer species first increases in size and then disappears, presumably because of its inclusion in higher order polymers. Depolymerization of siUcate species appears to be rapid, because crystalline Na2Si02 and Na2H2Si04 8H2O yield equivalent distributions of siUcate species in water upon dissolution. 

Hyperbranched polyurethanes are constmcted using phenol-blocked trifunctional monomers in combination with 4-methylbenzyl alcohol for end capping (11). Polyurethane interpenetrating polymer networks (IPNs) are mixtures of two cross-linked polymer networks, prepared by latex blending, sequential polymerization, or simultaneous polymerization. IPNs have improved mechanical properties, as weU as thermal stabiHties, compared to the single cross-linked polymers. In pseudo-IPNs, only one of the involved polymers is cross-linked. Numerous polymers are involved in the formation of polyurethane-derived IPNs (12). 

Block copolymers are synthesized by a variety of methods (45,46) most important are sequential polymeriza tion and step growth. In sequential polymerization, a polymer (A) is first synthesized in such a way that it contains at least one group per molecule that can initiate polymerization of another monomer B. 

Sequential one styrene block is polymerized, then the mid-block monomer is added and polymerized, then more styrene is added and the second styrene block polymerized. This process is used to produce 100% triblock rubbers, for maximum strength [5]. Termination is commonly with alcohols, which produces a lithium alkoxide salt as the by-product. 

Block copolymer—These copolymers are built of chemically dissimilar terminally connected segments. Block copolymers are generally prepared by sequential anionic addition or ring opening or step growth polymerization. 

Azoperoxydic initiators are particularly important due to their capacity to decompose sequentially into free radicals and to initiate the polymerization of vinylic monomers. The azo group is thermally decomposed first to initiate a vinyl monomer and to synthesize the polymeric initiator with perester groups at the ends of polymer chain (active polymer) [31,32]. 

Under certain condition, however, reactions are still preferably conducted in solution. This is the case e.g., for heterogeneous reactions and for conversions, which deliver complex product mixtures. In the latter case, further conversion of this mixture on the solid support is not desirable. In these instances, the combination of solution chemistry with polymer-assisted conversions can be an advantageous solution. Polymer-assisted synthesis in solution employs the polymer matrix either as a scavenger or for polymeric reagents. In both cases the virtues of solution phase and solid supported chemistry are ideally combined allowing for the preparation of pure products by filtration of the reactive resin. If several reactive polymers are used sequentially, multi-step syntheses can be conducted in a polymer-supported manner in solution as well. As a further advantage, many reactive polymers can be recycled for multiple use. 

Cases of addition-abstraction" polymerization have also been reported where propagation occurs by a mechanism involving sequential addition and intramolecular 1,5-hydrogen atom transfer steps (Section 4.4.3.4). 

Even in the absence of added transfer agents, all polymerizations may be complicated by transfer to initiator (Sections 3.2.10 and 3.3), solvent (Section 6.2.2.5), monomer (Section 6.2.6) or polymer (Section 6.2.7). The significance of these transfer reactions is dependent upon the particular propagating radicals involved, the reaction medium and the polymerization conditions. Thiol-ene polymerization consists of sequential chain transfer and reinitiation steps and ideally no monomer consumption by propagation (Section 7.5.3). 

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