Sequential polymerization, preparation

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). 

The additional complexity present in block copolymer synthesis is the order of monomer polymerization and/or the requirement in some cases to modify the reactivity of the propagating center during the transition from one block to the next block. This is due to the requirement that the nucleophilicity of the initiating block be equal or greater than the resulting propagating chain end of the second block. Therefore the synthesis of block copolymers by sequential polymerization generally follows the order dienes/styrenics before vinylpyridines before meth(acrylates) before oxiranes/siloxanes. As a consequence, styrene-MMA block copolymers should be prepared by initial polymerization of styrene followed by MMA, while PEO-MMA block copolymers should be prepared by... 

Sequential Oxidation at 25° C. Because of the slow polymerization rate of DPP at 25°C, most sequential polymerizations at this temperature were carried out with isolated homopolymers, separately prepared under conditions appropriate for the homopolymerization. Oxidation at 25°C of a 1 1 mixture of DPP and MPP homopolymer yielded a copolymer that did not crystallize, formed stable solutions in m-xylene, and had the NMR spectrum characteristic of a random copolymer. 

The living ROP of e-CL is usually initiated by aluminum isopropoxide, [Al(0 Pr)3] in toluene at 0-25 °C. Under these conditions this initiator exists as an aggregate of trimers and tetramers. However, freshly distilled Al(0 Pr)3 consists mainly of trimers, and is a more reactive initiator for ROP. The initiation rate is high compared to the rate of propagation so that a narrow molecular weight distribution is obtained in the polymer. There is no termination reaction and 3 chains grow per Al atom. Block polymers have been prepared by sequential polymerization of e-CL (monomer A) and DXO (monomer B) using Al(0 Pr)3 as an initiator in THF at 0 °C to yield AB or BA di-block copolymers [95]. 

The first report is available from Shen et al. who studied the preparation of BR/IR block copolymers by sequential polymerization of BD and IP [92]. Shen et al. found that the polymerization of the second monomer batch resulted in an increase of solution viscosity by 100%. The viscosity increase was considered as strong evidence in favor of block copolymer formation. Further evidence came from stress strain measurements in which the respective BD/IP block copolymers were compared with blends of BR and IR (at the same molar masses). It was found that the block copolymer exhibited higher elongation at break and higher tensile strength. Unfortunately, Mn data were not provided. Therefore, these results are not fully relevant regarding requirement No. 5 for a living polymerization. 

An interesting result with respect to applications obtained with the IPN hydrogels is that these are two- phase systems (two glass transition temperatures), with the hydrophilic domains behaving essentially like the pure hydrophilic component.6,7,9 Thus, the two basic functions of these IPN hydrogels with respect to applications, namely hydrophilicity and mechanical stability, are separately taken over by the two IPN components, the hydrophilic and hydrophobic domains, respectively. Figure 1 shows TSDC and DMA results for the water content dependence of the a relaxation (dynamic glass transition) of PHEA in sequential IPNS prepared from PHEA and poly(ethyl methacrylate) (PEMA) as the hydrophobic component.9 In these IPNs a porous PEMA network was prepared first, and PHEA was then polymerized in the pores. In addition to the... 

Although alkoxyamines were mainly used, a number of other initiators was studied [57, 58, 75-77], Druliner [57] describes a series of electron-transfer initiators able to generate long-lived oxygen-centered radicals such as 10 which are associated with the growing end of the acrylate and methacrylate chains. Nearly pure block copolymers are prepared from sequential polymerization of MA, BuA and MM A ... 

Back-biting reaction occurring during cationic polymerization of lactams is detrimental to preparation of block copolymers of two different lactams by sequential polymerization. Block copolymers can be obtained only in those systems in which the rate of polymerization of the second monomer is much higher than the rate of chain transfer to polymer resulting in transamidation [219]. 

The statistical anionic copolymerization of acrylates and methacrylates is also controlled in the presence of LiOEEM (30), as testified by the copolymerization of MMA and tBuA in THF at —78°C. Block copolymers were also prepared by the sequential polymerization of at least two methacrylates and acrylates. For instance, PMMA- >/c cA -PbBuA and PMMA-fcZocA -PnNonA were synthesized . The addition order of the comonomers is important. Indeed, when living PnBuA is the macroinitiator of the MMA polymerization, the expected block copolymer is contaminated by homo-PnBuA, which is not the case when the polymerization sequence is reversed. A fuUy acrylic-based thermoplastic elastomer, PMMA-fcZocA -P(2EtHA)-fcZocA -PMMA, was prepared by the sequential LiOEEM-ligated polymerization of MMA, 2-EtHA and MMA. ... 

Sequential polymerization of two acrylic monomers was discussed in Sections VI.E.4, VI.E.7 and VI.F. This technique was extended to three comonomers with formation of ABC-type triblock copolymers. For example, PMMA-fcZoc -PtBMA- Zoc -PMMA triblock copolymer was synthesized by the sequential DPFlLI-initiated polymerization of MMA, tBMA and MMA, respectively . Symmetrical BAB-type diblock copolymers were also prepared in two steps, polymerization of the A monomer being first initiated by a difunctional compound. The B blocks formed in the second step necessarily have the same average degree of polymerization. 

Various block copolymers with functional groups can be prepared by direct block copolymerization of functional monomers or by sequential polymerizations of their protected forms, followed by deprotection (Figure 18). 

Ring-opening metathesis polymerization was also used recently for the preparation of amphiphilic star-block copolymers [25]. Mo (CH-f-Bu) (NAr) (0-f-Bu)2 was used as the initiator for sequential polymerization of norbornene-type, unfunctionalized and functionalized, monomers. The living diblocks were reacted with endo-ris-endo-hexacyclo- [ 10.2.1.1.3/115>8.02>l 1. O 1-9] heptadeca-6,13-diene, a difunctional monomer in a scheme analogous to the use of DVB in anionic polymerization, to form the central core of the star (Scheme 10). 

Anionic Methods. Of the various methods used for nonpolar block polymer synthesis, the anionic approach is by far the most useful and most precise in producing a given segmented chain design [A-B-A, (A-B)j, A-B-C, etc.]. The linear A-B-A block polymer may be synthesized by two basic routes, sequential polymerization of all three blocks or sequential polymerization of two blocks, followed by coupling. The "sequential" method is illustrated in Reaction la for the preparation of S-B-S the "coupling" process is depicted in Reaction lb. 

This polymer was synthesized via NMRP (Nitroxide Mediated Radical Polymerization) (Benoit et al. 1999) by sequential polymerization of 2VP and a mixture of NIPAAm and DMIAAm. Using the macroinitiator method, the preparation of well-defined linear block copolymers consisting of a homo polymer block P2VP (pH-sensitive) and a random copolymer block of PNlPAAm (temperamre sensitive) with DMIAAm (photo crosslinker) was possible. 

Problems related to the reactivity of oxetanes in copolymerization are discussed in Sect. 15 (Copolymerization). This paragraph briefly describes the preparation of block copolymers of BCMO by sequential polymerization. 

The synthesis of block copolymers by sequential polymerization requires a living polymer prepared from the less nucleophilic monomer (first block) and the addition of a more nucleophilic monomer to the active species located on this first block. The general order of nucleophilicities of heterocyclic monomers is as follows Siloxanes orthoesters < acetals < ethers < sulfides < oxazolines < amines. Depending on substitution and ring strain some changes may occur in these positions. 

Another pyrrolidone-based phosphine has been incorporated into amphiphilic, water-soluble diblock co-polymers based on 2-oxazalone derivatives (Scheme 61). The synthesis involved the initial preparation of a diblock co-polymer precursor with ester functionalities in the side chain. This was achieved by sequential polymerization of 2-methyl-2-oxazoline to form the hydrophilic block that provides water solubility, and subsequently a mixture of ester-functionalized oxazoline 147 and 2-nonyl-2-oxazoline, the latter increasing the hydrophobicity of the second polymer block. Having made the backbone, the ester functionalities were converted into carboxylic acids giving polymer 148, which was then reacted with the phosphine ligand to give the desired supported material, 149. This was used in asymmetric hydrogenation reactions with success. 

In this two-stage process, B is sequentially polymerized onto A, and then the two chains are coupled to yield an ABBA block copolymer. Triblocks of SBS have been prepared using this method, with methylene dichloride as the coupling agent. The disadvantage is the formation of radical anions, which can lead to contamination of the triblock with multiblock species. 

Moreover, the reactivation of a cobalt-terminated polymer in the presence of second monomer leads to block copolymerization. In this respect, CMRP has aheady contributed to the preparation of the valuable copolymers listed in Table 4.1. For example, well-defined poly(acrylate) block copolymers were prepared via a sequential polymerization of acrylic monomers with cobalt porphyrin la or cobaloximes 2 [14, 20]. The synthesis of well-defined poly (acrylate)-b-poly(VAc) block copolymers was also achieved with complex la [26]. Co(acac)2 (3a see Figure 4.1) is the most prolific complex for the preparation of block copolymers, until now. Indeed, the sequential CMRP of VAc with NVP [33], AN [48], or vinyl pivalate (VPi) [49] leads to the corresponding block copolymers, in controlled fashion. Throughout the polymerization, the experimental conditions were necessarily adjusted, taking into consideration the reactivity of the second monomer. As an illustration of this, well-defined PVAc-b-poly(acrylonitrile) (PAN) copolymers could only be prepared via a bulk polymerization of VAc at 30 °C, followed by the AN polymerization at 0°C in solution in DMF [48]. In this case, the DMF not only serves as the solvent but also binds the metal and adjusts its reactivity. As a rule, the PVAc sequences of these copolymers were hydrolyzed in order to provide poly(vinyl alcohol) (PVA)-containing derivatives, such as hydrosoluble PVA-b-poly... 

Block-Type Brushes by Sequential Polymerization The sequential hving polymerization of two macromonomers or a macromonomer with a conventional comonomer forms either block-block- or block-coil-type brush structures. For example, giant rod-coil amphiphilic block copolymer bmshes were prepared via a stepwise metallocene-catalyzed polymerization [58]. In the first step, a concentrated solution of methacryloyl end-functionalized PS macromonomer (DP = 18.3, MWD = 1.05) was polymerized by the organosamarium(iii) catalyst in THF. After PS macromonomer was completely consumed, the active center remained living, and tert-butyl methacrylate tert-butyl methacrylate (tBMA) as a comonomer was added to grow the second block. After termination by ethanol, the poly(tert-butyl methacrylate) (PtBMA) coil block was hydrolyzed into a hydrophihc block, poly(methacrylic acid) (PMAA). The final product consisted of a hydrophobic PS brush block and a hydrophihc PMAA coil. The hydrophilic PMAA coil collapsed in nonpolar solvents, which forced the block-coil CPBs to self-assembled into giant micelles with PMAA as the core component and the stiff PS brush block as the shell to stabihze the micelles. 

The ABA triblocks which have been most exploited commercially are of the styrene-diene-styrene type, prepared by sequential polymerization initiated by alkyllithium compounds as shown in Eqs. (99-101) [263, 286]. The behavior of these block copolymers illustrates the special characteristics of block (and graft) copolymers, which are based on the general incompatibility of the different blocks [287]. Thus for a typical thermoplastic elastomer (263), the polystyrene end blocks (-15,000-20,000 MW) aggregate into a separate phase, which forms a microdispersion within the matrix composed of the polydiene chains (50,000-70,000 MW) [288-290]. A schematic representation of this morphology is shown in Fig. 3. This phase separation, which occurs in the melt (or swollen) state, results, at ambient temperatures, in a network of... 

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