Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Stereo-controlled polymerization

More recently, a number of different copolymer structures have been prepared from butadiene and styrene, using modified organolithiums as polymerization initiators ( 4). Organolithium initiated polymerizations have gained prominence because stereo-control is combined with excellent polymerization rates, and the absence of a chain termination reaction facilitates control of molecular weights and molecular weight distributions ( 5). [Pg.74]

The chain-end stereocontrol for olefin polymerizations leads generally to lower stereoselectivities (differences in activation energy for insertion of the two enantiofaces generally lower than 2 kcal/mol) than the chiral site stereo-control.18131132 For this reason, the corresponding catalytic systems have not reached industrial relevance for propene homopolymerization. However, some of them are widely used for propene copolymerization with ethene. [Pg.48]

From this equation, it is clear that concentration of the solvent, S, influences a number of sites on the template which are occupied by the monomer, M. As the result of monomer units association with the template, the orientation of the substrate takes place and some special type of structure can be created. The structures, in which the monomer is aligned in a regular manner on the polymer template, were described by Chapiro in the case of polymerization of acrylic acid and acrylonitrile and details are described below. The ordered structure increases concentration of monomer at the reaction site, affects distances between pre-oriented monomer molecules, and changes a steric hindrance. This change in structure leads to the change in the kinetics of the polymerization reaction and it is responsible for stereo-control of the propagation step. [Pg.22]

The structure of ligands in metallocene complexes determines activity, stereoselectivity, and molecular weight of 1-alkene polymerizations, by controlling the preferential conformation of the growing polymer chain which in turn controls the stereochemistry of monomer coordination ( enantiomorphic site control ). The difference between this and the chain-end control mechanism mentioned earlier is that stereo errors due to misinsertions can be repaired.101,106... [Pg.1272]

Syndiotactic polystyrene is produced by well stereo-controlled coordination polymerization by titanium compounds. The polymer has a high percentage of mr pentad structure. The 13C NMR chemical shift for the phenyl-1 carbon and backbone methylene carbon are approximately 145.3 and 44.9 ppm, respectively [1]. In general, these polymers are found to be >99% pure in syndiotactic structure as defined by NMR. The most stereoselective catalysts produce a >99.6% pure syndiotactic structure (Figure 18.2)... [Pg.390]

Anionic polymerizations of polar monomers in polar solvents are only poorly stereospecific, whereas, according to conditions, the anionic polymerization of apolar monomers in apolar solvents can be strongly stereo-controlled. All possible levels of stereocontrol lie within the two extremes. [Pg.154]

Stereo control can also be achieved in the homogeneous free-radical polymerization of vinyl monomers by varying the polymerization temperature. Some typical monomers that behave in this manner include methyl methacrylate,(39) vinyl acetate,(41) vinyl chloride,(42) isopropyl and cyclohexyl acrylates.(40) As the polymer temperature is lowered the crystallizabiUty of the polymers becomes more discernible. (46) This observation can be attributed to the fact that as the temperature is lowered there is a preference for units in the same configuration to be added to the growing chain. It has been found that in general there is a preference for syndiotactic sequences to develop as the temperature is lowered. As an example, the observed melting temperature of poly(vinyl chloride) increases from 285 °C to 310 °C as the polymerization temperature is lowered from —15 °C to —75°C, with a concomitant increase in the syndiotacticity.(47)... [Pg.166]

Figure 20 illustrates idealized classification of the stereo-controlled polymerizations of a racemic monomer that bears one center of chirality (p-BL case). For the sake of simplicity, only formation of isotactic (...-RRRRRRRR-... or. ..-SSSSSSSS-...) polymer formation is shown. In principle, this scheme conforms also to the rac-LA (containing two identical centers of chirality in one molecule) polymerization. In a more general case, formation of the syndiotactic (...-RSRSRSRS-...) and heterotactic (...-RRSSRRSS-...) polyester chains (see, e.g., structures 5) should additionally be considered vide infra). [Pg.237]

TABLE 8 Range of Polymer Structure/Properties Through Stereo-Control of Epoxide Polymerization... [Pg.67]

Paleos [43] was the first investigator to attempt control of microstructure of 2 through orientation of 1 (R = CH3) at a water-toluene interface. The water-organic solvent interface provides a reaction site, that may orient amphipathic molecules and ions in a particular way. Paleos was interested in obtaining stereoregular 2, i.e. pure isotactic or syndiotactic forms of 2, in this fashion but the experimental results were inconclusive [43-45]. Fife and Hu [46] have reinvestigated the possibility for stereo-controlled polymerization of 1 (R = C12H25) as a component of aqueous suspensions of microemulsions. This work will be explored in detail in Sect. 6.2.4. [Pg.76]

In principle, rac-lactide, a racemic mixmre of d- and L-lactide, may be polymerized in a stereoselective fashion. Depending on the stereoselection as the ROP proceeds, the resulting polymer may thus exhibit different stereoregularities these directly influence the thermal and mechanical properties of the produced PLAs. In this regard, isotactic PLA stereoblocks and PLA stereocomplexes, which are of interest for their thermal and mechanical properties, may be produced via the ROP of rac-lactide initiated by an achiral derivative, provided the polymerization proceeds via a chain-end stereocontrolled mechanism i.e., the last inserted lactide unit stereo-controls the insertion of the incoming monomer. This strategy has been first validated using salen-based aluminum complexes such as 16 (Scheme 16, top) to produce PLLA-PDLA isotactic stereoblocks [95, 96]. Alternatively, the chiral racemic salen aluminum complex 17 was found to be suitable for the parallel stereoselective synthesis of isotactic poly(D-lactide) and poly(L-lactide) from rac-... [Pg.142]

Activation enthalpies and entropies of stereo-control in free radical polymerization See corresponding chapter of this Handbook ... [Pg.766]

EHas HG, Riva M, Gold P. Stereo-controlled polymerizations. 5. free radical polymerization of vinylformate. Macromol Chem Phys. 1971 145 163-168. [Pg.252]

Structurally, plastomers straddle the property range between elastomers and plastics. Plastomers inherently contain some level of crystallinity due to the predominant monomer in a crystalline sequence within the polymer chains. The most common type of this residual crystallinity is ethylene (for ethylene-predominant plastomers or E-plastomers) or isotactic propylene in meso (or m) sequences (for propylene-predominant plastomers or P-plastomers). Uninterrupted sequences of these monomers crystallize into periodic strucmres, which form crystalline lamellae. Plastomers contain in addition at least one monomer, which interrupts this sequencing of crystalline mers. This may be a monomer too large to fit into the crystal lattice. An example is the incorporation of 1-octene into a polyethylene chain. The residual hexyl side chain provides a site for the dislocation of the periodic structure required for crystals to be formed. Another example would be the incorporation of a stereo error in the insertion of propylene. Thus, a propylene insertion with an r dyad leads similarly to a dislocation in the periodic structure required for the formation of an iPP crystal. In uniformly back-mixed polymerization processes, with a single discrete polymerization catalyst, the incorporation of these intermptions is statistical and controlled by the kinetics of the polymerization process. These statistics are known as reactivity ratios. [Pg.166]

From the standpoint of synthesis of polysaccharides, the most significant aspect of a polymerization mechanism is whether or not it involves regio- and stereo-selective control. However, the structural and stereochemical problems with polysaccharide synthesis are generally simpler than those that obtain with a racemic, unsymmetrical monomer, such as (R, S)-propylene oxide. For example, a variable percentage of head-to-head and tail-to-tail sequences is found in... [Pg.175]

When butadiene and 2,3-dimethylbutadiene are included in the channels of urea and thiourea, respectively, 1,4 addition invariably results to yield polymers with chemical and stereo regularities (Scheme 39). Note that addition in the 1,2 fashion is prevented sterically by the narrow channel. Similarly, high selectivity was obtained when butadiene, vinyl chloride, and styrenes were polymerized in the channels of cyclophosphazenes. Syndiotac-tic polymer alone is obtained from vinyl chloride included in urea channels this is apparently the first example of inclusion polymerization of a vinyl polymer in which control is exerted over the steric configuration of the developing tetrahedral carbon atom (Scheme 39). Highly isotactic polymer is obtained from 1,3-pentadiene when it is included in a perhydrotriphenylene matrix (Scheme 39). Note that addition could occur at either end (i.e., Q to... [Pg.151]

After the Natta s discovery of highly stereospecific polymerization processes, the interest in the preparation and properties of optically active polymers has greatly increased. In fact, the use of asymmetric catalysts or monomers to obtain optically active polymers may supply interesting informations on the mechanism of steric control in stereo-specific polymerization furthermore optical activity is an useful tool to study the polymer stereoregularity and the chain conformations of polymers in the molten state or in solution. [Pg.394]


See other pages where Stereo-controlled polymerization is mentioned: [Pg.11]    [Pg.75]    [Pg.135]    [Pg.14]    [Pg.300]    [Pg.500]    [Pg.429]    [Pg.191]    [Pg.181]    [Pg.63]    [Pg.18]    [Pg.874]    [Pg.217]    [Pg.219]    [Pg.226]    [Pg.228]    [Pg.98]    [Pg.49]    [Pg.210]    [Pg.237]    [Pg.274]    [Pg.143]    [Pg.177]    [Pg.188]    [Pg.381]    [Pg.75]    [Pg.1]    [Pg.6]    [Pg.14]    [Pg.89]    [Pg.443]   
See also in sourсe #XX -- [ Pg.300 ]




SEARCH



Controlled polymerization

© 2024 chempedia.info