Homopolymers single-site

As described briefly in Section V, the structurally and interaction symmetric binary blend is an Idealized model in which the A and B homopolymer (single site) chains are identical in every respect except they interact via a potential which disfavors mixing. By symmetry,... 

Description Polypropylene with a melt flowrate ranging from 0.1 to 1,200 can be produced with the Borstar PP process. Currently, Ziegler Natta catalysts are used, but there is a potential to use single-site catalysts latter. When producing homopolymers and random copolymers, the process consists of a loop reactor and a gas-phase reactor in series. One or two gas-phase reactors are combined with this arrangement when heterophasic copolymers are produced. Propylene, catalyst, cocatalyst, donor, hydrogen, and comonomer... 

Single-site metallocene catalysts are also highly reactive vis-a-vis cycloolefins such as cyclobutene, cyclopentene or norbornene. While homopolymers of these cycloolefins have melting temperatures (>380°C), much too high for technical processability, ethylene-cycloolefin copolymers (COC s) - e.g. ethylene-norbornene copolymers - are amorphous materials with glass transition temperatures, above which they become soft and processable [W. Kaminsky, J. Polym. Sci. A, Polym. Chem., 2004, 42, 3911]. 

Improved clarity of PP has provided the ability for replacement of PVC with PP in applications such as blisterpacks for hardware. In addition, new PP resins are being developed that use single-site metallocene catalysts (mPP). While virtually no difference exists in the processing behavior or finished product properties between conventional PP and mPP, these new materials are easier to nucleate. The use of nucleated mPP provides for a product with the higher physical properties of PP homopolymer and the clarity of nucleated random PP copolymer. 

Heterogeneous catalysts tend to produce polymers containing mixtures of copolymers and homopolymers.In contrast, the defined homogeneous catalysts containing a single site produce polymers possessing a more uniform structure, with a more random distribution of comonomer, and with molecular weights that can be more easily controlled. 

Figures 6 and 7 compare the sequence distributions and TREF chromatograms respectively for Ziegler-Natta- and metallocene-catalyzed ethylene-l-hexene copolymerizations. The lack of hlockiness in the metallocene resin in Figure 6 is demonstrated by the low levels of homopolymer triads, whereas intermolecular compositional homogeneity is shown by the narrow peak for the metallocene resin in Figure 7. It should be noted that while the narrow intermolecular distribution of comonomer content is a direct consequence of the single-site nature of the catalyst, the absence of hlockiness is a reflection of the relative magnitudes of the rate constants for insertion for the two monomers and does not speak to catalyst site diversity. The kinetics of insertion will be treated in the following section.

On a laboratory scale, single site catalysts based on metallocene/MAO are highly useful for the copolymerization of ethene with other olefins. Propene, 1-butene, 1-pentene, 1-hexene, and 1-octene have been studied in their use as comonomers, forming linear low-density polyethene (LLDPE) [188,189]. These copolymers have a great industrial potential and show a higher growth rate than the homopolymer. Due to thee short branching from... 

The structurally simplest polymer, and one of the most commercially important, is polyethylene. It consists of a linear chain of CH2 units, which we model as single spherical sites in the single-site homopolymer spirit. There exist well-developed ideal rotational isomeric state chain models where the bond rotational degrees of freedom are represented as discrete trans and gauche isomers. Numerical calculation of the required single-chain structure factor can be achieved via Monte Carlo simulation or using the recently developed computationally convenient approximate methods of McCoy and co-workers. ... 

However, a new concept of a tailored multi-site catalyst is indispensable in the next generation, because highly functionalized homopolymers and copolymers can be produced by the combined single-site catalysts. I conclude that this catalyst will open up new regions of the polyolefin industry in the future. 

Mixtures of two or more monomers can polymerize to form copolymers. Many copolymers have been developed to combine the best features of each monomer. For example, poly(vinyl chloride) (called a homopolymer because it is made from a single monomers) is brittle. By copolymerizing vinyl chloride with vinyl acetate, a copolymer is obtained that is flexible. Arrangement of the monomer units in a copolymer depends on the rates at which the monomers react with each other. Graft copolymers are formed when a monomer is initiated by free radical sites created on an already-formed polymer chain. 

Each lattice protein is compact and of a single type (i.e., a homopolymer) and features a binding site along one face (Fig. 14.1a). Complementary lattice... 

Terminal deoxynucleotidyl transferase normally adds homopolydeoxynucleotide tails to single-stranded DNA primers in the presence of a deoxynucleoside triphosphate and magnesium. If cobalt is used instead, not only does double-stranded DNA become an acceptable substrate, but ribonucleotides or homopolymer deoxyribo-nucleotide tracts may be added to all forms of duplex DNA at their 3 -ends, regardless of whether these are staggered or even.162 This allows terminal labelling for sequence analysis at the cleavage sites of restriction endonucleases,162- 183 or tail formation for in vitro studies on recombinant DNA.162... 

The combination of living cationic and anionic techniques provides a unique approach to block copolymers not available by a single method. Site transformation and coupling of two homopolymers are convenient and efficient ways to prepare well-defined block copolymers. 

Fig. 24 The side-chain models of amphiphilic polymers a amphiphilic homopolymer (poly-A), b regular alternating HA copolymer, c regular multiblock HA copolymer, and d protein-like HA copolymer. Each hydrophobic monomer unit (H) is considered as a single interaction site (bead) each amphiphilic group (A) is modeled by a dumbbell consisting of hydrophobic (H) and hydrophilic (P) beads...

With the single exception of PS-B, the PS homopolymers are more photostable than the copolymers. Among the random copolymers, the quantum yield for scission Increases as the proportion of S-unlts decreases. The least stable copolymers are those in which the S-unlts are separated by one or more MMA-units exclmer fluorescence Is not observed In these copolymers (1). As the ratio of exclmer fluorescence to fluorescence Increases, the quantum yield for scission decreases. Wherever a migrating singlet exclton Is trapped at an exclmer site. It Is less likely to contribute to the scission process. 

When a single ADP-ribose moiety (n = 1) becomes attached to an acceptor site, the reaction is referred to as mono(ADP-ribosyl)ation, and when a homopolymer chain of repeating units is attached, the process is referred to as poly(ADP-ribo-syl)ation (n > 1). Polymer chains may consist of over 200 ADP-ribose residues and may also contain branches, occurring some three times per 100 residues (see Figure 1). 

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