Reaction, chain, copolymer reactions

Chain transfer reaction during propagation gives homopolymers as well as block copolymers. Separation of the homopolymers is performed by extraction with suitable solvents. Homopolymer A together with a small amount of block copolymer rich in component A are extracted... 

For flexible chain copolymers based on acrylic and methacrylic acids (AA and MA) crosslinked with a polyvinyl component, the inhomogeneity of the structures formed depends on the nature of the crosslinking agent, its content in the reaction mixture and the thermodynamic quality of the solvent [13,14],... 

Since 1 is a monomer with low activity, copolymers 2 obtained at any stage of the copolymerization process, irrespective of the monomer ratio in the initial mixture, always contain a smaller amount of monomeric units of 1 than that in the corresponding monomer mixture. 1 being prone to enter the chain-transfer reaction, the increase of its content in the initial monomer mixture reduces substantially the reaction rate and decreases the molecular mass of the copolymers. It was found that copolymers 2 which contain 2—8% of monomeric units of 1 and are suitable for obtaining fibres must have a molecular mass between 45 000 and 50000. Such copolymers can be obtained with a AN 1 ratio in the initial mixture between 95 5 and 85 15. Concentrated solutions of copolymers, especially those with a molecular mass smaller than the above limit, are characterized by a very low stability which is a substantial shortcoming of these copolymers. 

Reactivity ratios for the copolymerization of AN with 7 in methanol at 60 °C, proved to be equal to rx AN= 3,6 0,2 and r%n = 0 0,06, i.e., AN is a much more active component in this binary system. The low reactivity of the vinyl double bond in 7 is explained by the specific effect of the sulfonyl group on its polarity26. In addition to that, the radical formed from 7 does not seem to be stabilized by the sulfonyl group and readily takes part in the chain transfer reaction and chain termination. As a result of this, the rate of copolymerization reaction and the molecular mass of the copolymers decrease with increasing content of 7 in the initial mixture. 

Siloxane-urea copolymers were synthesized by the reaction of the aminopropyl terminated PDMS oligomers with MDI or HMDI with no chain extenders (Reaction Scheme XI and Tables 14 and 15). Therefore, in these copolymers the hard segments consist of the aminopropyl end groups on the siloxane oligomers and MDI or HMDI backbones as shown below. The soft segment is pure polydimethylsiloxane. 

Another consequence of the absence of sponataneous transfer and termination reactions is that the polymer chains formed remain living 3), i.e. they carry at the chain end a metal-organic site able to give further reactions. Block copolymer synthesis is probably the major application 12 14), but the preparation of co-functional polymers, some chain extension processes, and the grafting onto reactions arise also directly from the long life time of the active sites. 

If the chains are long, the composition of the copolymer and the arrangement oi units along the chain are determined almost entirely by the relative rates of the various chain propagation reactions. On the other hand, the rate of polymerization depends not only on the rates of these propagation steps but also on the rates of the termination reactions. Copolymer composition has received far more attention than has the rate of copolymerization. The present section will be confined to consideration of the composition of copolymers formed by a free radical mechanism. 

We make polyethylene resins using two basic types of chain growth reaction free radical polymerization and coordination catalysis. We use free radical polymerization to make low density polyethylene, ethylene-vinyl ester copolymers, and the ethylene-acrylic acid copolymer precursors for ethylene ionomers. We employ coordination catalysts to make high density polyethylene, linear low density polyethylene, and very low density polyethylene. 

The preparation of a functional segmented block copolymer was also investigated (scheme ll).15 First hydroboration polymerization of the oligomer using thexylborane was carried out. Then the obtained organoboron polymer was subjected to a chain-transformation reaction (DCME rearrangement). DCME and lithium alkoxide of 3-ethyl-3-pentanol in hexane was added to a THF solution of the polymer at 0°C. 

For analyzing structure-property relationships, a variety of PEO-g-PVA copolymers were prepared, differing in the VAc-to-PEO ratio and the molar mass of PEO. The analysis of the copolymers by IR and 1H- and 13C-NMR showed the presence of both PEO and PVA. A small C=0 absorption was still present and was explained by a nonquantitative saponification. SEC showed polydispersities (Mw/M ) of around 5, with a small tailing to the low molar mass side. The latter was probably caused by the relatively low molar mass PVA homopolymer formed by the chain transfer reaction of VAc, both to the PEO and its acetate functionality. 

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). 

The Zr-FI catalyst selectively forms PE even in the presence of ethylene and 1-octene, while the Hf complex affords amorphous copolymers, resulting in the catalytic generation of PE- and poly(ethylene-c6>-l-octene)-based multiblock copolymers through a reversible chain transfer reaction mediated by R2Zn. The development of an FI catalyst with extremely high ethylene selectivity as well as a reversible chain transfer nature has made it possible to produce these unique polymers. Therefore, both Ti- and Zr-FI catalysts are at the forefront of the commercial production of polyolefinic block copolymers. 

This molecular weight response clearly indicates that chain-shuttled ethylene-octene block copolymers, rather than blends, are formed upon introduction of DEZ. The Mn can also be used in conjunction with the DEZ feed and polymerization rate to calculate the number of chains produced per Zn molecule. The low DEZ level of sample 4 results in the production of ca. 12 chains/Zn. However, the reaction is practically stoichiometric at higher DEZ (no H2), with production of sample 6 resulting in 1.9 chains/Zn (or ca. one chain per Zn-alkyl moiety). This example indicates that nearly every polymer chain exited the reactor bound to the CSA, with very little chain termination, demonstrating the efficiency of the chain shuttling reaction. 

The two reactor feeds were controlled to give copolymers with the desired densities, and a physical blend and a diblock OBC were produced. DEZ was added to the first reactor to achieve the desired melt index (/2 = 20 dg min1, equivalent to a Mn of -15-20 kg mol1). This material was fed to the second reactor, and production was continued under different conditions. The material collected after the second reactor had a lower melt index (/2 = 3.9 dg min1), indicating a higher molecular weight consistent with the chain extension reaction from the CCTP process. 

Recent advances in the development of well-defined homogeneous metallocene-type catalysts have facilitated mechanistic studies of the processes involved in initiation, propagation, and chain transfer reactions occurring in olefins coordi-native polyaddition. As a result, end-functional polyolefin chains have been made available [103].For instance, Waymouth et al.have reported about the formation of hydroxy-terminated poly(methylene-l,3-cyclopentane) (PMCP-OH) via selective chain transfer to the aluminum atoms of methylaluminoxane (MAO) in the cyclopolymerization of 1,5-hexadiene catalyzed by di(pentameth-ylcyclopentadienyl) zirconium dichloride (Scheme 37). Subsequent equimolar reaction of the hydroxyl extremity with AlEt3 afforded an aluminum alkoxide macroinitiator for the coordinative ROP of sCL and consecutively a novel po-ly(MCP-b-CL) block copolymer [104]. The diblock structure of the copolymer... 

Scheme 8.13 Dynamic covalent polymers based on carbine dimerization, (a) Preparation of difnnctional carbene 58 and polymerization of 58 via carbene dimerization (b) Chain transfer reaction of 59 by the agency of monofnnctional carbene 60, and (c) Formation of the organometallic copolymer 62 by the insertion of PdCl [45],...

Although the mechanism of copolymerization is similar to that discussed for the polymerization of one reactant (homopolymerization), the reactivities of monomers may differ when more than one is present in the feed, i.e., reaction mixture. Copolymers may be produced by step-reaction or by chain reaction polymerization. It is important to note that if the reactant species are Mi and M2, then the composition of the copolymer is not a physical mixture or blend, though the topic of blends will be dealt with in this chapter. 

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