Formation of Linear Polymers

When deuterated 1-heptyne was polymerized it reacted at a considerably slower rate than the undeuterated species (70). Meriwether et al. conclude from this important effect that cleavage of a carbon-hydrogen bond determines the rate and that only one hydrogen transfer is involved in the formation of product. They assume intermediate a-bonded species containing Ni—H and Ni—C bonds (XXXIII), (XXXIV), and (XXXV), 

Reactions between alkynes and phosphine-nickel carbonyl complexes show a remarkable insensitivity toward the solvent and may be performed 

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Polymerisation of conjugated dienes can frequently lead to the formation of linear polymers containing main chain double bonds. Examples of such diene... 

The preparation of poly(m-carborane-siloxane) polymers has also been successfully achieved directly from the carborane monomer.22 The reaction used is shown in scheme 9. Here, the direct salt elimination reaction between dilithiocarborane and a dichlorosiloxane (e.g., 1,5-dichlorohexamethyltrisiloxane) results in the formation of linear polymers with a molecular-weight (M ) typically of 6800 dalton. However, the reported literature detailing this approach is very limited indeed, and the reaction has not found significant use. This is most probably because only relatively low molecular-weight polymers can be produced, ultimately restricting the flexibility to produce materials of controlled mechanical properties. 

The Diels-Alder reaction mechanism is discussed in greater detail in reference 3. As seen above the reaction between a benzocyclobutene terminated resin and a BMI results in the formation of linear polymer chains. 

The ring-opening effect of alkyllithium on cyclic sulfides (Section VI,A,5) can be utilized for the formation of linear polymers of the general formula 221 when an excess of thietane is used. " ... 

The permeability of solutes across lipid bilayers is a product of the partition coefficient and the transverse diffusion coefficient [30]. Bilayer polymerization can alter solute diffusion by modifying either or both of these processes. In order to examine the effect of polymerization on bilayer permeability a nonionic solute of moderate permeability, [3H-glucose], was encapsulated in the vesicles prior to polymerization, removed from the exterior after polymerization, and its permeation across the bilayer was measured periodically [31]. Quantitative measurements of the 3H-glucose leakage revealed that the formation of linear polymer chains from methacryloyl lipids reduced the permeability coefficient to 0.3 to 0.5 of that of the unpolymerized lipid vesicles. A larger reduction (two orders of magnitude) was only found when crosslinked polymer networks were formed [31]. 

According to Shashoua et al (1), the linear polymerization preferentially proceeds at temperatures below -30°C. At room temperature and at elevated temperatures, no formation of linear polymers was observed, but cyclotrimerization of isocyanates occurred. 

B16. Butler, G. B., and A. Crawshaw The formation of linear polymers from diene monomers by a cyclic polymerisation mechanism. II. Polyacrylic anhydride and the derived polyacrylic acid. J. Am. Chem. Soc. 80, 5464 (1958). 

In this context it is interesting to note that benzonitrile, Ph—C=N, trimerizes to a triazine on a Raney nickel surface. It was assumed that Jt-bonded nitriles were involved in the reaction mechanism.10 This reaction resembles the well-known template synthesis of phthalocyanine complexes from phthalodinitrile. Formation of linear polymers [—C(R)—N—] occurs on heating aryl or alkyl cyanides with metal halides.11... 

Different kinetic processes lead to different dependences of the values in Eq. (2.66) on the degree of conversion. In Ref.105 formulas for the MW(P) and cp(P) dependences were obtained for the principal reaction kinetics in the formation of linear polymers. Introducing these dependences into Eq. (2.66), yields the unique dependence of viscosity on the degree of conversion, q(P). 

In asi-casein, it is arguable that because the hydrophobic interaction surfaces are well separated from Ca2+ ion-binding sites, the electrostatic and hydrophobic free energies of association can be treated as separate and additive, leading to the Z2 dependence of the rate of aggregation under many circumstances. Likewise, the nearly bifunctional nature of the aggregation reaction is consistent with the formation of linear polymers, as observed in the absence of Ca2+ (Thurn etal., 1987b), and may involve the apposition of hydrophobic surfaces formed from the N- and C-terminal peptides. 

Crawshaw, A., and G. B. Butler The formation of linear polymers from... 

An additional experiment was done using two equivalents of bis(2-iodoethyl) ether under exactly the same conditions. The product H NMR spectrum was found identical with those in the former case. The conclusion is, that under those conditions the transition state leading to the cryptand skeleton is highly favoured over the formation of linear polymers or the 1 2 adduct. 

Let us discuss briefly the other possibilities of controlling the distribution of linear and cyclic polymers in cationic ring-opening polymerization. In cationic ring-opening polymerization, formation of linear polymer and cyclization may be described by the scheme ... 

Cyclophosphazenes (46) nndergo both thermal ringopening polymerization with formation of linear polymers (4 7) see Polyphosphazenes) and ring-ring eqnilibria (resnlting in ring expansion). 

On increasing the concentration, larger complexes will be formed and this can only occur by the formation of linear polymers of the type ... 

Incidentally, bulk polymerization is also the chief method used for commercial polycondensations. Polycondensations are not as exothermic as free radical catalyzed vinyl-type polymerizations, so thermal control is less of a problem. Bulk polycondensation also favors formation of linear polymer rather than the cyclic products that are favored by dilute solution polymerization, particularly if AB-type monomers are being used. Finally, since a high degree of polymerization (i.e., high DP, or high molecular weight) is only... 

The polymerisation of DVF was first investigated by two independent research groups (2-7) who reported that soluble polymers could readily be obtained using free radical and cationic initiators. These authors proposed that cyclopolymerisation had occurred with the formation of linear polymers having three-carbon bridged ferrocene units (I), and some acyclic units (II), in the chain. Evidence for structure (I) as the predominant unit in the polymer chain was provided by the low level of vinyl unsaturation detectable by NMR or infrared spectroscopy and the observation that bands attributable to a bridged ferrocene (8) were to be found in the infrared spectra of these polymers. 

It is well established that under certain reaction conditions isocyanates can form cyclic trimers or linear polymers. Shashoua et al. (1) have found that catalysis and reaction temperature are key factors which determine the composition of the resulting reaction products. They observed that the formation of linear polymers proceeded only at low temperatures (< -20°C) and the formation of cyclic trimers at ambient or higher temperatures. Under certain steric conditions formation of polycyclic structures were observed by Butler et al. 02,3) and Iwakura (4). 

Both D3 and D4 can be polymerized either anionically or cationically. The main advantages of the anionic polymerization of D3 are formation of a narrow MWD polymer and formation of linear polymer virtually free of cyclic products (the process can be stopped before the linear-cyclic polymer equilibrium is reached). The cationic polymerization is better suited for the preparation of telechelics, particularly with acetoxy or alkoxy end-groups. Moreover the polymerization of monomers containing functional groups like =Si—H, can also be performed only cationically, on account of the lability of the = Si—H bond toward bases. 

A number of cationic polymerizations of D3 and D4 have been described in the literature. More recently CF3S03H (TfOH) has been used in CH2C12 solvent for both D3 and D4 providing satisfactory conditions of the cationic synthesis of polydimethylsiloxanes. It seems, on the basis of analysis of literature data, that TfOH is the most suitable initiator, at least for small-scale laboratory synthesis. In this system the formation of linear polymer is accompanied by cyclic products and their proportion, as it will be discussed later in this section, depends on the initial monomer concentration ([DJ0). At higher [DJ0, the proportion of cyclic products is lower. Thus, it is desirable to conduct polymerizations at highest possible [D4]0. 

In the polymerization of the cyclic siloxanes, D3 and D4, the formation of linear polymer is accompanied by cyclic oligomers. Thus, polydimethylsiloxane prepared at temperatures not exceeding 200 °C contains up to 15-18 vol% cyclic compounds. This gives [D] 2.2 mol l-1 [where D = -(-Si(CH3)20—] and therefore this is the lowest... 

These two steps are the basic reactions in silicone polymerization. It will be recognized that fhe presence of two —OH groups on silicon allows the formation of linear polymers three or more —OH groups allows the formation of cross-linked polymers. There is no evidence that hydroxyls on the same silicon ever condense to give =Si=0 for this reason the polymer is free to grow. Acid or base catalysis is generally used to speed up the reaction. 

So far, a formation of linear polymer chains has been assumed. Although coupling in a-a position is most likely, it is also possible between a-P positions, whereupon a network is generated (Figure 11.7). 

The presence of the solid surface affects the formation of crosslmked and linear polymers in different ways [22]. In the case of crosslmked polymers, sufficiently large branched molecules are formed at comparatively early stages of the process. In the formation of linear polymers, the critical molecular weight of macromolecules at which the surface becomes capable of effecting its conformation set is higher than for crosslinked polymers. 

In this system, ATG performs other functions as well. In the course of the adhesive cure, ATG copolymerizes with MMA to form a copolymer that contains isocyanate able to form intermolecular crosslinks consisting of disubstituted urea groups when it reacts with moisture that diffuses into the adhesive interlayer. Such separation in time of the processes of formation of linear polymer and of the intermolecular... 

Free metasUidc acid is contained in waterglasses with M > 2 and thus formation of linear polymers becomes possible ... 

Step-growth polymerization can be complicated by cyclization reactions that accompany formations of linear polymers. Such ring formations can occur in reactions of monomers with either the same type of functional groups or with different ones. Some illustrations of cyclization reactions follow ... 

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