Эротические рассказы

Supramolecular Polymers and Assemblies. Andreas WinterЧитать онлайн книгу.

Supramolecular Polymers and Assemblies - Andreas Winter


Скачать книгу
association constant (M−1) for an intermolecular reaction.

      In the case of a supramolecular polymerization in which a heteroditopic AB‐type monomer is used, EM defines the limit monomer concentration below which the (macro)cyclization pathway dominates the linear chain growth. This empirical approach allows one to predict the different cyclization reactions and, even more importantly, gives an absolute measure for a monomer's cyclization ability at the cost of its polymerization (valid only for reversible, non‐covalent interactions).

      For thermodynamically controlled step‐growth polymerizations, Jacobsen and Stockmayer predicted a critical concentration limit [87]: the system is exclusively composed of cyclic species below this value; above this value, an excess of monomer exclusively gives linear chains while the concentration of cyclic species stays constant (Figure 1.10b). These authors related the equilibrium constant for the cyclization to the probability for, thus directly connecting EM and ceff. It was additionally shown that this constant would decrease with N−5/2; in other words, a macrocycle composed of N subunits can reopen in N different ways. This study was extended by Ercolani et al., who also considered the size distribution of macrocycles under dilute conditions; thereby, a broad range of Ka values for the supramolecular macrocyclization were taken into account [83]. According to this, only for high Ka values (>105 M−1) can a critical concentration limit be observed.

      where EMn‐mer: effective molarity of the n‐mer, Kintra(n‐mer): intermolecular binding constant for the n‐th ring closure, Kinter: association constant (M−1) for an intermolecular reaction, EM1: effective molarity of the bifunctional monomer.

      Source: Flory and Suter [91].

      Dormidontova and coworker addressed the issue of the spacer's rigidity with respect to the ring‐chain equilibrium of supramolecular polymers [92]. Applying Monte Carlo simulations on such supramolecular polymerizations, these authors showed that the critical concentration was strongly dependent on the rigidity of the spacer (in these modeling studies, H‐bonding interactions were representatively studied). Keeping all further parameters constant (e.g. the length of the spacer or the energy for the interaction of the end groups), the critical concentration decreased in the following order: rigid > semi‐flexible > flexible. Thus, for rigid and semi‐flexible systems, the probability of their end groups meeting within a bonding distance and, thus, the formation of rings, is much smaller as for flexible systems.

      Various groups have reported on critical temperatures in ring‐chain equilibria (Tc). These values define the transition between macrocyclic and linear species of high molar mass [71, 72, 93]. Like the supramolecular IDP elaborated in Section 1.3.1, one has to also distinguish two limiting cases for the ring‐chain equilibrium polymerization [56]:

      1 Above a certain ceiling temperature, polymers of high molar mass are thermodynamically less stable than cyclic monomers or oligomers.

      2 Below a certain floor temperature, polymers of high molar mass are thermodynamically less stable than cyclic monomers/oligomers.

      In other words, a ceiling temperature can be found in those (supramolecular) polymerizations where negative changes in the enthalpy and entropy of propagation occur; in the second case, the changes in these measures are positive and, consequently, the floor temperature defines the limit below which (supramolecular) polymerization cannot occur.

      Covalent ROPs typically involve the opening of strained cycles (e.g. the cationic polymerization of tetrahydrofuran [THF] and dioxolane [42]). In general, such polymerizations represent enthalpy‐driven processes for which ceiling temperatures can be observed (basically, all species are of cyclic nature above this value). Very few examples are known for ROPs exhibiting a floor temperature [94]. Examples for such processes that are characterized by a gain in entropy are the ROP of cyclic S8 in liquid sulfur [93] and the ROMP of unstrained, macrocyclic olefins [70].


Скачать книгу
Яндекс.Метрика