Control of molecular weight

The quantitative dependence of Z , and hence molecular weight, on the extent of reaction in condensation polymerization is shown by Eq. (5.30). From the corresponding data presented in Table 5.2 it is evident that to produce a relatively high-molecular-weight polymer (X 100), it is necessary to allow the reaction 

It is further evident from Table 5.2 that Xn, and hence molecular weight, increases very rapidly with conversion in the high conversion range (p 0.99). An increase in conversion from 0.990 to 0.999, for example, leads to a 10-fold increase in Xn- It is thus not a realistic proposition to control polymer molecular weight by adjusting the extent of conversion. An alternative procedure to control Xn would be to introduce, deliberately, an imbalance in the ratio of the two types of functional groups in the feed. 

For example, the use of excess diol in the polymerization of a diol with a diacid yields a polyester (I) with hydroxyl end groups which are incapable of further reaction, since the diacid has been completely reacted  

The use of excess diacid leads to a similar result, as the polyester (II) now has only carboxyl groups at both ends after the diol is completely reacted  

Another method of achieving the desired molecular weight is by addition of a small amount of a monofunctional monomer. The monofunctional monomer effectively caps the chain end it reacts with and prevents further growth of the chain. 

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The polymeric products can be made to vary widely in physical properties through controlled variation in the ratios of monomers employed in thek preparation, cross-linking, and control of molecular weight. They share common quaHties of high resistance to chemical and environmental attack, excellent clarity, and attractive strength properties (see Acrylic ester polymers). In addition to acryHc acid itself, methyl, ethyl, butyl, isobutyl, and 2-ethylhexyl acrylates are manufactured on a large scale and are available in better than 98—99% purity (4). They usually contain 10—200 ppm of hydroquinone monomethyl ether as polymerization inhibitor. 

Detailed modifications in the polymerisation procedure have led to continuing developments in the materials available. For example in the 1990s greater understanding of the crystalline nature of isotactic polymers gave rise to developments of enhanced flexural modulus (up to 2300 MPa). Greater control of molecular weight distribution has led to broad MWD polymers produced by use of twin-reactors, and very narrow MWD polymers by use of metallocenes (see below). There is current interest in the production of polymers with a bimodal MWD (for explanations see the Appendix to Chapter 4). 

Zucchini, U. and Cecchin,G. Control of Molecular-Weight Distribution in Polyolefins Synthesized with Ziegler-Natta Catalytic Systems. Vol. 51, pp. 101 — 154. 

There are methods to manipulate the backbones of polymers in several areas that include control of microstructures such as crystallinity, precise control of molecular weight, copolymerization of additives (flame retardants), antioxidants, stabilizers, etc.), and direct attachment of pigments. A major development with all this type action has been to provide significant reduction in the variability of plastic performances, more processes can run at room temperature and atmospheric pressure, and 80% energy cost reductions. 

Comparisons of relative rate constants obtained with Mv s of the total polymer and M s of the HMWF for the same samples show similar trends negligible transfer and termination control of molecular weights for the f-BuCl/Et2AlCl/MeCl system in the —40° to —60 °C range and also for the f-BuBr/Et2AlCl/MeCl at —50 °C (Table 7). For the samples prepared with the f-BuCl/Et AlCl system Mayo plots based on Mv s show zero intercept while that based on Mn s of the HMWF shows a small but finite intercept, z., ktr/kp = 1.91 x 10-5 and 2.14 x 10-s at —50° and -60 °C. Similarly, for the samples prepared with the t-BuBr/Et2AlCl system the Mayo plot based on Mn s of HMWF shows zero intercept while the Mayo plot based on Mv s show a very small intercept, ie., ktr/kp = 5.0 x 10-s at —50 °C. The reasons for this small discrepancy are not known. 

In the more general case of joint control of molecular weight by both transfer and radical termination, it is appropriate to consider that two distributions are formed simultaneously. One of these distributions consists of molecules terminated by chain transfer the other of pairs of chains joined by the combination of radicals. For any conversion increment, the two coexisting distributions will depend on the same parameter p representing the probability of continuation of the growth of any chain, i.e. 

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 effect of the nitrone stmcture on the kinetics of the styrene polymerization has been reported. Of all the nitrones tested, those of the C-PBN type (Fig. 2.29, family 4) are the most efficient regarding polymerization rate, control of molecular weight, and polydispersity. Electrophilic substitution of the phenyl group of PBN by either an electrodonor or an electroacceptor group has only a minor effect on the polymerization kinetics. The polymerization rate is not governed by the thermal polymerization of styrene but by the alkoxyamine formed in situ during the pre-reaction step. The initiation efficiency is, however, very low, consistent with a limited conversion of the nitrone into nitroxide or alkoxyamine. 

Borstar A catalytic process for polymerizing ethylene. Use of two reactors, a loop reactor and a gas-phase reactor, allows better control of molecular weight distribution. The loop reactor operates under super-critical conditions to avoid bubble formation. Either Ziegler-Natta or metallocene catalysts can be used. The first commercial unit was installed in Porvoo, Finland, in 1995. 

Cyriac A, Lee SH, Varghese JK, Park ES, Park JH, Lee BY (2010) Immortal C02/propylene oxide copolymerization precise control of molecular weight and architecture of various block copolymers. Macromolecules 43(18) 7398-7401... 

The reaction actually involves the sodium salt of bisphenol A since polymerization is carried out in the presence of an equivalent of sodium hydroxide. Reaction temperatures are in the range 50-95°C. Side reactions (hydrolysis of epichlorohydrin, reaction of epichlorohydrin with hydroxyl groups of polymer or impurities) as well as the stoichiometric ratio need to be controlled to produce a prepolymer with two epoxide end groups. Either liquid or solid prepolymers are produced by control of molecular weight typical values of n are less than 1 for liquid prepolymers and in the range 2-30 for solid prepolymers. 

Figure 3-19 shows a generalized solution process for polymerization of styrene. Actual processes may have up to five reactors in series processes with only one reactor are sometimes used. Styrene, solvent (usually ethylbenzene in amounts of 2-30%), and occasionally initiator are fed to the first reactor. Solvent is used primarily for viscosity control with the amount determined by the exact configuration of the reactor and the polymer molecular weight desired. A secondary function of solvent is control of molecular weight by chain... 

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