Control of Molecular Weight and Polydispersity

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. 

Alternative bimolecular methods have been reported that involve mixing appropriate ratios of monomer with free-radical initiators (such as benzoyl peroxide) and an excess of the nitroxide stable free-radical moiety. Such bimolecular methods do not afford the same degree of control of molecular weight and polydispersity since the stoichiometry of the mediating system cannot be accurately dehned, which is a crucial factor in these controlled polymerization systems. A wide variety of unimolecular nitroxide based initiator systems have been described in the literature with those based upon the 2,2,6,6-tetramethylpiperidinyl-l-oxy (TEMPO) group proving to be the most commonly used. 

A special case of control over the structure of copolymers may include the first stereoblock copolymers made by CRP. By applying either RAFT or ATRP to polymerization of acrylamides in the presence of rare-earth triflates such as Y(OTf)3 and Yb(OTf)3, it was possible to enhance isotacticity of A(A/-dmiethylacrylamide (DMA) from 50% meso to 90% meso dyads (170). At the same time control of molecular weights and polydispersity was preserved. Similar results were obtained for RAFT of A(-ispropylacrylamide (171). The ATRP and RAFT of DMA was applied to the first one-pot stereoblock synthesis by radical mechanism. RAFT or ATRP of DMA were started without Lewis acid to produce the first atactic block. Subsequently, the complexing agent was added at the desired conversion to continue the chain growth with the preferential isotactic placement (170). 

Gong C, Miravet J, Frechet JMJ (2000) Intramolecular cyclization in the polymerization of ABx monomers approaches to the control of molecular weight and polydispersity in hyperbranched poly(siloxysilane). J Polym Sci A 37 3193-3201... 

Most of the stereoselective initiators are heterogeneous systems that have a low effidency, and therefore a predse control of molecular weight and polydispersity is not possible. A number of stereoselective and stereoelective polymerizations of MT have also been studied in a homogeneous phase using chiral cadmium thiolates of cysteine esters and cadmium carboxylates of cysteine and methionine (Figure 4). 

The degree of control over molecular weight and polydispersity. 

Akkara, J.A., Kaplan, D.L., and Ayyagari, M. (2000) Process to control the molecular weight and polydispersity of substituted polyphenols and polyaromatic amines by enzymatic synthesis in organic solvents, microemulsions, andbiphasic systems, United States Dept, of the Army, USA US 6096859 US 20010003774 US 6362314. 

An alternative approach is the so-called hypergrafting that relies on the use of a linear block copolymer with a poly-ftmctional (usually relatively short) second block that acts as an initiator in the grafting polymerization of the branching monomer even for a step-growth mechanism. In this case the polyfimctionality of the initiator permits control over molecular weights and polydispersity and suppresses homopolymetization. 

For MMA polymerization, the addition of a Lewis acid, specifically scandium triflate [Sc(OTf)3], is known to increase the fraction of isotactic triads and enhance the rate of polymerization. Similar stereocontrol was observed for RAFT polymerization with cumyl dithiobenzoate and Sc(OTf)3. However, these RAFT polymerizations gave only poor control over molecular weight and polydispersity. Our NMR analyses confirm that the poor... 

It is well known that the useful properties of siloxanes stem from the nature of the siloxane bond (93), the structural characteristics of the polymer, the synthetic control over molecular weight and polydispersity, and the character of the framing groups (R). Polysiloxanes are presently available commercially as linear chains (93), rings, ladders, and three-dimensional networks (e.g., polyhedral oligomeric... 

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