Poly , syndiotactic association

Clustering (association or aggregation) as a result of dissimilar parts within one chain molecule is well-known in the field of biopolymers a recent example in the field of synthetic polymers is provided by poly (p-nitrophenylmethacrylate) which in dimethylformamide (105) forms threefold clusters, presumably because of the difference in polarity of backbone and ester groups. Liquori (118) has presented proof for a double-stranded helix of isotactic and syndiotactic polymethylmethacrylate. 

The poly(methyl methacrylate)s prepared in this experiment, as well as polymers formed in model reactions with silanes that contain bulky substituents (such as phenyldimethyl and diphenylmethyl groups), have predominantly syndiotactic structures identical to polymers prepared by the conventional group-transfer process. This result supports a two-step dissociative mechanism for a group-transfer process, because steric hindrance from bulky silyl groups should increase the proportion of isotactic triads in the hypothetical associative concerted propagation step. 

The influence of preformed stereoregular polymethylmethacrylate on the polymerization mechanism is particularly interesting. Grignard compounds at — 50 C in toluene give syndiotactic poly(methylmethacrylate) when preformed isotactic poly(methyl-methacrylate) is present, and vice versa [30,31]. In this replica polymerization, the primary structure formed is the 1 1 (isotactic/syndiotactic) complex. Further association between this complex and syndiotactic macromolecules results in the 1 2 (isotactic/syndiotatic) complex [32]. In the absence of preformed polymer, isotactic poly(methylmethacrylate) was obtained under the same conditions. 

This conclusion has been reached in the copolymerization of acrylic acid (AA) and methacrylic acid (MAA) with N-vinylpyrrolidone (34, 35) (a monomer noted for its thermal stability (36) and discussed in Chapter 9) and with acrylamide (37). The incorporation of the acid monomer in the copolymer decreases with increasing solution pH. The r values with MAA are particularly low at pH <5 because of hydrophobic associations of the methyl groups. Laser-Raman studies (38) have also indicated intramolecular association among the methyl groups of syndiotactic poly(methacrylic acid) (PMAA) in aqueous solution. The addition of NaCl, at a moderate pH, increases the amount of neutralized, weak acid monomer incorporated (Table III). A more gradual change is observed with pH in the MAA-AM combination than in the AA-AM pair because of the hydrophobic interactions cited. The relationships are nearly quantitative with the ionization of the acids as reflected by the pK of the monomer and polymer acid sequences. 

Syndiotactic poly (methyl methacrylate) is formed at higher concentrations of tetrahydrofuran (>7.5% by volume). The kinetics indicated that four molecules of tetrahydrofuran are associated with each active chain end. In contrast, isotactic polymer forms with only one molecule of tetrahydrofuran associated with... 

A similar two-step mechanism of gelation through coil-helix transition was confirmed for synthetic polymers with stereo-regularity. For instance, in solutions of syndiotactic poly(methyl methacrylate) (PMMA) in toluene, a fast intramolecular conformational change is followed by the intermolecular association leading eventually to gelation [49,50]. 

The first living metathesis polymerizations were established using metallacyclobutane complexes 3 and 4 as catalysts (Figure 20.11) and NB as the monomer. The latter complex has been employed for the synthesis of moderately regular (80 20 transicis) poly(anft-7-methylnorbornene) (poly(anti-7-MeNB)) that contains a small excess of syndiotactic structures. Trans double bonds are primarily associated with r dyads (r m = 75 25), whereas cis double bonds are predominantly associated with m dyads. The overall r m ratio of the hydrogenated derivative of the polymer (poly-H-(fl fi-7-MeNB), Figure 20.12) is 64 36. 

Study of the template action of poly(methyl methacrylate) on the polymerization of the monomer has been pioneered by Challa and his co-workers. The observed phenomena are attributable to stereo-association - under suitable conditions between isotactic (i-PMMA) and syndiotactic (s-PMMA) poly(methyl... 

N.m.r. has been used to probe various aggregation phenomena. Heatley and Begum observed that mieellization of styrene-butadiene-styrene tri-block copolymers in non-solvents for butadiene is accompanied by marked hindrance to butadiene mobility. Bovey et a/. have shown that C Ti s for poly(vinylidene fluoride) and PMMA are the same in a mixed solution as in single solutions. The unusual compatibility of these two polyn rs therefore does not arise from complex formation. Self-association of stereo-r ular PMMA s in dilute solution has been studied. In toluene, isotactic PMMA is 10—17% associated at 27 C whereas a highly syndiotactic sample is 76% associated. 

Van der Waals complexes result from association of non-ionic macromolecules via dipole-induced dipole or dispersion interactions. For example, the macromolecular complex of isotactic and syndiotactic poly(methyl methacrylate) forms as a result of van der Waals interactions [1]. Similar mechanisms give rise to complexes involving copolymers of poly(methacrylic acid) (PMAA) with poly(methyl methacrylate) (PMMA) [1,2]. 

It should be noted that the steric effects of the pendant groups considered above are simply additional contributions to the main chain effects. Similarly cis-trans isomerism in polydienes and tacticity variations in certain a-methyl substituted polymers alter chain flexibility and hence affect Tg. Well-known examples of cis-trans variations are polybutadiene cis Tg = — 108°C) and trans(Tg = — 18°C) or polyisoprene cis Tg= —73°C) and trans T = —53°C). An example of tacticity variation is poly(methyl methacrylate) for which the isotactic, atactic, and syndiotactic stereostructures are associated with Tg values of 45, 105, and 115°C, respectively. 

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