ISSN:0022-233X    

DOI:10.1080/00222338208056652

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

The precise tailoring of polymer molecules (macromolecular engineering) by cationic methods can only be achieved in the absence of adventitious protogenic events, i.e., uncontrolled initiation, chain transfer to monomer. It is postulated that these undesirable processes can be aborted in cationic polymerization charges by the use of certain sterically hindered amines. This group of compounds has been used to scavenge protons in preparative small molecule organic chemistry, the pertinent literature of which is briefly examined. Orienting polymerization experiments carried out in the presence of various sterically hindered amines, in particular with 2,6-di-tert-butyl-pyridine, gave encouraging results in terms of conversion, molecular weights, and molecular weight distributions, and gave impetus for further detailed investigations.

ISSN:0022-233X    

DOI:10.1080/00222338208056653

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

The effect of the sterically hindered base 2,6-di-tert-butyl-pyridine (DtBP) on the polymerization of α-methylstyrene (αMeSt initiated by “H2O”/BCl3in a wide temperature range) has been investigated. αMeSt polymerizations carried out under identical conditions except in the presence of DtBP give much lower yield, much higher molecular weight, and noticeably narrower distributions than those obtained in the absence of this hindered base. The possibility of DtBP acting as an inhibitor is ruled out. Yields, molecular weights, and molecular weight distributions virtually do not change by changing DtBP concentration from 1.0 × 10−1to 1.0 × 10−3M. Differences in conversion levels do not explain the observed phenomena. PaMeSt samples obtained in the presence of trace amount of DtBP were of much higher molecular weight than those prepared in the absence of hindered pyridine, even at the same conversion level. These findings are explained by proposing that DtBP is a specific proton scavenger able to trap protons emerging during chain transfer to monomer, and that other elementary events remain unaffected by the presence of this hindered base.

ISSN:0022-233X    

DOI:10.1080/00222338208056654

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

The effect of 2,6-di-tert-butylpyridine (DtBP) on a-methyl-styrene (αMeSt) polymerization induced by the “H20”/BC13initiating system has been studied. Yields, molecular weights, (MW's), and molecular weight distributions (MWD's) obtained in the presence of [DtBP] >1.0 × 10−3Mare invariant with DtBP concentration. Results are explained by the concept of terminative chain transfer, i.e., a process in which the proton which arises in the course of chain transfer to monomer is intercepted by DtBP so that the kinetic chain is terminated. Polymerization goes to completion in the presence of very small amounts of DtBP, i.e., 4.8 × 10−4M or less; however, MW's and MWD's of the poly(a-methylstyrene) (PaMeSt) formed are still profoundlyjiffected. Thus, in the absence of DtBP, Mn= 1.2 × 104and Mw/Mn= 3.8, whereas in the presence of [DtBP] = 1.0 ×l(10−4,MM = 4.5 × 10−4and Mw/Mn= 1.87.

ISSN:0022-233X    

DOI:10.1080/00222338208056655

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

The mechanism of cationic olefin polymerization has been investigated by studying the effect of 2,6-di-tertbutylpyridine (DtBP) on the polymerization of isobutylene by the C6H5C(CH3)2Cl/BCl3initiating system. In the absence of DtBP, initiation is by cumylation, chain transfer to monomer is absent, and termination is by chlorination. In the presence of DtBP, conversions and molecular weights decrease as a function of DtBP concentration, indicating that while DtBP does not interfere with nonprotic initiation by the C6H5C⊕(CH3)2carbocation, it functions as a terminating agent. According to Mayo plots, chain transfer to monomer is absent with or without DtBP in the system; however, the ratio kt/kp(the slope of the plot) obtained in the presence of DtBP is ∼25 times larger than that obtained in the absence of this hindered base which also indicates termination by DtBP. The exclusive presence of CH2˭C(CH3) ∼ endgroups in polyisobutylene prepared in the presence of DtBP suggests that this base is able to abstract a proton from the propagating ∼C⊕(CH3)2center. In contrast, according to initiator efficiency data, DtBP does not abstract a proton from the initiating C6H5C⊕(CH3)2cation.

ISSN:0022-233X    

DOI:10.1080/00222338208056656

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

The effect of 2,6-di-tert-butyl-pyridine (DtBP) on isobutylene polymerization induced by AlCl3in n-hexane, CH2Cl2, and n-hexane/CH2Cl2mixtures has been investigated. Polymerization carried out in the presence of DtBP invariably yielded much reduced yields, much elevated molecular weights, and noticeably narrower molecular weight dispersities than those carried out under identical conditions except in the absence of this hindered pyridine. As expected, conversions increase while [Mbar]n's decrease and [Mbar]w/[Mbar]n's broaden with increasing [AlCl3] in the absence of DtBP; however, in the presence of DtBP, increasing [AlCl3]'s lead to increased conversions, increasing [Mbar]n's, and narrowing [Mbar]w/[Mbar]n's. At constant [AlCl3], increasing [DtBP] tends to decrease yields but at the same time increases [Mbar]nand narrow [Mbar]w/[Mbar]n. Directionally these effects remain similar by changing the polarity of the medium. The proposition of proton scavenging during chain transfer to monomer by the hindered pyridine helps to explain these observations.

ISSN:0022-233X    

DOI:10.1080/00222338208056657

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

The influence of 2,6-di-tert-butylpyridine (DtBP) on the polymerization of α-methylstyrene (αMeSt) induced by the “H2O”/BC13initiating system in the -20 to −60°C range has been studied in detail. Adventitious H2O (“H2O”) is the initiating cationogen and initiation most likely proceeds by a concerted route in the absence of free protons or the acid H⊕BC13OH⊕Polymerizations are extremely rapid and kinetic termination is absent (conversions are 100%) in the absence of DtBP. In the presence of DtBP, polymerizations are still very fast; however, conversions are reduced. Significantly, conversions increase with decreasing temperatures, which suggests the operational presence of terminative proton entrapment. Molecular weights increase with decreasing temperatures in the presence and absence of DtBP and the slopes of the linear Arrhenius plots (In [Mbar]wversus 1/T) are parallel; the molecular weights obtained in the presence of DtBP are close to a factor of 10 higher than those produced in the absence of this hindered pyridine. The virtual identity of the slopes of Arrhenius plots indicates close similarity between the nature and rate of molecular-weight-determining events in the absence and presence of DtBP, i.e., kp/ktr,mand kp/ktr,G⊕DtBP profoundly affects molecular weight dispersity: [Mbar]w/[Mbar]n= 3.0–4.0 in the absence of DtBP whereas [Mbar]w/[Mbar]n= 1.5 -1.8 in the presence of DtBP. The number w of polymer molecules formed (yield/[Mbar]n) in the absence of DtBP whereas [Mbar]w/[Mbar]n= 1.5–1.8 in presence of DtBP. The number of polymer molecules formed (yield/[Mbar]n) = 1.5–1.8 in the presence of DtBp. The number of polymer molecules formed (yield/[Mbar]n) in the absence of DtBP decreases with decreasing temperature while those formed in the presence of DtBP remain constant. According to Mayo, (1/[Mbar]nversus 1/ [M] plots chain transfer to monomer in the peresence of DtBP is vey low (ktr,m/kp= 6.4 × 10−4and 2.8 × 10−4at −30 and −50°C) but not zero. Conceivably two kinds of chain transfer to monomer reactions may exist (direct and indirect) and only one (i.e., the indirect one) may be trappable by DtBP. The effect of [DtBP] on the percent converstion and [Mbar]nwas investigated: Above a fairly well defined [DtBP], neither conversions nor [Mbar]n's were affected by [DtBP]. With increasing [DtBP] molecular weight dispersions rapidly decrease and [Mbar]w[Mbar]n's seem to level off at ∼ 1.5 at relatively high [DtBP]. Changing the polarity of the solvent characteristically affects the mechanism of α MeSt polmerzation in the presence and absence of DtBP results in a strong increase in [Mbar]w/[Mbar]n(from ∼ 1.5 to ∼ 4.0), in the presence of DtBP [Mbar]w/[Mbar]nremains virtually unchanged at ∼1.5.

ISSN:0022-233X    

DOI:10.1080/00222338208056658

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

The polymerization of α-methylstyrene has been investigated using the “H2O”/SnCl4initiating system in the presence of 2,6-di-tert-butylpyridine (DtBP) in CH2Cl2solvent at −60°C. In the absence of DtBP and at low DtBP concentrations conversions are about 100%, but with increasing amounts of DtBP conversions drop to 3–5%. Polydispersity decreases with increasing [DtBP], The [Mbar]nversus [DtBP] plot shows a maximum; thus [Mbar]nincreases from ∼70,000 to ∼120,000 by increasing [DtBP] from 6 × 10−7to 4.8 × 10−4M and decreases to ∼ 10,000 on further increasing DtBP. The effect of DtBP on this polymerization system can be explained by assuming nonprotic initiation, termination only by DtBP, depressed chain transfer to monomer, and complex formation between the propagating carbocation and DtBP.

ISSN:0022-233X    

DOI:10.1080/00222338208056659

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

The polymerization of α-methylstyrene by the Me5C6CH2C1/ SnCl4nonprotic initiating system has been studied in the presence and absence of 2,6-di-tert-butylpyridine (DtBP) in CH2C12in the −30 to −80°C temperature range. Model experiments demonstrate that initiation involves direct carbocationation of the olefin by Me5C6C⊕H2. According to kinetic studies, polymerizations in the absence of DtBP are very rapid (no induction) and terminationless, and molecular weights are determined by chain transfer to monomer. In the presence of DtBP, polymerizations do not go to completion due to terminative proton entrapment. Reducing the temperature increases the yields and molecular weights. Evidently a decrease in temperature reduces the rate of proton expulsion and thus the rate of terminative proton entrapment is also reduced. That chain transfer to monomer is absent in the presence of DtBP is demonstrated by the constancy of the number of polymer molecules formed and initiator efficiencies obtained under a variety of experimental conditions, and by Mayo plots. In addition to being an initiator, Me5C6CH2C1 is also an efficient chain transfer agent. The chain transfer activity of Me5C6CH2Cl has been quantitatively determined in the -30 to -66°C range. The effect of temperature on poly(α-methylstyrene) molecular weights has been investigated and the Arrhenius plot yielded ΔHĐ[Pbar]n= −2.15 ± 0.2 kcal/mol.

ISSN:0022-233X    

DOI:10.1080/00222338208056660

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

The syntheses of chlorobutyl rubber-g-poly(α-methylstyrene) and poly(chloroprene-g-α-methylstyrene) have been accomplished with 90–97% grafting efficiencies. The synthesis principle involves the initiation of α-methylstyrene polymerization by chlorobutyl rubber or polychloroprene rubber in conjunction with BCI3 or SnCU coinitiators in the presence of 2,6-di-t-butylpyridine proton trap. Selective solvent extraction coupled with molecular weight determination and molecular size distribution indicate satisfactory separation procedures and reliable grafting efficiency data. The close agreement between theoretical and experimental graft copolymer molecular weights suggests that backbone scission or gelation does not occur during grafting. According to graft composition and molecular weight data, 6 to 11 poly(α-methylstyrene) branches have been attached per chlorobutyl or polychloroprene rubber backbones.

ISSN:0022-233X    

DOI:10.1080/00222338208056661

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor